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Femtosecond Electron Imaging and Spectroscopy 4
(FEIS 4)
May 13-15, 2019, Lincoln, NE
Conference Abstracts
1
Compressive Sensing and the Full Spectrum of TEM Time Resolution
B. W. Reed1*, A. A. Moghadam1, R. S. Bloom1, S. T. Park1, and D. J. Masiel1
1 Integrated Dynamic Electron Solutions, Pleasanton, CA, USA
Time-resolved transmission electron microscopy (TEM) continues to grow and diversify as more and more
instruments are created around the world, each optimized for particular ranges of science targets. Not so
long ago, instrument designs neatly divided into picosecond/femtosecond laser-driven stroboscopic systems,
nanosecond/microsecond laser-driven single-shot/movie-mode systems, and conventional in situ
microscopes with continuous sources. New technologies are blurring these lines. These include faster
detectors, compressive-sensing systems, and alternative methods of obtaining time resolution. Concepts
developed over one range of time resolution carry over to others, and the entire 15-order-of-magnitude span
of in situ TEM time scales more and more looks like a smooth continuum rather than a set of disconnected
specializations.
We will especially focus on the insights afforded by the development of compressive sensing (CS) video
systems for TEM. These systems allow conventional Hz-scale cameras to access the microsecond regime
by encoding multiple frames of compressed video into each camera acquisition. But the general concept of
compressive sensing—using prior knowledge of statistical regularities to make the best possible use of
every single measurement—has applications beyond any single time scale or choice of technology. And,
according to an extensive set of simulations and theoretical studies, many of the apparent advantages of
certain CS-related undersampling/inpainting approaches actually derive from the data analysis and not the
undersampling itself.[1] In other words, how you analyze the data is at least as important as how you collect
it, and emerging CS-inspired data analysis methods have the potential to revolutionize methodology across
the spectrum.
[1] W. Van den Broek et al., IEEE Transactions on Computational Imaging,
DOI:10.1109/TCI.2019.2894950
This material is based in part upon work supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Award Number DE-SC0013104.
2
XFEL Science Opportunities and Plans for LCLS-II and LCLS-II-HE
(high-energy upgrade)
Robert W. Schoenlein
Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
Email: [email protected]
The unique capabilities of LCLS, the world’s first hard X-ray FEL, have had significant impact on
advancing our understanding across a broad range of science, from fundamental atomic and molecular
physics, to condensed matter, to catalysis, to structural biology. This talk will outline some of the ongoing
developments at LCLS.
A major upgrade of the LCLS facility (LCLS-II project) is now underway. LCLS-II is being developed as
a high repetition rate X-ray laser with two simultaneously operating, independently tunable FELs. The
baseline design features a 4 GeV continuous wave superconducting linac (CW-SCRF) that is capable of
producing uniformly spaced (or programmable) ultrafast X-ray laser pulses at a repetition rate up to ~1
MHz spanning the energy range from 0.25 to 5 keV. The superconducting linac will be installed in the first
third of the SLAC linac tunnel. The final third of the SLAC linac will continue to operate as a warm Cu
accelerator at energies up to 15 GeV, providing tunable X-rays with photon energy up to 25 keV at 120 Hz.
Four new instruments are planned to exploit the new capabilities of LCLS-II. One instrument will support
AMO science, strong-field science, and a new dynamic reaction microscope. Two instruments will rely on
a monochromator to support high-resolution and moderate-resolution soft X-ray spectroscopy at close to
the Fourier transform limit. A fourth instrument will operate in the tender X-ray range (1-7 keV) and will
be capable of combining pulses from both the soft X-ray and hard X-ray FELs.
Figure 1: Projected photons per pulse (left) and average brightness (right) for LCLS-II and proposed LCLS-II-HE, including
future X-ray facilities: the European XFEL and diffraction-limited storage rings (DLSRs).
Looking to the future, there is a compelling opportunity to upgrade the energy of LCLS-II (LCLS-II-HE).
By adding CW-SCRF cryomodules, the electron beam energy can be doubled to 8 GeV, thus increasing the
spectral reach of the hard X-ray undulator (HXU) to more than 12 keV. Anticipated improvements in
electron beam emittance will extend the energy reach to 20 keV. This will enable the study of atomic-scale
dynamics with the penetrating power and pulse structure needed for in situ and operando time-resolved
studies of real-world materials, functioning assemblies, and biological systems.
This talk will present some of the important science opportunities and instrumentation being planned for
LCSL-II and LCLS-II-HE.
3
Ultrafast interlayer electron transfer in van der Waals heterostructures
Hui Zhao
Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA
[email protected]; http://ultrafast.ku.edu
Recently, heterostructures formed by two-dimensional materials have drawn considerable
attention. Such new materials can potential combine novel properties of participating atomic layers
and form emergent properties suitable for a number of applications. In developing these materials,
one key issue is to understand and control interlayer charge and energy transfer.
In this talk, I will present recent progress on experimental studies of ultrafast charge and energy
transfer between monolayer semiconductors . The heterostructure samples were fabricated by
stacking different monolayer materials that were obtained by mechanical exfoliation or chemical
vapor deposition. Charge and energy transfer processes were studied by a layer-selective
femtosecond pump-probe technique. At first, previous studies by several groups on charge transfer
in van der Waals hetero-bilayers, such as MoS2/MoSe2, WS2/graphene, MoS2/graphene, and
MoS2/MoTe2, will be introduced. I will then present latest results on charge and energy transfer in
van der Waals bilayers, with emphases on a type-I heterostructure of MoS2/ReS2 and energy
transfer in homo-bilayers, such as MoSe2/MoSe2. We will discuss charge transfer in van der Waals
trilayers, and show evidence of the coherent nature of the transfer. Such measurements were further
expanded to a set of samples, where the electron transfer rate and recombination time are
systematically probed as the number of involved atomic layers evolves from one to four. We will
show that by band-alignment engineering, it is possible to control the flow and population of
electrons in different atomic layers. As an example, we achieved unipolar optical doping of
graphene with significantly extended photocarrier lifetimes.
4
Imaging collective state dynamics with femtosecond coherent electrons
Faran Zhou1, Joseph Williams1, Christos D. Malliakas2,3, Mercouri G. Kanatzidis2,3, Alexander
F. Kemper4, Chong-Yu Ruan1*
1 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA.
2 Department of Chemistry, Northwestern University, Evanston, IL 60208, USA.
3 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA.
4 Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
* e-mail: [email protected]
Nonequilibrium phase transition plays a pivotal role in a broad physical context from condensed matter to
cosmology. On a smaller scale, the quantum simulator experiments afforded by ultracold atoms have
started a new era of non-equilibrium systems. Central to these fundamental questions is the ability to
directly observe the dynamics of a many-body system as it unfolds upon a sudden change. In principle,
non-equilibrium systems may unveil thermodynamically hidden routes towards new or desirable states of
matter. In condensed matter systems, this is particularly relevant in the search for high-temperature
superconductors or insulator-metal transitions. Nonetheless, tracking the formation of non-equilibrium
phases in condensed matter is challenging and requires a resolution of the long-range cooperativity on the
ultra-short timescale.
Here, we demonstrate a charge-density-wave state that, upon a sudden interaction quench, exhibits a
critical behavior far from equilibrium. We capture the entire nonequilibrium self-organization dynamics
through highly sensitive, femtosecond coherent electron scattering experiments with atomic resolution. We
show that ultrafast quench over a continuous phase transition can replay the spontaneously symmetry
breaking (SSB) events. The interaction quench alters the system preference in SSB into a new macroscopic
broken-symmetry ground state not allowed thermodynamically. While this phase transition entirely occurs
on nonequilibrium timescales (100-500 fs), yet the dynamics toward the new macroscopic states is governed
by universality. This is supported by the observation of divergence of correlations and an initial freezing of
phase ordering over Zurek time ~250 fs implicating the critical slowing down predicted by the Kibble-
Zurek theory. These key timescales is but a few cycles of critical vibration periods, suggesting a high degree
of cooperativity to forge the new phase in this interaction-driven SSB phase transition.
Our quantitative measurements not only provide a benchmark for studying non-equilibrium phase
transitions in condensed matter systems, but also opens an intriguing perspective of controlling quantum
ultrashort timescales that are useful for practical applications.
5
Mega-Electron-Volt Ultrafast Electron Diffraction For Probing Electric-Field-Driven Structural Phase Transition
Xiaozhe Shen1, Aditya Sood2,3, Suji Park3, Suhas Kumar4, Marc Zajac2, Yifei Sun5, Jie
Yang1, Renkai Li1, Stephen Weathersby1, Shriram Ramanathan5, William Chueh2, Aaron
Lindenberg2,3, and Xijie Wang1
1SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
2Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA
3SIMES, SLAC National Accelerator Laboratory, Menlo Park, CA, USA
4Hewlett Packard Labs, Palo Alto, CA, USA
5Department of Materials Science and Engineering, Purdue University, IN, USA
Dramatic property-phase transitions can occur in materials under electric field excitation, such as the insulator-
to-metal transition in VO2 1, the antiferroelectric‐to‐ferroelectric transition in lead zirconate titanate stannate
ceramics2, etc. A time-resolved study of atomic structural changes during the electric-field-driven phase
transition is essential to understand the underlying mechanism, and ultimately, to control the phase transition
for applications. Here, we report the development of a mega-electron-volt ultrafast electron diffraction
apparatus for probing in-situ atomic structural changes of materials undergoing an electric-field-driven phase
transition. The apparatus has demonstrated a momentum transfer range of 9 Å-1 with a resolution of <0.17 Å-1.
The temporal resolution of the apparatus is determined by the rise time of the driving electric-field pulse.
Details of the experimental setup and performance characterization will be reported.
References: 1. Hyun-Tak Kim, et al., Appl. Phys. Lett. 86, 242101 (2005)2. L. Shebanov, et al., J. Appl. Phys.76, 4301 (1994)
6
Probing electron-phonon interplay in strongly correlated systems
Yimei Zhu
Department of Condensed Matter Physics and Materials Science
Brookhaven National Laboratory, Upton, NY 11973 USA
In this presentation I will give an overview of our recent work on probing competing
degrees of freedom of charge, orbital and lattice and electron-phonon coupling using
MeV-ultrafast electron diffraction (UED). A few examples will be given. The first is
the study of electron and lattice dynamics of optimally doped Bi2Sr2CaCu2O8+ō,
combined with time- and angular-resolved photon-emission spectroscopy (tr-ARPES).
By quantitatively measuring the diffuse scattering after photoexcitation we illustrate
the preferential coupling of electrons to the in-plane Cu-O bond-stretching phonons,
revealing unique electron-phonon interactions beyond the N-temperature model and
providing insight in resolving longstanding controversies surrounding equilibrium
interactions in the high-Tc superconductor. Another study is understanding ultrafast
dynamics of photoinduced melting and interplay between of local electronic
nematicity and lattice distortions in FeSe crystals, combined with TEM and x-ray
pair-distribution-function (PDF) for quantitative structural analysis. Finally, I will
report our latest development on the GHz pulser device implemented into a 200keV
commercial transmission electron microscope for ultrafast stroboscopic imaging. The
work is in collaboration with Euclid Inc. and NIST and preliminary results on metal-
insulator transition of strongly correlated oxides will be discussed. The author would
like to acknowledge collaborations with researchers from the UED-ATFII/BNL team,
the UED/SLAC team, and the Euclid-NIST team. Work at Brookhaven was supported
US DOE-BES under Contract No. DE-SC0012704.
7
Ultrafast carrier and structural dynamics of supported monolayer MoS2
Ding-Shyue (Jerry) Yang, Xing He, Mazhar Chebl
Department of Chemistry, University of Houston, Houston, Texas 77024 United States
Two-dimensional materials, such as graphene and transition metal dichalcogenides, have been considered
promising for novel (opto)electronic and energy applications due to their unique properties at the
mono- to few-layer limit. A thorough understanding of their carrier dynamics and energy transport
behavior is therefore needed. Here, we present ultrafast carrier, structural, and energy-transport dynamics
observed in sapphire-supported monolayer MoS2 by using time- resolved transient reflectivity and
ultrafast electron diffraction (UED). The exciton‒exciton annihilation process up to a carrier density
near the Mott transition was observed, followed by the carrier‒phonon coupling in a few picoseconds. In
particular, both monolayer MoS2 and the sapphire substrate surface were probed by reflection UED,
which enables direct monitoring of the structural dynamics at the interface. The characteristic time constants
of the steps involved in the dissipation of the photoexcitation energy were obtained. A thermal boundary
conductance of 10.0 MWm-2
K-1
was determined for the energy transport across the MoS2‒sapphire
interface.
