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Ultrafast dynamics in real time
http://www.fhi-berlin.mpg.de/pc/electrondynamix
Julia StählerFritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Physical Chemistry, Berlin, Germany
why ultrafast dynamics?
electricalenergy
light
light
electricalenergy
BLACK BOX
How do we see ?
example: photochemistry of vision
Rhodopsin
Eye Rod cells: A single rodcontains 108 rhodopsins(renewed every 10 daysthroughout a person’s life)
cis-trans isomerization
Retinal
hν
11-cis all-trans
ultrafast processes in excited solids
solids and interfaces exhibit various fundamental processes on atto- to picosecond timescales
adapted from Petek & Ogawa, Prog. Surf. Sci. 56, 239 (1997)
e - e scattering
e - phonon scattering
linewidth (eV)1 0.1 0.01 0.001
0.1 1 10 100 1000
molecular motions
screening
e - correlation
timescale (fs)
dephasing
femtoseconds
10-15 s 10-10 s 10-5 s 1 s 105 s 1010 s 1015 s
processorspeed
flash month end of lastice age
age ofuniverse
pump-probe experiment
outline
▪ introduction: elementary excitations in solids
▪ charge carrier dynamics in metals and semiconductors
▪ surface femtochemistry
coupling of electrons, spins, and phonons
electrons
Tel
lattice
Tph
spins
elementary scattering processes and energy flow:
hν (~eV)
electron-electron
scattering
electron-phonon
couplingτe-ph
τe-magn
electron-magnon
coupling,
‚spin flip‘ scattering
spin-lattice relaxation
phonon-magnon
scattering
τph-magn
phonon
decay
elementary excitations
▪ single (quasi)-particle excitations :e.g. photoholes, injected excess electrons,…
▪ two-particle excitations :e.g. excitons, Cooper pairs,…
▪ collective excitations :e.g. magnons, plasmons, phonons, polariton,…
spin wave
(assuming rapid decay of coherence)
dephasing/ optical absorption
elementary processes:electronic polarization
cooling by e-ph coupling, heat transport
electron thermalization
at high excitation densities: description by distribution function and time evolution, e.g., according Boltzmann equation
t = 0
electron thermalization
electrons in a metal
fs-laser excitation
transient electron distribution function
(assuming rapid decay of coherence)
dephasing/ optical absorption
elementary processes:electronic polarization
cooling by e-ph coupling, heat transport
electron thermalization
at high excitation densities: description by distribution function and time evolution, e.g., according Boltzmann equation
t = 0
electron thermalization
electrons in a metal
fs-laser excitation
transient electron distribution function
photoabsorption in a metal
elementary scattering processes:
electron-electron scattering electron-phonon, electron-magnon scattering, ….
lifetime of an excited electronabove the Fermi sea
▪ Fermi liquid theory (Landau 1957)
▪ solution for homogeneous electron gas,self energy (Quinn & Ferell 1958)
screened Coulomb interaction:
screening in metals
τ~ ωP-1
(10-16 s in metals)
time requiredfor screening
build-up?
quasiparticles
system of interacting real particles acts as if it were
composed of weakly interacting fictitous bodies (quasiparticles)
bareparticle
+‘cloud’ of
other particles
=
quasiparticle
fundamental concept in condensed-matter physics:
one many-body problem multiple one-body problems
quasiparticle lifetimes
▪ Interactions between quasiparticles limit how long the corresponding quantum states retain their identity, i.e., the lifetime of the excitation.
▪ In combination with the velocity, this lifetime determines the mean free path, a measure of the interaction in the solid.
hυ EF
lifetime-determining factors:▪ initial state (energy, momentum)▪ final state (“phase space”)▪ screening (response function)
outline
▪ introduction: elementary excitations in solids
▪ charge carrier dynamics in metals and semiconductors
▪ molecule dynamics at surfaces
image potential states at metal surfaces
z
Echenique & Pendry (1978)classical image potential:
courtesy: Ulrich Höfer, Marburg
binding energy (Rydberg series):
lifetime:
probing the energy levels: photoemission
▪ photoelectron spectroscopy:absolute binding energiessurface sensitivityUHV conditions
Y. Gao. Mat. Sci. Eng. R 68, 39 (2010)
2PPE spectroscopy
2-photonphotoelectron spectroscopy
experimental setup I
NOPA< 20 fs,
laser setup
RegA 9050< 40 fs, 3 µJ/pulse
@ 300kHz
275 nm
200 nm
OPA 9450< 60 fs,
stretcher
compressorVerd
i V18
Micr
a < 40
fs,
4 nJ/p
ulse @
76 M
Hz
2PPE of image potential states
Berthold et al, PRL 88, (2002) 056805n = 1
n = 2
courtesy: Ulrich Höfer, Marburg
time-resolved 2PPE of image potential states
courtesy: Ulrich Höfer, Marburg
Cu(100)▪ direct measurement of the lifetime using 2PPE▪ excellent agreement with many-body theory(GW approximation)
Echenique et al., Surf. Sci. Rep. 52, 219 (2004)
time-resolved 2PPE of image potential states
populationdynamics
Berthold et al, PRL 88, 056805 (2002)
ferromagnetic surfaces: spin splitting
Passek et al., PRL 75, 2746 (1995)Aeschlimann et al., PRL 79, 5158 (1997)
origin: different density of states of majority and minority electrons
spin splitting
spin-, angle-, and energy-resolved 2PPE
spin-polarized LEED detector
minority spin
majority spin
parallel momentum (1/Å)
bind
ing
ener
gy(e
V)
example: 3ML Fe film on Cu(100)
Schmidt et al., PRL 95, 107402 (2005)
spin-dependent image state lifetimes
origin: different density of states
Passek et al., PRL 75, 2746 (1995)Aeschlimann et al., PRL 79, 5158 (1997)
Schmidt et al., PRL 95, 107402 (2005)
Magnon-enhanced intraband scattering
magnon emission
Schmidt et al., PRL 95, 107402 (2005)
VB
CB
semiconductor surfaces
EF
▪ potential step at the surface:electrons leaking into the vacuum
Φ
▪ band bending & adsorption:change of surface dipole
Evac
HOMO
LUMO
Evac
zincoxide: a promising challenge
▪ inorganic semiconductor: ZnO-wide band gap semiconductor-transparent-high carrier mobility-room temperature luminescence
-wurtzide structure-two polar, one non-polarlow-index surface
Is the pristine ZnO(10-10) surface stable?
Wang et al., PRL 95, 266104 (2005)
▪ hydrogen termination probable▪ potential metalicity of the
ZnO(10-10) surface:
hydrogen termination
Ozawa and Mase, PRB 83, 125406 (2011)
our work
▪ hyrogen-induced CB down-bending observed
Fermi edge
time-resolved spectroscopy
Tisdale et al., JPC C 112