*We acknowledge the support from the R. A. Welch Foundation and Samsung Global Research Outreach
Program and the partial support from a National Science Foundation CAREER Award.
8
Imaging the ultrafast structural dynamics during photochemical ring-opening
by megaelectronvolt ultrafast electron diffraction
T. J. A. Wolf1, D. M. Sanchez1,2, J. Yang1,3, R. M. Parrish1,2, J. P. F. Nunes4,5, M. Centurion5, R.
Coffee3,J. P. Cryan1, M. Gühr1,6, K. Hegazy1,7, A. Kirrander8, R. K. Li3, J. Ruddock9, X. Shen3, T.
Vecchione3, S.P. Weathersby3, P. M. Weber9, K. Wilkin5, H. Yong9, Q. Zheng3, X. J. Wang3, M. P.
Minitti3, T. J. Martínez1,
1Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, USA.
2Department of Chemistry, Stanford University, Stanford, USA.
3SLAC National Accelerator Laboratory, Menlo Park, USA.
4Department of Chemistry, University of York, Heslington, York, UK.
5Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, USA.
6Institut für Physik und Astronomie, Universität Potsdam, Potsdam, Germany.
7Department of Physics, Stanford University, Stanford, USA.
8EaStCHEM, School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, United Kingdom.
9Department of Chemistry, Brown University, Providence, USA.
The photochemical electrocyclic ring opening of 1,3-cyclohexadiene is a prototypical, ultrafast reaction in
agreement with the Woodward-Hoffmann rules and a model reaction for the biosynthesis of vitamin D.[1,2]
The reaction mechanism involves coupled dynamics of the electrons and nuclei of the molecule in the vicinity
of a conical intersection between the excited state and the ground state. It has been intensively investigated
by spectroscopic methods, which were primarily sensitive to transient changes in the electronic structure
during internal conversion through a conical intersection between the excited state and the ground state.[3]
The corresponding changes in the nuclear structure were so far investigated in ultrafast x-ray and electron
diffraction studies lacking either the temporal or spatial resolution to resolve the dynamics in terms of atomic
distance changes in real space and time.[4,5] Employing MeV ultrafast electron diffraction, we resolved these
structural changes for the first time with sub-Å spatial and femtosecond temporal resolution. We observe the
ring opening in time-dependent changes of atomic pair distribution functions, which we can directly compare
to ab-initio multiple spawning simulations of the reaction dynamics. We, furthermore, observe a substantial
speed-up of the ring-opening motion after the molecule has undergone internal conversion through the conical
intersection with the ground state due to the steep gradient of the ground state towards the minima of the
photoproduct 1,3,5-hexatriene. 1,3,5-hexatriene exhibits several different isomers with low barriers in between.
The ring-opening motion transforms in the ground state into the isomerization motion between the hexatriene
isomers. We observe a coherent oscillation of the nuclear wavepacket between different isomer structures.
References
[1] S. Deb, P. M. Weber, Ann. Rev. Phys. Chem. 62, 19 (2011).
[2] B. C. Arruda, R. J. Sension, Phys. Chem. Chem. Phys. 16, 4439 (2014).
[3] A. R. Attar et al., Science 356, 6333 (2017).
[4] C.-Y. Ruan, et al., Proc. Natl. Acad. Sci. USA 98, 7117 (2001).
[5] M. P. Minitti et al., Phys. Rev. Lett. 114, 255501 (2015). 9
Study of the photolysis dynamics of o-nitrophenol by MeV ultrafast electron
diffraction
J. P. F. Nunes1, J. Yang2, T. J. A. Wolf3, M. Williams2,3,4, R. Parrish2,3,4, B. Moore1, K. Wilkin1,
X. Shen2, M. Lin2,3, K. Hegazy3, R. Li2, S. Weathersby2, M. Gühr5, Todd Martinez2,3,4, X.
Wang2, M. Centurion1
1Department of Physics and Astronomy, University of Nebraska-Lincoln, 855 N 16th Street, Lincoln,
USA, [email protected]
2SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, USA
3Stanford PULSE Institute, SLAC, 2575 Sand Hill Rd, Menlo Park, CA, 94025, USA.
4Department of Chemistry, Stanford University, 333 Campus Dr., Menlo Park, CA, 94305, USA.
5 Institut für Physik und Astronomie, Universität Potsdam, Potsdam, Germany
The photolysis of nitroaromatic compounds, such as o-nitrophenol, has been hypothesized as a potential
source of nitrous acid (HONO) in the atmosphere.1 Previous spectroscopic2 and computational studies3
have reported the formation of HONO to occur in o-nitrophenol’s first excited state S1, and to be
mediated by an intramolecular proton transfer between the OH and NO2 groups. Moreover, this proton-
transfer is believed to be coupled to an out-of-plane rotation of the HONO group, which allows the
system to relax back to ground state through a conical intersection.4
The work here presented uses MeV ultrafast electron diffraction (UED) to structurally resolved o-
nitrophenol’s photolysis reaction coordinate and aims to complement the findings of previous studies,
which despite their unrivaled temporal resolution and sensitivity the electronic structure, could not
directly access the structural dynamics of the nuclei.
The ultrafast gas phase electron diffraction apparatus at the SLAC National Accelerator Laboratory was
used to capture the photolysis dynamics of o-nitrophenol along the S1 and S4 states, accessible by
illumination with 330 and 266 nm light, respectively. A comparative analysis of transient momentum and
real-space features in the 330 and 266 nm UED datasets has allowed us to explore the differences in the
photolysis dynamics along the S1 and S4 states, and with the support of ab-initio multiple spawning
(AIMS) simulations shed some light of the mechanisms behind the formation and release of HONO in o-
nitrophenol.
References:
[1] I. Bejan, U. Abd El Aal, I. Barnes, T. Benter, B. Bohn, P. Wiesen, J. Kleffmann, Phys. Chem. Chem.
Phys., 2006, 8, 2028−2035.
[2] Y.-Q. Wang, H.-G. Wang, S.-Q. Zhang, K.-M. Pei, X. Zheng, D. L. Phillips, J. Chem. Phys., 2006,
125, 214506.
[3] K. P. K. Namboodiri, S. Viswanathan, R. Ganesan, V. C. J. Bhasu, J. Comput. Chem., 1981, 2,
392−401.
[4] H. A. Ernst, T. J. A. Wolf, O. Schalk, N. Gonzalez-Garcia, A. E. Boguslavskiy, A. Stolow, M.
Olzmann, A.-N. Unterreiner, J. Phys. Chem. A, 2015, 119, 9225−9235.
10
Investigating Ring-Opening Reactions by Time-Resolved Photoelectron Spectroscopy with a Free-Electron Laser
S Pathak1, M Ashfold2, R Boll3, C Callegari4, B Erk5, R Feifel6, R Forbes7, M Di Fraia4
C Hansen2, D Holland8, R Ingle2, R Mason9, O Plekan4, K Prince4, A Rouzée10
R Squibb6, J Tross1, D Rolles1
1 J.R. Macdonald Laboratory, Kansas State University, USA 2 University of Bristol, UK, 3 European XFEL, Germany, 4 FERMI, Italy,
5 Deutsches Elektronen-Synchrotron, Germany, 6 Gothenburg University, Sweden, 7 University of Ottawa, Canada 8 Daresbury Laboratory, UK, 9 University of Oxford, UK, 10 Max-Born-Institut, Germany,
Studying ring-opening reactions is crucial to understand several key processes in nature. For example,
studies involving the ring-opening reaction in cyclohexadiene to understand vitamin D generation1. Also,
electrocyclic ring opening reactions are a promising candidate in the development of molecular switches.
Due to the advancements in computational chemistry, several computational models and molecular
dynamics simulation methods exist, which can predict the evolution of these ring-opened molecular
structures during the chemical reaction. In contrast, the experimental verification of these photo-induced
ring-opened products are still very limited. Here we report our results on the UV-induced ring-opening
and subsequent unimolecular dissociation of a heterocyclic ring molecule.
Our time-resolved photoelectron spectroscopy experiment is performed using short-pulse and narrow-
bandwidth extreme ultraviolet radiation provided by the seeded free-electron laser (FEL), FERMI in Italy.
High spectral resolution at FERMI (compared to other FELs) allows us to distinguish several electronic
states involved in the reaction. The key idea is to excite the ring- closed molecule by a 266 nm ultraviolet
(UV) pulse resulting in the formation of ring-opened photo-product, which is then probed by the FEL
pulse. The ion and electron spectra are recorded using a magnetic-bottle spectrometer, which allows us to
study the photo-products evolution as a function of delay between UV and FEL pulses. Theoretical
predictions suggest the existence of several ring-opened isomers with about 1-2 eV less binding energy
as compared to the parent molecule2. Our experiment probes the ultrafast electronic pathways leading to
their creation. The recorded photo-ions also map the vibrationally excited photo-products, confirming the
observation of the ring-opened products.
[1] Arruda, B. C. & Sension, R. J. Ultrafast polyene dynamics: the ring opening of 1,3-cyclohexadiene
derivatives. Phys. Chem. Chem. Phys. 16, 4439–4455 (2014).
[2] D. Murdock et al., Transient UV pump–IR probe investigation of heterocyclic ring-opening dynamics
in the solution phase: the role played by nσ* states in the photoinduced reactions of thiophenone and
furanone, Phys. Chem. Chem. Phys. 16, 21271 (2014).
Supported by the National Science Foundation through grant PHYS-1753324.
11
Imaging Structural Dynamics in Isolated Molecules by MeV Ultrafast Gas
Electron Diffraction
J. Yang1,2, X. Zhu2,3, T. J. A. Wolf1,2, Z. Li.2,4,5 J. P. F. Nunes6, R. Parrish2,3, K. Hegazy2, R. K.
Li1, X. Shen1, S. Weathersby1, K. Wilkin6, M. Gühr7, T. J. Martinez2,3, M. Centurion6, X. J.
Wang1
1SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, USA
2Stanford PULSE Institute, SLAC, 2575 Sand Hill Rd, Menlo Park, CA, 94025, USA.
3Department of Chemistry, Stanford University, 333 Campus Dr., Menlo Park, CA, 94305, USA.
4Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, Hamburg, Germany.
5Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany.
6Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, USA
7Institut für Physik und Astronomie, Universität Potsdam, Potsdam, Germany
Abstract
Direct experimental measurements of excited state reaction trajectories in isolated molecules are very
important in atomic-level description of photochemistry and photophysics, but they are experimentally
challenging due to the extreme spatiotemporal resolution that is required. Mega-electron-volt (MeV)
ultrafast gas electron diffraction (UGED) was recently demonstrated to have sufficient spatial (0.5 Å) and
temporal resolution (~100 fs) to capture photoexcited atomic motion in isolated diatomic molecules [1-3].
In this presentation, I will discuss the first few MeV UGED experiments. In the first two proof-of-
principle experiments, we observed the rotational revivals and vibrational wavepacket of diatomic
molecules N2 and I2 [1, 2]. In addition, I will present the first polyatomic MeV UGED experiment on
CF3I. In this experiment, an intense UV pulse simultaneously accessed two excitation channels on CF3I
molecule—a one-photon excitation to the dissociative 3Q0 state and a two-photon excitation to a Rydberg
7s state. The reaction trajectories within each channel were separately resolved with high precision by
exploiting the anisotropy from photoselection rules. In the one-photon channel, the dissociation and
subsequent umbrella opening of the CF3 group were resolved. In the two-photon excitation, non-adiabatic
wavepacket splitting at a conical intersection was observed [3]. This experiment demonstrated that MeV
UGED is capable of mapping reaction trajectories in nonadiabatic dynamics through a conical
intersection.
References:
1. Yang, J., et al., Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules. Physical
Review Letters, 2016. 117(15): p. 173002.
2. Yang, J., et al., Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond
megaelectronvolt electron pulses. Nature Communications, 2016. 7: p. 11232.
3. Yang, J., et al., Imaging CF3I conical intersection and photodissociation dynamics with ultrafast
electron diffraction. Science, 2018. 361(6397): p. 64-67.
12
Dynamics of nanoscale electron-phonon coupling and thermal transport
M. Gorfien1, H. Rahmani1, X. Wang2, and J. Cao1,3
1 Department of Physics and National High Magnetic Field laboratory, Florida State University, FL,
USA 2 Institute of Physics, Chinese Academy of Sciences, Beijing, China
3 School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China
Nanometer-sized materials and devices display size-dependent, novel optical and electronic
properties. These unique properties are shaped by various elementary interactions such as electron-
electron coupling and electron-phonon coupling under the quantum confinement condition, and the
interactions of the nanoparticles with the environments (surrounding matrix, absorbed molecules,
and nearby nanoparticles). The dynamical behaviors in nanoscales give direct links to these
coupling processes. By capturing these acts at the critical steps in time domain, the physical
processes that are unique to their size, composition and environment can be determined. In this talk,
I will present two of our recent research activities along this line. One is to study the dynamics of
electron-phonon coupling in semiconductor quantum dots (QD) and role of phonon bottleneck
effect using ultrafast electron diffraction (UED). For 5-nm PbSe QDs, we found the energy
relaxation between the carriers and lattice took place in about 10 ps, showing no significant phonon
bottleneck effect. Meanwhile, the heat transport between the QD and substrate significantly deviates
from Fourier’s Law. This work is relevant to the development of QD based solar cells to reach
higher conversion efficiency beyond the Shockley–Queisser limit. The other is to investigate the
nanoscale thermal transport across heterojunction semiconductor interface. By using UED together
with theoretical modeling, we monitored the kinetics of heat flow across a single GaAs/AlGaAs
quantum well and determined the interface thermal conductance G. Interestingly, we found that G
is a linear function of sample temperature, even past the Debye temperature, under the highly non-
equilibrium conditions created by ultrafast heating. This work is relavant to understanding and
controlling of the ever-increasingly aggressive thermal management issues in nano electronic
devices.
13
A “nanoscale-femtosecond” spin polarized electron source and the Hanbury-
Brown Twiss electron anti-correlation
Sam Keramati, Evan Brunkow, Eric Jones, Timothy Gay, and Herman Batelaan.
Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
In the summer of 2018, we realized in collaboration with Tim Gay’s group at UNL a femtosecond spin-
polarized source [1]. This source followed our earlier femtosecond source design [2], but the tungsten tip
was replaced with GaAs shard, circular polarized light was used, and a Mott detector was added [3]. The
reason we wanted this source was to attempt to increase quantum degeneracy of free electrons. The problem
is that in earlier studies on continuous electron sources, the role of Coulomb and Pauli forces could not be
distinguished [4,5]. As Coulomb interaction does not depend on spin and Pauli forces do depend on spin,
we now may have a tool to solve the problem.
For pulsed electron beams we theoretically investigated the effect of Coulomb versus Pauli forces [6], and
we investigated the effect of partial coherence of the electron beam [7]. The conclusion remains that with
the advent of ultra-short electron pulses, high degeneracy is expected. In the presentation we review the
spin polarized source, the quantum degeneracy theory, and experimental steps towards degeneracy.
[1] Femtosecond Spin-Polarized Source of Electrons from p-GaAs. Evan Brunkow, Eric R. Jones,
Herman Batelaan and Timothy J. Gay., Appl. Phys. Lett. 114, 073502 (2019).
[2] Laser-induced ultrafast electron emission from a field emission tip. B. Barwick, C. Corder, J.
Strohaber, N. Chandler-Smith, C. Uiterwaal and H. Batelaan, New J. Phys. 9, 142 (2007).
[3] A cylindrically symmetric “micro-Mott” electron polarimeter. N. B. Clayburn, E. Brunkow, S. J.
Burtwistle, G. H. Rutherford, and T. J. Gay, Review of Scientific Instruments 87, 053302 (2016).
[4] Hanbury Brown–Twiss Interferometry with Electrons: Coulomb vs. Quantum Statistics. Gordon
Baym and Kan Shen (arXiv.org > quant-ph > arXiv:1212.4008), Book: In Memory of Akira Tonomura,
pp. 201-210 (2014).
[5] Correlation in a coherent electron beam Tetsuji Kodama, Nobuyuki Osakabe, and Akira Tonomura,
Phys. Rev. A 83, 063616 (2011).
[6] Quantum description and properties of electrons emitted from pulsed nanotip electron sources. P.
Lougovski, H. Batelaan, Phys. Rev. A 84, 023417 (2011).
[7] Partially coherent quantum degenerate electron matter waves, Sam Keramati, Eric R. Jones, Jeremy
Armstrong, Herman Batelaan, quant-ph > arXiv:1811.09743 (2018).
14
Generation and characterization of attosecond electron bunch trains via the
interaction with infrared femtosecond laser pulses
N. Schönenberger1*, M. Kozák1,2, P. Yousefi1, A. Mittelbach1, P. Hommelhoff1
1 Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany, EU
2 now with Charles University Prague, Prague, Czech Republic, EU
We demonstrate the generation and characterization of attosecond electron bunch trains in our ultrafast
scanning electron microscope [1]. Utilizing linear and higher order effects of the fields of femtosecond
pulsed infrared lasers, we show the generation and subsequent characterization of attosecond electron
bunch trains from ~400 fs electron pulses, generated via photoemission from a standard Schottky type
emitter. Achievable bunch lengths go down as far as ~150 as [2, 3].
[1] Ultrafast scanning electron microscope applied for studying the interaction between free electrons
and optical near-fields of periodic nanostructures, M. Kozák, J. McNeur, N. Schönenberger, J.
Illmer, A. Li, A. Tafel, P. Yousefi, T. Eckstein, P. Hommelhoff, Journal of Applied Physics 124
(2018), 023104
[2] Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in
vacuum, M. Kozák, T. Eckstein, N. Schönenberger, P.Hommelhoff, Nature Physics 14 (2018), S.
121–125
[3] Ponderomotive Generation and Detection of Attosecond Free-Electron Pulse Trains, M. Kozák, N.
Schönenberger, P. Hommelhoff, Physical Review Letters 120 (2018), Art.Nr.: 103203
15
Attosecond coherent control of a free-electron wave function
via semi-infinite light fields and plasmon polaritons
G. M. Vanacore1, I. Madan1, G. Berruto1, P. Biagioni2, I. Kaminer3, B. Barwick4,
V. Grillo5, E. Karimi6, F. J. Garcia de Abajo7,8, F. Carbone1
1Institute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES),
Ecole Polytechnique Federal de Lausanne, Station 6, CH-1015 Lausanne, Switzerland
2Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano,
Italy 3Faculty of Electrical Engineering and Solid State Institute, Technion, Haifa 32000,
Israel 4Ripon College, 300 W. Seward St., Ripon, WI 54971, United States
5CNR-Istituto Nanoscienze, Centro S3, Via G Campi 213/a, I-41125 Modena, Italy
6Department of Physics, University of Ottawa, 25 Templeton St., Ottawa, Ontario, K1N 6N5 Canada
7ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology,
08860 Castelldefels (Barcelona), Spain
8ICREA-Institucío Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010
Barcelona, Spain
E-mail: [email protected]
The interaction between light and electrons can be exploited for generating radiation, such as in
synchrotrons and free electron lasers, or for controlling electron beams for the dynamical investigation of
materials and molecules. Using electromagnetic fields the coherent control of an electron wave function can
be pushed to unexplored timescales, enabling new applications in light-assisted quantum devices and
diagnostics at extremely small timescales, such as those governing intramolecular electronic motions and
nuclear processes.
In this contribution, I will describe a novel method for the coherent longitudinal and transverse phase
manipulation of a free-electron wave function. Using appropriately synthesized optical light fields I will
demonstrate how to modulate the energy, linear momentum and orbital angular momentum (vorticity) of
the electron wave function with attosecond precision.
A relativistic pulsed electron beam was made to interact with an appropriately synthesized electromagnetic
field. The field was generated either by a sequence of two fs laser pulses reflected at the surface of a mirror
(semi-infinite field), or by the coherent superposition of the surface plasmon polaritons (SPPs) optically-
generated from nanofabricated structures (near field). The energy-momentum exchange resulting from the
electron-field interaction was directly mapped via momentum-resolved ultrafast electron energy-loss
spectroscopy. When the two phase-locked light pulses were delayed by fractions of the optical cycle, we
observed coherent oscillations in the electrons energy-momentum states. This effect
is the result of coherent constructive and destructive phase modulation of the electron wave function
while varying the relative phase between the two driving optical pulses.
In addition, our method offers the possibility to manipulate the phase-controlled interaction of the
electrons with both a semi-infinite light field and a plasmon polariton propagating on a plasmonic
waveguide. Here, I will describe the case of SPPs generated at the edge of a circular nanocavity carved in a
Ag layer deposited on a Si3N4 thin film, and demonstrate that in the case of circularly-polarized
illumination the resulting near-field distribution transiently creates a vortex plasmon carrying a well-16
defined orbital angular momentum (OAM), which can be efficiently transferred to the interacting electrons
as a result of the coherent interaction.
The potential of our approach for longitudinal and transverse phase modulation at the attosecond
timescale and below should pave the way to achieve unprecedented insights into non-equilibrium
phenomena in advanced quantum materials, and should play a decisive role in the rational design and
engineering of future photonics and electronics applications.
17
An ultracold and ultrafast electron source
Jim Franssen, Tim de Raadt, Daniel Nijhof, Peter Mutsaers, Jom Luiten
Eindhoven University of Technology, The Netherlands
At Eindhoven University of Technology (TU/e) a pulsed, ultra-cold electron source (UCES) is
being developed, based on femtosecond, near-threshold photoionization of a laser-cooled and trapped atomic gas. Source temperatures of ~10 K or ~1 meV, 2-3 orders lower than
conventional field or photoemission sources, are routinely achieved, resulting in picosecond
bunches containing ~103 electrons with a high degree of coherence. An important goal is to
produce bunches with much more charge, preferably in excess of 106 electrons, while retaining
a high degree of coherence. This would enable single-shot electron crystallography of
macromolecules, which was originally the main driving force behind UCES development at
TU/e.
Recently, the ColdLight project has been started at TU/e, in which the UCES will be used to
realize a fully coherent Inverse Compton Scattering (ICS) soft X-ray source. In this fascinating
new application the unique properties of the UCES are used to the fullest: The ultra-low
emittance of the UCES enables generation of fully spatially coherent soft X-ray pulses. The two-
step photoionization scheme of the laser-cooled gas allows intricate structuring of the electron
bunch, in particular longitudinal pre- bunching at optical wavelengths. Combining the pre-
bunching with standard RF bunch compression results in micro-bunching at soft X-ray
wavelengths and thus full temporal coherence as well. It is intriguing to note that the soft X-ray
emission by such bunches should be enhanced by super-radiance, suggesting the possibility of
realizing a compact and easily tunable soft X-ray Free Electron Laser.
In this contribution recent experimental progress will be presented, in particular longitudinal
phase space characterization of the ultracold bunches and the recent
commissioning of a compact, turn-key, grating-MOT based UCES.
18
Entanglements of free-electrons and cavity-photons in the strong coupling regime
Ofer Kfir
University of Göttingen, IV. Physical Institute, Göttingen 37077, Germany [email protected]
Abstract: This work investigates the entanglement between cavity-photons and a beam of electrons, at an arbitrary (weak and strong) coupling strength, and proposes a road-map to approach this regime experimentally.
The coherent interaction of electrons and photons provides for a mechanism to controllably manipulate their quantum state, with dramatic implications. An important example is the employment of PINEM (photon-induced near-field electron microscopy [1]) to imprint an oscillatory phase profile of a strong laser onto an electronic wavefunction [2]. From the point of view of quantum optics, although the PINEM spectrum can be described as the absorption and emission of multiple photons, in practice, the weak coupling between the light and the electrons necessitates high laser intensities, corresponding to a classical optical state.
This work proposes a road-map to approach strong couplings between an electron-beams and optical-cavity modes, and investigates possible electron-photon entanglements in this regime. The proposed experimental scheme utilizes a long interaction of the electron beam with a transparent dielectric cavity (see Fig. 1a). For phase-matched propagation, the coupling strength builds-up coherently over many micrometers. The confinement allows for even a single photon to have substantial electric fields in the vicinity of the cavity walls, and in addition, tilts the optical polarization towards the propagation axis, which exerts accelerating or decelerating force on an electron. Importantly, the phase matching constraint narrows the interaction spectrum dramatically, and thus sets the excitation spectrum to ℏ��, regardless of an external driving laser.
To illustrate possible entanglement features of strong couplings, I consider a relativistic electron beam strongly coupled (� = 1) to an either empty cavity or to a cavity populated with a four-photon coherent state (Fig. 1c-d for � = 0 and � = 2, respectively). These few photons make a dramatic difference. In the case of an empty cavity (� = 0) the system’s energy is well defined, and a strict entanglement between the electron-energy loss and photon Fock-state generation appears. The uncertainty arising from the addition of few photons to the cavity results in an elaborate entanglement pattern, manifested in the two-particle (electron-photon) probability map. That is, the electron spectrum varies dramatically when coinciding with one or with two photons. In the limit of a cavity populated with a large-amplitude coherent-state which is weakly coupled to an electron beam, the two entities are decoupled. In such cases the electron spectrum conforms to Bessel-like amplitudes, typical for PINEM [2] and the photonic coherent state varies negligibly.
A quantitative analysis predicts that a coupling strength of � = 0.5, which would already enable the observation of strong-coupling phenomena, is achievable with commercially available electron microscopes. The integration capabilities of whispering-gallery-mode cavity with fiber optics would allow for the injection and collection of photons in such a system with high fidelity and in a controlled manner. Thus, entanglements arising in this approach could introduce electron beams into the realms of quantum-optics and quantum-information.
References
1. B. Barwick, D. J. Flannigan, and A. H. Zewail, "Photon-induced near-field electron microscopy," Nature 462, 902–906 (2009). 2. A. Feist, K. E. Echternkamp, J. Schauss, S. V. Yalunin, S. Schäfer, and C. Ropers, "Quantum coherent optical phase modulation in an ultrafast
transmission electron microscope," Nature 521, 200–203 (2015).
Figure 1. (a) An illustration of the proposed apparatus for strongly coupled electrons and photons. An electron beam traversing near the long arm of a stadium-shaped whispering-gallery-mode cavity can coherently exchange energy with a co-propagating cavity-photon. (b) Due to the material- and modal-dispersion of the optical mode in the cavity, phase-matched interaction is limited to a narrow band. For example, in a cavity optimized for excitations of 1.55 eV (�=800 nm), a co-propagation along 100 µm would have a bandwidth of 0.04 eV. Thus, the interaction frequency �� is inherent to the system, regardless of an external laser excitation. (c) In the case of an initially empty cavity, the conservation of energy entangles the generated photons (horizontal axis, spectrum below) and electron-energy losses (vertical axis, spectrum on the right). The two-particle probability map is strictly diagonal since the system’s energy is well defined. The population probabilities are calculated for a coupling strength � = 1. (d) Populating the cavity with 4 photons in the form of a coherent state |�⟩ with � = 2, introduces a rich entanglement structures of the photons and electrons. The statistics of the initial coherent state (lower graph, dashed line) transforms substantially, while the electron can either gain or lose energy. The electron spectrum is smooth due to the entanglement to different photon states – in the limit of weak coupling driven by an intense coherent state, the electron spectrum converges to the known Bessel-function amplitudes of PINEM.
(a) (b) (c) (d) L=100 µm
19
MeV Ultrafast Electron Probe: Science and Challenges
Xijie Wang
SLAC National Accelerator Laboratory
20
Towards 10 fs resolution in MeV ultrafast electron diffraction
Dao Xiang
School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China
The temporal resolution of ultrafast electron diffraction depends on laser pulse width, electron beam pulse
width and timing jitter between the two pulses. In this talk, we discuss various advanced techniques
(e.g. bunch compression [1-4] and time-stamping [4-6], etc.) that may allow one to reach 10 fs resolution
in MeV UED.
References
1. T. van Oudheusden et al., Compression of Subrelativistic Space-Charge-Dominated Electron
Bunches for Single-Shot Femtosecond Electron Diffraction, Phys. Rev. Lett. 105, 264801 (2010).
2. J. Maxson et al., Direct Measurement of Sub-10 fs Relativistic Electron Beams with Ultralow
Emittance, Phys. Rev. Lett. 118, 154802 (2017).
3. C. Lu et al., Coulomb-Driven Relativistic Electron Beam Compression, Phys. Rev. Lett. 120,
044801 (2018).
4. L. Zhao et al., Terahertz Streaking of Few-Femtosecond Relativistic Electron Beams, Phys. Rev. X
8, 021061 (2018).
5. R. K. Li et al., Terahertz-based subfemtosecond metrology of relativistic electron beams, Phys.
Rev. Accel. Beams 22, 012803 (2019).
6. L. Zhao et al., Terahertz Oscilloscope for Recording Time Information of Ultrashort Electron
Beams, Phys. Rev. Lett. 122, 144801 (2019).
21
Demonstration of transmission high energy electron microscopy
F. E. Merrill, J. Goett, J. W. Gibbs, S.D. Imhoff, F.G. Mariam, C.L. Morris, L.P. Neuirch, J.
Perry, D. Poulson, R Simpson, P.L. Volegov, P.L. Walstrom, C.H. Wilde, Los Alamos National Laboratory
C. Hast, K. Jobe, T. Smith SLAC National Accelerator Laboratory
U. Wienands Argonne National Laboratory
A.J. Clark Colorado School of Mines
D. Tourret IMDEA Materials Institute
High energy electrons have been used to investigate at LANL the extension of transmission electron
microscopy1. This technique, transmission high energy electron microscopy (THEEM), provides two
additional capabilities to electron microscopy. First, high energy electrons are more penetrating than low
energy electrons, and thus, they are able to image through thicker samples. Second, the accelerating mode
of a radio-frequency linear accelerator provides fast exposures, down to 1 ps, which are ideal for flash
radiography, making THEEM well suited to study the evolution of fast material processes under dynamic
conditions. Initial investigations with static objects and during material processing have been performed to
investigate the capabilities of this technique. The system design requirements for these measurements will
be presented along with the data that was collected in these demonstration measurements.
1. Merrill, F. E., et al. "Demonstration of transmission high energy electron microscopy." Applied Physics
Letters 112.14 (2018): 144103.
22
Development of RF-enabled high-brightness femtosecond electron
microscope
Shuaishuai Sun, Xiaoyi Sun, Daniel Bartles, Elliot D Wozniak, Joseph Williams, Faran Zhou,
Chong-yu Ruan
Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824,
United States
Ultrafast electron diffraction and microscopy has been proved to be a powerful tool for studying ultrafast
structural, electronic and magnetic dynamics in materials. However, significant challenges exist in reaching
adequate sensitivities at the femtosecond temporal scale due to the space-charge-led broadening effects at
high-charge densities. Here we report a new development of ultrafast electron microscope that combines
the longitudinal optical system using RF cavities to reach time and energy compression, thus achieving
higher performance by optimizing the phase space evolution of space-charge-dominated beams in the
targeted resolution window. With this key concept, we demonstrate that high beam optical properties with
sub-100 fs RF-compressed electron pulses can be delivered for ultrafast electron diffraction; while in the
imaging mode, a sub-50 nm imaging resolution is reached largely constrained by the point spread function
of CCD camera. The current level of performance clearly suggests that the collective space charge limits
presented in the earlier ultrafast electron microscope experiments can be overcome, and opens the prospect
of performing ultrafast electron energy loss spectroscopy with the new RF optical components in a
commercial TEM system.
23
Dynamics of nanoscale electron-phonon coupling and thermal transport
M. Gorfien1, H. Rahmani1, X. Wang2, and J. Cao1,3 1 Department of Physics and National High Magnetic Field laboratory, Florida State University,
FL, USA 2 Institute of Physics, Chinese Academy of Sciences, Beijing, China
3 School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, China
Nanometer-sized materials and devices display size-dependent, novel optical and electronic properties. These unique properties are shaped by various elementary interactions such as electron-electron coupling and electron-phonon coupling under the quantum confinement condition, and the interactions of the nanoparticles with the environments (surrounding matrix, absorbed molecules, and nearby nanoparticles). The dynamical behaviors in nanoscales give direct links to these coupling processes. By capturing these acts at the critical steps in time domain, the physical processes that are unique to their size, composition and environment can be determined. In this talk, I will present two of our recent research activities along this line. One is to study the dynamics of electron-phonon coupling in semiconductor quantum dots (QD) and role of phonon bottleneck effect using ultrafast electron diffraction (UED). For 5-nm PbSe QDs, we found the energy relaxation between the carriers and lattice took place in about 10 ps, showing no significant phonon bottleneck effect. Meanwhile, the heat transport between the QD and substrate significantly deviates from Fourier’s Law. This work is relevant to the development of QD based solar cells to reach higher conversion efficiency beyond the Shockley–Queisser limit. The other is to investigate the nanoscale thermal transport across heterojunction semiconductor interface. By using UED together with theoretical modeling, we monitored the kinetics of heat flow across a single GaAs/AlGaAs quantum well and determined the interface thermal conductance G. Interestingly, we found that G is a linear function of sample temperature, even past the Debye temperature, under the highly non-equilibrium conditions created by ultrafast heating. This work is relavant to understanding and controlling of the ever-increasingly aggressive thermal management issues in nano electronic devices.
24
How do non-equilibrium phonons thermalize?
Hermann Dürr
Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden
Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool
down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the
solid. By combining ultrafast diffuse electron scattering with ab initio calculations we gain a detailed
understanding of the complex non-equilibrium energy transfer between electrons and phonons in laser-
excited Ni metal. Our experimental results show that the wavevector resolved population dynamics of
phonon modes is distinctly different throughout the Brillouin zone. We find that zone-boundary phonon
modes become occupied first. As soon as the energy in these modes becomes larger than the average
electron energy a backflow of energy from lattice to electronic degrees of freedom occurs. Subsequent
excitation of lower-energy phonon modes drives the thermalization of the whole system on the
picosecond timescale. We determine the evolving non-equilibrium phonon occupations which are found
to deviate markedly from thermal occupations.
25
Low-Frequency Phonon-Mode Coupling and Single-Layer De-Phasing in
Anisotropic Materials
David J. Flannigan
Department of Chemical Engineering and Materials Science
University of Minnesota, Minneapolis, USA
Development of ultrafast electron and X-ray scattering methods has enabled direct routes to probing atomic-
to-microscale structural dynamics in myriad chemical and materials systems. This in turn has led to new
physical insights into molecular and crystal-lattice responses associated with chemical-bond dynamics,
phase transformations, electron-lattice correlations, and nanoscale structural motion. Importantly, spatially-
localized dynamics that are single contributors to ensemble-averaged signals are significantly influenced
by ever-present lattice discontinuities, nanoscale morphological structures, and interfaces, the distribution
of which is heterogeneous over disparate nanoscale volumes. Thus, direct probing of local responses is
likely to provide a richer, more detailed picture of the formation, evolution, and decay of ultrafast non-
equilibrium energy transport and conversion in a host of functional materials. Here, I will discuss how we
have used fs electron imaging with an ultrafast electron microscope (UEM) to directly visualize coherent,
low-frequency acoustic-phonon dynamics in a variety of materials, with particular emphasis placed on
understanding the influence of lattice discontinuities and structural anisotropies. After a brief overview of
the instrumentation and the general experimental approach [1], I will describe how the concepts of static,
real-space imaging with conventional electron microscopes can be directly extended to UEM to visualize
local coherent phonon dynamics. In TMDs (MoS2, WSe2, TaS2) we have found that fs photoexcitation leads
to the generation of coherent phonon wavetrains preferentially at vacuum-crystal interfaces and extended
crystal step edges [2-4]. This arises via an initial impulsive expansion along the c-axis van der Waals
stacking direction occurring within the first few picoseconds after fs photoexcitation, as indicated by local
oscillatory bend-contour motions. Impulsive excitation of this interlayer low-frequency breathing mode
induces the launch of coherent ab-plane phonon wavefronts due to the rapid, picosecond development of a
phase lag between the neighboring layers owing to varying total transit times of the speed-of-sound c-axis
phonons. As with the stacking direction, the coherent intralayer modes propagate at the speed of sound (e.g.,
8 nm/ps) and initially along a single wavevector oriented perpendicular to the defect nucleation sites prior
to the first scattering events. Aspects of this behavior are in contrast to those in strongly-photoexcited Ge
(diamond cubic), in which a number of remarkable responses have been observed with UEM [5]; including
the launch of highly-coherent phonon wavefronts propagating with hypersonic phase velocities (e.g., 35
nm/ps), the significantly-delayed (i.e., 10s of picoseconds or more) generation of phonon wavetrains
relative to the precise moment of fs photoexcitation, and the time-varying phase-velocity dispersions
displaying single-exponential relaxation to the bulk speed of sound. This survey of recent results will serve
to illustrate the rich and detailed information obtainable with fs electron imaging, with particular emphasis
placed on the low-frequency modes highlighted here.
[1] Plemmons, D. A. et al. Chem. Mater. 2015, 27, 3178-3192.
[2] Cremons, D. R. et al. Nat. Commun. 2016, 7, 11230.
[3] McKenna, A J. et al. Nano Lett. 2017, 17, 3952-3958.
[4] Cremons, D. R. et al. Struct. Dyn. 2017, 4, 044019.
[5] Cremons, D. R. et al. Phys. Rev. Mater. 2017, 1, 073801.
26
Photophysics in the gas phase: uniting structural and electronic perspectives
Markus Gühr
Physics and Astronomy, Potsdam
and the LCLS nucleobase as well as the SLAC gas phase UED collaboration (see author lists below)
The conversion of light energy into other energy forms in molecules is the result of a concerted and
ultrafast motion of electrons and nuclei, often under breakdown of the Born-Oppenheimer
approximation. This talk is about ultrafast experiments aimed at resolving light induced ultrafast
molecular dynamics with x-ray probe pulses using free electron lasers as well as relativistic electron
pulses.
We present experiments on internal conversion of the nucleobase thymine, which we probe by
femtosecond resonant x-ray spectroscopy at the oxygen K-edge. We deduce a less than 100 fs ππ* →
nπ* transition, which plays a crucial role in the photoprotection of this nucleobase [1].
In addition, we present results from femtosecond electron diffraction experiments on electronically
excited states of small molecules, that unravel wavepacket dynamics with Angstrom level spatial
resolution and femtosecond domain temporal resolution [2,3].
[1] Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant
absorption, T. J. A. Wolf, R. H. Myhre, J. P. Cryan, S. Coriani, R. J. Squibb, A. Battistoni, N. Berrah, C.
Bostedt, P. Bucksbaum, G. Coslovich, R. Feifel, K. J. Gaffney, J. Grilj, T. J. Martinez, S. Miyabe, S. P.
Moeller, M. Mucke, A. Natan, R. Obaid, T. Osipov, O. Plekan, S. Wang, H. Koch and M. Gühr, Nature
Communications 8, 29 (2017)
[2] Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules
J. Yang, M. Guehr, X. Shen, R. Li, T. Vecchione, R. Coffee, J. Corbett, A. Fry, N. Hartmann, C. Hast,
K. Hegazy, K. Jobe, I. Makasyuk, J. Robinson, M. S. Robinson, S. Vetter, S. Weathersby, C. Yoneda,
X. Wang, M. Centurion, Phys. Rev. Lett. 115, 173002 (2016)
[3] Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron
diffraction, J. Yang, X. Zhu, T.J.A. Wolf, Z. Li, J.P.F. Nunes, R. Coffee, J.P. Cryan, M. Gühr, K.
Hegazy,
T.F. Heinz, K. Jobe, R. Li, X. Shen, T. Veccione, S. Weathersby, K.J. Wilkin, C. Yoneda, Q. Zheng, T.J.
Martinez, M. Centurion, X. Wang, Science 361, 64 (2018)
27
Structural and Spectroscopic Probing of Excited State Molecular Dynamics
Thomas WeinachtStony Brook University
Pump-probe measurements aim to capture the motion of atoms and molecules in real time as chemical and
physical transformations take place, effectively making ”molecular movies” with short light pulses.
However, it is not possible to make movies directly from experimental observations due to a number of
fundamental and technical limitations. Thus, it is through a combination of experimental measurements and
theoretical calculations that one can actually construct molecular movies. I will present a combination of
spectroscopic (time resolved photoelectron spectroscopy - TRPES) and structural (relativistic ultrafast
electron diffraction - UED) measurements to follow the coupled electronic and nuclear dynamics involved
in the internal conversion and photodissociation of diiodomethane, CH2I2. The TRPES measurements
provide the time-dependent energy of the molecule, and the UED measurements give the positions of the
nuclei for each time. These measurements are combined with trajectory surface hopping calculations, which
are capable of calculating the measured observables for both measurements from the same dynamics
simulation. The measurements highlight the non-local dynamics captured by different groups of trajectories
in the calculations. This is the first time that both structural and spectroscopic measurements are combined
with theory capable of calculating the measurement observables in both cases, yielding an unprecedented
view of the complicated quantum dynamics involved in the molecular relaxation."
28
Photo-dissociation Dynamics in 1,2-Diiodotetrafluoroethane Captured with
Ultrafast Electron Diffraction
Kyle Wilkin1, Jie Yang2, Robert Parrish2,3,4, Markus Guehr3,5, Renkai Li2, Michael Minitti2,
Pedro Nunes1,6, Xiaozhe Shen2, Thomas Wolf3, Qiang Zheng2, Todd J. Martinez2,3,4, Xijie
Wang2, Martin Centurion1
1Department of Physics and Astronomy, University of Nebraska – Lincoln, 855 N 16th St., Lincoln, NE,
68588, United States.
2SLAC National Laboratory, 2575 Sand Hill Rd, Menlo Park, CA, 94025, United States.
3Stanford PULSE Institute, SLAC, 2575 Sand Hill Rd, Menlo Park, CA, 94025, United States.
4Department of Chemistry, Stanford University, 333 Campus Dr., Menlo Park, CA, 94305, United States.
5Institut für Physik und Astronomie, Universität Potsdam, Potsdam, 14476, Germany.
6Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
Abstract
We have observed coherent dynamics of 1,2-diiodotetrafluoroethane (C2F4I2) following single UV photon
excitation using ultrafast electron diffraction (UED). The experiment was performed at the SLAC UED
facility with a 3.7 MeV electron beam with 150 fs resolution [1-4]. The dissociation of the two iodine atoms
is known to be nonconcerted with the first iodine ejecting within 200 fs and the second dissociating after
about 30 ps. The difference in time constants between the two dissociations allows us to observe the
redistribution of energy before the excess energy causes the second dissociation. The structure of the
transient C2F4I is also determined. Previously, Ihee et al. determined the structure to be classical in nature
but were limited by a 5 ps temporal resolution [5]. We have determined the structure to be classical to
within one vibrational period of the relevant bonds (~200 fs) as well as observed coherent oscillations in
the transient after the first dissociation. We have imaged the dissociating iodine atom to 6 Å-1 as well as
captured oscillations corresponding to changes in the longest atomic distances in the transient (F-I).
[1] J. Yang et al., Physical Review Letters 117, 153002 (2016).
[2] J. Yang et al., Nature Communications 7, 11232 (2016).
[3] J. Yang et al., Science 361, 64 (2018).
[4] T. J. A. Wolf et al., Nature Chemistry Accepted (2019).
[5] B. M. G. Hyotcherl Ihee, Ramesh Srinivasan, Vladimir A. Lobastov, and Ahmed H. Zewail, Journal
of Physical Chemistry A 106, 4087 (2002).
29
Internuclear-distance and angle dependence of strong-field ionization rates of
UV-dissociated halomethanes
F. Ziaee1, K. Borne1, Kanaka Raju P. 1, R. Forbes2, B. Kaderiya1, Y. Malakar1, T. Severt1, I. Ben-
Itzhak1, A. Rudenko1, D. Rolles1
1- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan
KS, USA
2- Department of Physics, Stanford University, California, USA
The dependence of the strong-field ionization rates of iodine-containing halomethanes on the iodine-
carbon internuclear-distance and the orientation of molecular bonds with respect to the polarization direction
of an infrared laser field is investigated utilizing a UV pump-NIR probe technique. Excitation at 258
nm initiates a resonant single-photon absorption cleaving the carbon-iodine bond. A subsequent NIR
laser pulse ionizes the dissociating molecule at different delays. Measuring single and double ionization
rates as a function of pump-probe delay allows the determination of their internuclear-distance dependence.
We extract the distance between the charges and the shape of the dissociative potential energy curves from
the measured kinetic energies for the internuclear distances of above 10 a.u, where the dicationic and
tricationic potential energy curves are purely Coulombic [1]. Furthermore, by determining the delay-
dependence of the fragment ion angular distributions, the gradual transition of the ionization from the
molecular to the atomic limit is probed.
[1] M. E. Corrales et al., J. Phys. Chem. A 116, 2669 (2012).
Supported by the U.S. Department of Energy under grant no. DE-FG02-86ER13491.
30
Internuclear-distance and angle dependence of strong-field ionization rates of UV-dissociated halomethanes
F. Ziaee1, K. Borne1, Kanaka Raju P. 1, R. Forbes2, B. Kaderiya1, Y. Malakar1, T. Severt1, I. Ben- Itzhak1, A. Rudenko1, D. Rolles1
1- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan KS, USA
2- Department of Physics, Stanford University, California, USA
The dependence of the strong-field ionization rates of iodine-containing halomethanes on the iodine-carbon internuclear-distance and the orientation of molecular bonds with respect to the polarization direction of an infrared laser field is investigated utilizing a UV pump-NIR probe technique. Excitation at 258 nm initiates a resonant single-photon absorption cleaving the carbon-iodine bond. A subsequent NIR laser pulse ionizes the dissociating molecule at different delays. Measuring single and double ionization rates as a function of pump-probe delay allows the determination of their internuclear-distance dependence. We extract the distance between the charges and the shape of the dissociative potential energy curves from the measured kinetic energies for the internuclear distances of above 10 a.u, where the dicationic and tricationic potential energy curves are purely Coulombic [1]. Furthermore, by determining the delay-dependence of the fragment ion angular distributions, the gradual transition of the ionization from the molecular to the atomic limit is probed.
[1] M. E. Corrales et al., J. Phys. Chem. A 116, 2669 (2012).
Supported by the U.S. Department of Energy under grant no. DE-FG02-86ER13491.
31
Photoelectron-photoion coincidence setup for XUV-NIR pump-probe
experiments
S.J. Robatjazi, S. Pathak, W. L. Pearson, J. Powell, Kanaka Raju P, D. Rolles and A. Rudenko.
J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS, USA
Coincident photoelectron-photoion spectroscopy represents a powerful experimental tool to study ultrafast
molecular dynamics [1-3]. However, its applications for time-resolved measurements have often been
limited by rather low repetition rate of the available light sources, especially at short wavelengths entering
extreme-ultraviolet (XUV) and X-ray domains [4,5]. Here, we describe a newly developed Kansas Atomic
and Molecular Physics (KAMP) instrument, which combines a double-sided velocity map imaging (VMI)
spectrometer for photoion–photoelectron coincidence measurements with a femtosecond pump-probe setup
employing XUV and near-infrared (NIR) pulses at 10 kHz repetition rate. The VMI spectrometer equipped
with two time- and position-sensitive delay-line detectors is attached to a high-harmonics generation (HHG)
setup based on a commercial KM Labs eXtreme Ultraviolet Ultrafast Source [6]. The latter is capable of
delivering HHG radiation of less than 30 fs pulse duration in the photon energy range of ~17-100 eV. Most
of the major setup elements such as the interaction chamber, VMI spectrometer, detectors and a gas target
arrangement are compatible with the CAMP [7] and LAMP [8] instruments installed at FLASH and LCLS
free-electron laser facilities, respectively, enabling efficient testing and implementation of the new
equipment components for atomic and molecular physics experiments at these facilities.
We will present the results of the instrument’s commissioning, including ion-electron coincidence spectra
from XUV-NIR pump-probe measurements on valence- and inner-shell ionization of Xe and Kr atoms, and
then focus on the outcome of the time-resolved study of ionization and fragmentation of CO2 molecules. In
this experiment, the neutral CO2 target is ionized by a train of high harmonics (11th to 19th ) of the NIR
laser beam at 790 nm, and the ensuing dynamics are probed by the time-delayed NIR pulse. Coincident
measurement of the photoelectrons allows us to separate contributions from higher-order harmonics, and
to focus on the dynamics driven only by the 11th and 13th harmonics. For those events, we map the yields
of CO2+ parent ions as well as CO+ and O+ fragments resulting from the XUV-NIR dissociative ionization
as a function of XUV-NIR delay, and analyze coincident ion and electron spectra for each channel. Further
filtering on photoelectron energies allows us to disentangle contributions from different excited cationic
states, and enables deeper understanding of ultrafast dynamics observed in earlier, non-coincident
measurement on CO2 dissociative ionization by XUV-NIR pump-probe pulses [9].
[1] E. Gagnon et al., Science 317, 1374 (2007).
[2] A. Sandhu et al., Science 322, 1081 (2008).
[3] A.E. Boguslavsky et al., Science 335, 1336 (2012).
[4] A. Rudenko et al., J. Phys. B: At. Mol. Opt. Phys 43, 194004 (2010).
[5] J. Ullrich, A. Rudenko and R. Moshammer, Annu. Rev. Phys. Chem. 63, 635 (2012).
[6] https://www.kmlabs.com/product/xuus/
[7] B. Erk et al., J. Synch. Rad. 25, 1529 (2018).
[8] T. Osipov et al., Rev. Sci. Instr. 89, 035112 (2018).
[9] H. Timmers et al, Phys. Rev. Lett. 113, 113003 (2014).
This project is supported by the Chemical Sciences, Geosciences, and Bio-Sciences Division, Office of
Basic Energy Science, Office of Science, U.S. Department of Energy.
32
Ultrafast point-projection electron microscopy
Christoph LienauInstitut für Physik Carl von Ossietzky Universität, 26129 Oldenburg, Germany
Ultrafast optical spectroscopy is now able to track even the fastest elementary processes such as the motion of electrons
and/or holes in biomolecules or organic solar cells. Despite tremendous progress in sub-diffraction optical microscopy,
a direct spatially resolved imaging of such processes is still out of reach since the spatial resolution of even the most
advanced near-field imaging techniques is far beyond the Angström-resolution achieved, e.g., in aberration-corrected
electron microscopy.
Recently, point-projection
electron microscopy, realized by
placing an object directly behind a
nanoscopic electron source and
recording a diffraction image on a
distant screen, emerged as an
interesting concept for improving
the time resolution in ultrafast
electron microscopy into the
regime of few tens of
femtoseconds or possibly even
beyond. It avoids the need for
electron lenses, makes the
experimental setup compact and
simple and minimizes temporal
dispersion of the electron pulses.
Here, we use plasmonic
nanofocusing to create an
isolated, few-femtosecond, few
nanometer-sized electron source
for ultrafast point-projection
microscopy [1]. We implement
this new electron source in an
ultrafast point-projection
microscope (Fig. 1) and use it for
taking movies of the
photoemission of electrons from the hot spot of a single plasmonic nanostructure with 20-nm spatial resolution and a
temporal resolution of better than 20 fs. To our knowledge this is the first time that such high space-time resolution
has been achieved in electron microscopy [2]. We show how this unique new technique allows us to trace the ballistic
motion of electrons that are ejected from a plasmonic hot spot with a mere 20-nm spatial diameter. We can directly see
the spreading of the electron cloud and extract quantitative information about the released electron wavepacket such
as their momentum and kinetic energy distribution.
We will introduce this new time-resolved electron microscopy technique and present first steps towards time-resolved
electron holography with few nanometer spatial resolution [3].
References: [1] J. Vogelsang, J. Robin, B. J. Nagy, P. Dombi, D. Rosenkranz, M. Schiek, P. Gross, C. Lienau, “Ultrafast Electron Emission from a Sharp Metal
Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons.”, Nano Letters 15, 4685-4691 (2015).
[2] J. Vogelsang, G. Hergert, D. Wang, P. Gross, C. Lienau, Observing charge separation in nanoantennas via ultrafast point-projection electron microscopy. Light-Science & Applications 7, 55 (2018).
[3] J. Vogelsang, N. Talebi, G. Hergert, A. Woste, P. Gross, A. Hartschuh, C. Lienau, Plasmonic-Nanofocusing-Based Electron Holography. Acs
Photonics 5, 3584-3593 (2018).
Fig. 1 Schematic of the ultrafast point-projection microscope imaging the
spatio-temporal dynamics of electron photoemisson from the hot spot of a single
plasmonic nanoantenna.
33
Challenges and opportunities for ultrafast relativistic electron probes at MHz repetition rates
Daniele Filippetto1, Fuhao Ji1, Dan Durham2, Martin Centurion3, R. Kaindl1, Andrew Minor2,
Pietro Musumeci4, Khalid Siddiqui1, Dan Slaughter1, Xiaojun Wang3.
1. Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, USA,
2. Department of Material Science and Engineering, UC Berkeley, Berkeley, California 94720, USA
3. Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA
4. Department of Physics and Astronomy, UCLA, Los Angeles, California 90095, USA
Ultrafast electron diffraction and microscopy (UED/UEM) have seen a rapid increase of interest in the
last decade due to the availability of high-field electron sources providing femtosecond pulses. The latest
particle accelerator technology is used for producing dense probes of relativistic electrons, up to one million
in less than 100 femtoseconds, and rapidly approaching the single- digit femtosecond range. Yet, the
scientific reach of such techniques is severely limited in the spatial domain, as consequence of the low
electron flux and average brightness.
UED experiments on solid targets collect data from large areas (tens to hundreds of micrometers), smearing
out the dynamical information around local defects, boundaries and grains, crucial for advancing the
fundamental understanding of material properties and behavior. In gas and liquid phase UED, experiments
are either limited to small and simple molecules or to time resolutions above the picosecond, ruling out
many systems or relevant scientific interest and public impact. A new UED beamline (HiRES) is under
development at Lawrence Berkeley National Laboratory. It makes use of a unique electron source to
deliver at the sample up to 1012 electrons per second with sub-picosecond resolution, an increase of 3
orders of magnitude over state-of-art MeV UED setups. Along the beamline, the beam is optimally
shaped for the particular experimental requirement. The large electron flux translates in the ability of
providing nanometer-scale probe sizes, for ultrafast nano-diffraction and U-STEM, or mesoscale lateral
coherent lengths, opening up new possibilities in gas and liquid phase UED.
Here we discuss the status of UED/UEM instrumentation and present the first results on electron beam
characterization and tight focusing at LBNL, together with the experimental plan using solid and gas-phase
targets on the HiRES beamline and on the development of novel photocathodes for UED/UEM applications.
F. Ji, D Durham, A. Minor. P. Musumeci, J. Navarro and D. Filippetto, arXiv:1901.03443
34
Demonstration of a Tunable Electron Beam Pulser in 200 kV and 300kV JEOL
TEMs
Chunguang Jing*, Ao Liu, Eric Montgomery, Yubin Zhao, Wade Rush, Alexei Kanareykin Euclid TechLabs, 365 Remington Blvd., Bolingbrook, IL 60440, USA
Xuewen Fu, Yimei Zhu Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton,
NY 11973, USA.
Karl B. Schliep, Michael B. Katz, June W. Lau National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Abstract: For the last several decades, time-resolved transmission electron microscopes (TEM) exploring
the pico- and sub-pico-second timescale have relied on the photoemission technology to generate the single
or train of electron bunches. However, the complexity of additional laser system and the availability of high
repetition rate laser limit applications of the laser-driven approach. Lately we have made substantial progress
on pioneering a new kind of time-resolved TEM, complementary to the existing laser-based techniques.
Using a tunable RF beam-chopper, we are able to retrofit an existing TEM providing a pulsed electron beam
at a continuously tunable repetition rate up to 12GHz and a tunable bunch length down to a few pico-seconds.
In recent few months two electron pulsers have been installed and operated, one in a 200KeV (2010F) JEOL
TEM and the other in a 300KeV (3010) JEOL TEM. In the presentation we will briefly discuss the working
principle and experimental progress to date.
Acknowledgement: We thank JEOL US for its generous support and in-kind contribution in this project. We
thank IDES for helping the column interface design and fabrication for the electron pulser. The work has
been supported by the DoE SBIR Program under Grant DE-SC0013121, and by NIST Contract SB1341-16-
CN-0035. The former Euclid employees, Sergey Baryshev and Jiaqi Qiu had significant contribution in the
early stage of Project.
References:
1. S. Baryshev, et al, Apparatus for GHz rate high duty cycle pulsing and manipulation of low and medium
energy DC electron beams, patent, US9697982B2
2. C. Jing, et al, UltraBroad Band Continuously Tunable Electron Beam Pulser, patent, pending, Publication
number: 20170162361
3. C. Jing, et al, A Metallic Broad Band Modulator for Tunable Electron Pulsers, patent, pending.
35
Femtosecond-Laser-Induced Spin-Polarized Electron Emission from a GaAs
Tip
Evan Brunkow, Eric R. Jones, Herman Batelaan, and T. J. Gay
Jorgensen Hall, University of Nebraska, Lincoln, Nebraska 68588-0299 USA
We show that focusing circularly-polarized 800nm light pulses of 100 fs duration on the tips of p-GaAs
crystalline shards results in electron emission that is both fast and spin polarized. The emission process was
shown to have a 400 fs duration by demonstrating the nonlinearity of the intensity dependence of emission
and by using pump-probe techniques that exhibit additivity. The three samples we investigated produced
electron polarizations of 13.1(9) %, 13.3(7) %, and 10.4(2) %. Emission currents ranged between 50 pA
and 3 nA with a sample bias of -100V and average laser power of 100mW. The GaAs shard tips also exhibit
electron emission that is linearly dichroic, a feature common to sharp field emission tips. This technology
represents a source of spin-polarized electron pulses that are “fast," in the sense that the electron emission
process is of comparable duration to the photon pulses that initiate it. The source does not require ultrahigh
vacuum conditions or activation to negative electron affinity (NEA) conditions.
This work has been funded by NSF awards PHY - 1206067 and 1505794 (TJG), EPS - 1430519 (HB and
TJG), and PHY - 1602755 (HB).
36
MeV laser plasma electron acceleration for ultrafast electron diffraction
Ping Zhang, Yunhao Fan, Matthew Robinson, Donald Umstadter, Martin Centurion and Matthias
Fuchs
Department of Physics and Astronomy, University of Nebraska-Lincoln
Laser-plasma electron accelerators can generate relativistic electron beams from a comparably compact
setup [1]. The generated electron bunches are measured to be only a few femtoseconds long [2,3]. We have
used laser-plasma acceleration to generate weakly relativistic electron beams with a high bunch charge of
a few nC (~1010 electrons/pulse). The generated electrons have a thermal energy distribution with a
temperature ~1MeV. To generate the electron beams, we have developed and characterized new thin pulsed
gas jet targets of the thickness less than 100 𝜇𝜇m. With these targets and a 2 TW laser (~40fs, 80mJ), we
are able to create plasma densities close to the plasma critical density. By modifying the plasma density,
we can tune the electron acceleration mechanism from the self-modulated laser wakefield acceleration
(SMLWFA) [1] regime to direct laser acceleration (DLA) [4]. In SMLWFA, a high-amplitude plasma wave
is driven by longitudinal laser pulse modulation through the forward Raman instability. Electrons are
accelerated by the ~ GeV/cm fields of the plasma wave to relativistic energies with a typically broad
spectrum and comparably narrow beam divergence. Correspondingly strong Raman scattering light is
observed. At higher plasma densities direct laser acceleration dominates, where electrons are accelerated
by the laser ponderomotive force and emitted with a relatively large divergence. In this regime, Raman
scattered light is highly suppressed.
Despite the broad energy spectrum and large beam divergence, in our experiments, the generated electron
beams in the SMLWFA and DLA have an extremely high spectral brightness due to the high charge. As
the generated electron beam is perfectly synchronized to the driver laser, this source has the potential to be
used for ultrafast electron diffraction.
[1] E. Esarey, C. B. Schroeder, and W. P. Leemans. Physics of laser-driven plasma-based electron
accelera- tors. Reviews of Modern Physics, 81(3), 08 2009.
[2] A. Buck, et al. Real-time observation of laser-driven electron acceleration. Nat Phys, 7(7):543–548, 07
2011.
[3] O. Lundh et al. Few femtosecond, few kiloampere electron bunch produced by a laser-plasma
accelerator. Nat Phys, 7(3):219–222, 03 2011.
[4] C. Gahn et al., Multi-MeV Electron Beam Generation by Direct Laser Acceleration in High-Density
Plasma Channels Rev. Lett. 83, 4772-4775, 1999.
37
Dressing a metal on the nano-femto scale
Hrvoje Petek, University of Pittsburgh
Ultrafast laser pulses can interact with metal surfaces in a perturbative or nonperturbative regime. We
explore electron dynamics in metals under highly nonlinear conditions by interferometric time-resolved
multiphoton photoemission.1 Multidimensional coherent photoelectron spectroscopy and microscopy
reveals how an optical field can modify the electron band structure of a metal and introduce new
quasiparticles. The main topic of my talk will be on ultrafast photoemission electron microscopy imaging
of topological spin texture in a metal that can be synthesized on ~20 fs time scale and can be modulated at
0.5 fs-1 frequency.
[1] M. Reutzel, A. Li, and H. Petek, Coherent Two-Dimensional Multiphoton Photoelectron Spectroscopy
of Metal Surfaces, Phys. Rev. X 9, 011044 (2019).
38
Coherent control of a surface-specific structural phase transition examined by ultrafast LEED
Jan Gerrit Horstmann1, Bareld Wit1, Gero Storeck1, and Claus Ropers1 1IV. Physical Institute, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Solid state surface systems are particularly interesting because of their modified electronic, lattice and spin structures, resulting in strongly altered physical and chemical properties compared with the bulk. We have recently developed ultrafast low-energy electron diffraction (ULEED) in a laser-pump/electron-probe scheme to explore optically-induced structural dynamics at surfaces on their intrinsic time scales [1,2].
In this contribution, we report the coherent control of a structural phase transition in a solid-state surface system. For this, we investigate the structural transition between the insulating (8×2) and the metallic (4×1) phase of Indium nanowires on Si(111). As recently demonstrated, this transition can be driven in the limit of critically damped atomic motion [3] by means of optical excitation with single intense light pulses. However, owing to their low dimensionality, atomic wire systems often exhibit a limited number of structural modes with potentially long dephasing times.
Here, we employ an optical two-pulse excitation scheme (Fig. 1a) to exert control over the phase transition, thereby harnessing the vibrational coherences present in the system. A detailed analysis of the delay-dependent phase transition efficiency recorded by ULEED (Fig. 1b) proves the critical role of two specific phonon modes for the transition [4] (Fig. 1c) and allows for a deeper insight into the underlying structural pathways. The mode-selective coherent control of solid-state surfaces could enable unprecedented switching speeds of chemical and physical functionalities, as well as metastable and non-equilibrium states.
Fig. 1: a, Scheme of ULEED combined with a two-pulse optical excitation to control the phase transition in In-nanowires on Si(111). b, Oscillations in the delay-dependent intensities of (4×1) and (8×2) diffraction spots confirm the coherent control of the phase transition. c, Fourier transform of the (8×2) spot suppression, revealing the critical role of a shear mode at 0.54 THz and a rotation mode at 0.81 THz for the transition.
References: [1] S. Vogelgesang, G. Storeck, J. G. Horstmann, T. Diekmann, M. Sivis, S. Schramm, K. Rossnagel, S. Schäfer and C. Ropers, Nat. Phys. (2017), advance online publication, doi:10.1038/nphys4309. [2] G. Storeck, S. Vogelgesang, M. Sivis, S. Schäfer and C. Ropers, Struct. Dyn. 4, 044024 (2017) [3] T. Frigge, B. Hafke, T. Witte, B. Krenzer, C. Streubühr, A. Samad Syed, V. Mikšić Trontl, I. Avigo, P. Zhou, M. Ligges, D. von der Linde, U. Bovensiepen, M. Horn-von Hoegen, S. Wippermann, A. Lücke, S. Sanna, U. Gerstmann and W. G. Schmidt, Nature 544, 207 (2017). [4] S. Wippermann and W. G. Schmidt, PRL 105, 12 (2010)
39
Ultrafast nano-imaging of the order parameter in a structural phase transition
Thomas Danz, Till Domröse, Claus Ropers 4th Physical Institute – Solids and Nanostructures, University of Göttingen, Friedrich-Hund-Platz 1, 37077
Göttingen, Germany
Email: [email protected]
Ultrafast transmission electron microscopy (UTEM) is an emerging approach to study ultrafast processes with high spatial resolution [1]. Using the imaging, diffraction, and spectroscopy capabilities of such an instrument, we investigate structural, electronic, and magnetic dynamics in a laser pump/electron probe scheme. To this end, the Göttingen UTEM is equipped with a nanoscopic tip emitter, delivering highly coherent electron pulses with down to 200 fs pulse duration, 0.6 eV energy width, and sub-1-nm focused spot diameter by linear photoemission [2].
These exceptional beam properties allow for a versatile use of the Göttingen UTEM, as demonstrated in recent years. This includes, for example, imaging magnetic dynamics via Lorentz microscopy [3], mapping strain dynamics with scanning techniques [4], and the quantum coherent manipulation of electron beams in optical near-fields [5], ultimately leading to the generation of attosecond electron pulses [6].
Some of the most interesting possibilities are connected to the investigation of various kinds of structural and electronic phase transitions in correlated materials, such as transition metal dichalcogenides (TMDCs), e.g., 1T-TaS2. This quasi-2D material favors a periodic modulation of the electronic density, i.e. the formation of charge density waves (CDWs) coupled to a periodic lattice distortion (PLD) [7]. Various ultrafast electron diffraction experiments have elucidated the optically induced dynamics of transitions between several CDW/PLD phases [8-11].
Here, we report on the first ultrafast real-space imaging of CDW dynamics (see Fig. 1A). We conduct a UTEM study on a freestanding, single-crystalline 1T-TaS2 film with 50 nm thickness [12]. In the experiments, the material is pumped out of the nearly commensurate room-temperature CDW phase towards the high-temperature incommensurate CDW phase. Specifically, we employ dark-field microscopy to follow the formation, evolution, and relaxation of CDW domain patterns on their intrinsic femtosecond to nanosecond timescales, yielding nanoscale access to the order parameter of the structural phase transition (see Fig. 1B and C). More generally, we discuss approaches to drive phase transitions in freestanding thin films with high repetition rates and low cumulative heating.
[1] A.H. Zewail, Science 328, pp. 187-193 (2010). [2] A. Feist, Th. Danz et al., Ultramicroscopy 176, pp. 63-73 (2017). [3] N. Rubiano da Silva et al., PRX 8, 031052 (2018). [4] A. Feist et al., Structural Dynamics 5, 014302 (2018). [5] A. Feist et al., Nature 521, pp. 200-203 (2015). [6] K.E. Priebe et al., Nat. Photonics 11, pp. 793-797 (2017).
[7] K. Rossnagel, J. Phys.: Condens. Matter 23, 213001 (2011). [8] M. Eichberger et al., Nature 468, pp. 799-802 (2010). [9] K. Haupt et al., PRL 116, 016402 (2016). [10] S. Vogelgesang et al., Nat. Phys. 14, pp. 184-190 (2018). [11] A. Zong et al., Sci. Adv. 4, eaau5501 (2018). [12] Th. Danz et al., J. Phys.: Condens. Matter 28, 356002 (2016).
Fig. 1. (A) Schematic of the experimental setup in the Göttingen ultrafast transmission electron microscope. (B and C) Ultrafast electron micrographs of the specimen before and after time zero, showing domains of the room- (bright) and the high-temperature phase (dark).
40
Development of a Dynamic Environment Transmission Electron Microscope at the University of Illinois
Omid Zandi, Allan Sykes, Eric Thornburg, Ryan Cornelius, Frank Alcorn, Jian-Min Zuo, Jim Mabon, and Renske van der Veen
Department of Chemistry and Materials Research Laboratory, University of Illinois at Urbana-Champaign, USA
Abstract
We present the development and characterization of a Dynamic Environmental Transmission Electron Microscope (DETEM) at the University of Illinois at Urbana-Champaign (UIUC). The setup is based on a 100-300 keV Hitachi H-9500 TEM, equipped with a thermionic LaB6 gun, and a Gatan Imaging Filter (GIF) Quantum ER with Gatan K2 IS direct electron detection camera. The microscope is interfaced with a fs laser system (Light Conversion Pharos with Orpheus-F), that allows excitation of the sample with wavelengths between 210-2600 nm and variable repetition rates up to 1 MHz, as well as two ns lasers (Spectra Physics VGEN HE and Bright Solutions Wedge HF). Various modes of operation are implemented in order to span a broad range of time scales: fs and ns stroboscopy for reversible dynamics (300 fs-1 ns), ns-resolved single-shot probing for irreversible dynamics (~20 ns), as well as low-repetition rate (400 Hz) pump-probe microscopy using the K2 IS camera synchronized to the pulsed electron beam. The latter mode should allow us to circumvent the “rolling-shutter” problem commonly encountered in fast microscopies. The unique in situ capabilities of the DETEM instrument (gas exposure, liquids and high temperatures), in combination with electron-energy loss spectroscopy (EELS) and sensitive direct-electron detection, will enable transformative studies in the fields of photocatalysis, field-induced nanoparticle assembly, and photoinduced phase transitions. First results in these areas will be presented as well.
41
Quantitative UED study on atomic trajectory and electron dynamics in 1T-
TaSeTe
Jing Tao
Brookhaven National Laboratory
42
Laminar flow and Coulomb cooling of high intensity beams
Brandon Zerbe, Xukun Xiang, Phil Duxbury (Presenter)
Physics and Astronomy Department
Michigan State University
Abstract: Much of the theoretical beam physics literature either explicitly or implicitly assumes that the
beam dynamics can be treated using the assumptions of uniformity and symmetry; for example the envelope
equations; or the analytical models of Michalik and Sipe. Motivated by the fact that photocathodes typically
generate bunches with Gaussian-like profiles we have developed a formulation to extend these approaches
to treat general density distributions. This formulation reproduces the known results for uniform density
distributions and extends them to non-uniform cases. An intriguing new result is that an initial Gaussian
density distribution can evolve to a bimodal density distribution for cylindrical beams or for spherical
distributions; even in the non-relativistic limit. The formation of bimodal radial density distributions is
correlated with a high-density ring of electrons with high local emittance, which can be removed using an
aperture. This can lead to a significant enhancement in beam brightness, which is the “Coulomb cooling”
effect. Gaussian beams typically evolve through laminar flow prior to formation of bimodal structures, but
then undergo crossover which violates the laminar condition. The conditions for laminarity are extended to
treat cases with an initial radial chirp; and an extension to relativistic planar bunches enables an exact
treatment of the shape of an ultra-relativistic bunch. For cold initial conditions, a planar relativistic bunch
develops a bimodal structure while in the non-relativistic limit this does not occur.
References:
Michalik, A. M. & Sipe, J. E. Analytic model of electron pulse propagation in ultrafast electron diffraction
experiments. Journal of Applied Physics 99, 054908 (2006)
Zerbe, B. S., Xiang, X. , Ruan, C.-Y., Lund, S. M., Duxbury, P. M. Dynamical bunching and density peaks
in expanding Coulomb clouds Phys. Rev. AB 064201 (2018)
Zerbe, B. S., Xiang. X., Duxbury, P. M. Realizing laminar-like flow in charged bunches with density
evolution equations. Preprint (2019)
Zerbe, B. S., Duxbury, P. M. Density shocks in the relativistic expansion of highly charged one component
plasmas, Preprint (2019)
43
Dynamics of uniform spheroidal electron bunches driven by linear chirp
Xukun Xiang (presenter), Brandon Zerbe, Phil Duxbury
Physics and Astronomy Department
Michigan State University
Abstract:
A simple mean-field model is proposed for non-relativistic compression of uniform spheroidal electron
bunches driven by a linear chirp in order to better understand the focusing process in ultrafast electron
microscopy/diffraction beamlines. In high intensity bunches, a significant initial chirp is required to drive
longitudinal crossover, and we define a critical chirp which is the threshold at which crossover occurs.
Below the critical chirp the bunch narrows and then expands without crossover. During focusing of high
intensity beams, and particularly near the critical chirp, energy transfer occurs between the longitudinal and
transverse degrees of freedom, and this behavior is captured quite well by simple mean field theory.
However the simple mean field approach does not treat the emittance dynamics; which we instead evaluate
by using a statistical study of non-interacting elections and by using large-scale N-electron simulations.
Reference:
Lin, C. C., Mestel, L. & Shu, F. H. The Gravitational Collapse of a Uniform Spheroid. The Astrophysical
Journal 142, 1431 (1965).
Luiten, O. J., van der Geer, S. B., de Loos, M. J., Kiewiet, F. B. & van der Wiel, M. J. How to Realize
Uniform Three-Dimensional Ellipsoidal Electron Bunches. Physical Review Letters 93, (2004).
Grech, M. et al. Coulomb explosion of uniformly charged spheroids. Physical Review E 84, (2011).
Michalik, A. M. & Sipe, J. E. Analytic model of electron pulse propagation in ultrafast electron diffraction
experiments. Journal of Applied Physics 99, 054908 (2006)
44
Kinetic model of near-source emittance growth
Brandon Zerbe (Presenter), Phil Duxbury
Physics and Astronomy Department
Michigan State University
Abstract: Realizing low emittance bunches of electrons is paramount to obtaining high quality data in the
ultra-fast electron microscopy community. It is well known that the emittance of a bunch is limited by the
“intrinsic emittance” of the cathode, and that this intrinsic emittance is further degraded as a result of a
process Wangler called charge redistribution. More than 30 years ago, Anderson presented a dynamic
model of charge redistribution in the presence of a focusing field, but his model does not accurately capture
emittance growth. On the other hand, Reiser described free energy of the initial distribution as driving
emittance growth, but his model is not kinetic and has recently been criticized by Bernal as incorrectly
applying equilibrium thermodynamic concepts as these cases are non-equilibrium and emittance is not a
thermodynamic variable. Here we consider the free expansion of the uniform distribution of electrons, and
we show that emittance growth due to charge redistribution is occurring in this simple case. We demonstrate
emittance’s rightful place in non-equilibrium and equilibrium thermodynamic theory. Using our recent
description of charged-bunch density evolution, we show that a perturbation of the spherically-symmetric
uniform expansion results in a novel form of incomplete oscillation. We use these emittance insights
coupled with the perturbative description to develop a kinetic description of emittance growth, and we show
that this model accurately captures the emittance growth of the freely expanding uniform electron bunch.
References:
Anderson, O.A. Internal dynamics and emittance growth in space charge dominated beams. Part. Accel.
Vol. 21, 197-226 (1987)
Bernal, S. Conceptual difficulties of a thermodynamics description of charged-particle beams. In JACoW
Proceedings MOPMA045 (2015)
Reiser, M. Theory and Design of Charged Particle Beams. John Wiley & Sons (1994)
Wangler, T.P. Emittance growth from space-charge forces. In AIP Conference Proceedings Vol. 253,
No. 1, 21-40 (1991)
Zerbe, B. S., Xiang, X., Ruan, C.-Y., Lund, S. M., Duxbury, P. M. Dynamical bunching and density
peaks in expanding Coulomb clouds. Phys. Rev. AB 064201 (2018)
45
Unexpected chemical processes induced by electron recollision and their control
N. Ekanayake1, M. Nairat1, N. P. Weingartz1, B. M. Farris1, M. J. Michie1, T. Severt2, B. Kaderiya2, P. Feizollah, 2 B. Jochim2, F. Ziaee2, K. Borne2, P. Kanaka Raju2, K. D. Carnes2, D. Rolles2, A. Rudenko2, J. E. Jackson1, B.
G. Levine1, I. Ben-Itzhak, and M. Dantus1, 3
1. Department of Chemistry, Michigan State University, East Lansing MI, 48824, USA
2. Department of Physics, J. R. Macdonald Laboratory, Kansas State University, Manhattan, KS 66506, USA3. Department of Physics and Astronomy, Michigan State University, East Lansing MI, 48824, USA
In a recent study [1], we provided experimental and theoretical evidence for the existence of two reaction pathways for the formation of H3+ from methanol under strong-field ionization. In brief (see Fig. 1a), both reaction pathways are initiated by the ultrafast double ionization of the parent molecule and proceed through prompt formation of a roaming neutral H2 moiety from the methyl site. By roaming, we imply that H2 explores relatively flat regions of the potential energy surface far from the minimum energy path. The roaming H2 fragment abstracts a third proton from the methyl carbon or from the hydroxyl oxygen leading to the formation of H3+. We have examined the involvement of H2 roaming mechanisms in ionic species in the formation of H3+ from a series of alcohols under strong-field excitation. We have evaluated the femtosecond dynamics and mechanisms associated with H3+ formation for series of alcohols [2]: methanol (CH3OH), ethanol (CH3CH2OH), 1-propanol (CH3CH2CH2OH), 2-propanol (CH3CH(OH)CH3), and tert-butanol ((CH3) 3COH). Similarly, we have studied the substitution of oxygen with sulfur, comparing ethanol and ethanethiol [3]. Through systematic pulse shaping we were able to control the yield of H3+ fragments by almost an order of magnitude for pulses with identical spectra and peak intensities [4]. I will explain the observed differences and present the latest results.
Fig. 1 The strong-field triggered fragmentation of methanol includes a chemical reaction producing H3+ and HCO+ or CHO+.
References
[1] N. Ekanayake, M. Nairat, B. Kaderiya, P. Feizollah, B. Jochim, T. Severt, B. Berry, K. Raju P., K.D. Carnes, S. Pathak, D. Rolles, A. Rudenko, I. Ben-Itzhak, C.A. Mancuso, B.S. Fales, J.E. Jackson, B.G. Levine, and M. Dantus, Sci. Rep. 7, 4703 (2017).
[2] N. Ekanayake, T. Severt, M. Nairat, N. P. Weingartz, B. M. Farris, B. Kaderiya, P. Feizollah, B. Jochim, F. Ziaee, K. Borne, K. Raju P., K. D. Carnes, D. Rolles, A. Rudenko, B. G. Levine, J. E. Jackson, I. Ben-Itzhak, and M. Dantus, Nat. Commun. 9, 5186 (2018).
[3] N. Ekanayake , M. Nairat, N. P. Weingartz, M. J. Michie , B. G. Levine , and M. Dantus, J. Chem. Phys. 149, 244310 (2018).
[4] M. J. Michie, N. Ekanayake, N. P. Weingartz, J. Stamm, and M. Dantus, “Quantum coherent control of H3+ formation in strong fields,” J.
Chem. Phys. 150, 044303 (2019).
CH3OH2+ CHOH2+ + H2 COH+ + H3+
HCO+ + H3+
Roaming H2
Methanol
2.0×1014 W/cm2
46
“Molecular movies” of light-induced bond formation and breakage in
molecular halogen elimination from halomethanes
Balram Kaderiya, Kanaka Raju P., Wright L. Pearson, Xiang Li, Travis Severt, Farzaneh Ziaee,
Kurtis Borne, Yubaraj Malakar, I. Ben Itzhak, Daniel Rolles and Artem Rudenko
J.R. Macdonald Laboratory, Kansas State University, Manhattan, KS 66506
Mapping the light-induced cleavage and formation of molecular bonds constitutes one of the central topics
for strong-field physics and ultrafast photochemistry [1]. Some of the fundamental questions here are: What
determines whether a particular molecular bond is or will be broken? What is the role of different vibrational
modes in bond breaking and formation? How are the final products related to the intermediate molecular
configurations? Time-resolved momentum imaging of molecular fragmentation provides a powerful
experimental tool to address these questions [2,3].
Here, we apply coincident ion momentum imaging to study one of the prototypical photo-induced reactions
involving modifications of several molecular bonds, namely the elimination of molecular halogens from
halomethanes. In this experiment, an intense (~2×1014 W/cm2) 25-fs, 800-nm pump pulse ionizes and
fragments CH2I2 or CH2ICl molecules, and the subsequent dynamics are probed with a second, more intense
pulse, resulting in multiple ionization and Coulomb explosion (CE). We consider several two- and three-
body breakup channels resulting from singly-, doubly- and triply-charged final states, and particularly focus
on the pathway yielding three singly charged fragments, CH2+ and two halogen ions. A detailed analysis of
multidimensional experimental data along with classical CE simulations allows for reliable identification
of the contributions from different reaction channels triggered by the pump pulse. Those include concerted
CE of a bound molecule, a sequential breakup of the long-lived metastable CH2I++ intermediate [4], CE of
CH2I+ + I or CH2I + I+ dissociation pathways, and channels involving transient I2 or I2+ formation. We focus
on the latter group of channels, analyze possible formation mechanisms of molecular halogens, discuss their
relation to the vibrational motion of the precursor molecule [5] and create a “molecular movie” of this bond-
formation reaction.
[1] A.H. Zewail, J. Phys. Chem. A 104, 5660 (2000).
[2] Y.H. Jiang et. al., Phys. Rev. Lett. 105, 263002 (2010)
[3] H. Ibrahim et al., Nature Comm. 5, 4422 (2014).
[4] J. Rajput et al., Phys. Rev. Lett 120, 103001 (2018).
[5] D. Geißler, B.J. Pearson and T. Weinacht, J. Chem. Phys. 127, 204305 (2007).
This project is supported by the Chemical Sciences, Geosciences, and Bio-Sciences Division, Office of
Basic Energy Science, Office of Science, U.S. Department of Energy.
47
Ultrafast electron imaging of electronic and molecular motionsa
Hua-Chieh Shao and Anthony F. StaraceDepartment of Physics and Astronomy, The University of Nebraska, Lincoln,
Nebraska 68588-0299, USA
Owing to
advances
in
ultrafast
electron
technologies,
it
is
now
possible
to
generate
and
manipulate fs
electron
pulses
with
sub-˚angstr¨om
de
Broglie
wave-lengths.
Therefore,
it
is
now feasible
to
directly
image
and
visualize
electronic
and
molecular
transient
motions
during reactions.
We
report
theoretical
stud-ies
for
utilizing
ultrafast
electron
pulses
to
image electronic
and
molecular
motions.
In
particular,
ultrafast
electron
diffraction
is
studied as
a means
of
imaging
the
oriented
ro-vibrational
motions
of
deuterated
lithium
hydride (LiD)
and
hydrogen
(HD)
molecules.
The
molecular
motion
is
assumed
to
be
initiated by
a pump
pulse
that
impulsively
excites
an
electron
from
the
ground
state
to
some excited
electronic
state.
Then
the
ensuing
molecular
motion
in
the
excited
state
is
imaged by
time-delayed
1-fs
electron
pulses.
The
simulated
diffraction
images
show
a delay-dependent
ring
pattern
owing
to
the
interfer-ing
scattering
amplitudes
from
the
constituent atoms
as
the
molecule
vibrates.
Moreover,
violation
of
the
centrosymmetry
of
the diffraction
patterns
is
ob-served
in
the
time-resolved
measurements,
which
exhibit
asymmetric angular distributions that relate to the direction of motion of the atoms.
In addition, time-resolved (e, 2e) electron momentum spectroscopy has been considered as a means of probing the momentum profile of electronic motion in a population transfer in lithium atoms [1]. A ps chirped pulse adiabatically drives the valence electron from the 2s to 2p states of the Li atom, and such motion is imaged by 100- and 1-fs electron pulses through the mechanism of high-energy impact ionization. The simulations show that the momentum distribution of the valence electron at the moment of collision can be retrieved from the spectrum. However, the level of detail of the information about the motion depends on the pulse duration and the time scale of the electronic motion.
[1] H.-C. Shao and A.F. Starace, Time-resolved electron (e, 2e) momentum spectroscopy: Application to laser-driven population transfer in atoms, Phys. Rev. A 97, 022702 (2018).
a This work was supported in part by the U.S. National Science Foundation under Grant No. PHY-1505492. This work was completed utilizing the Holland Computing Center of the University of Nebraska, which receives support from the Nebraska Research Initiative.
48
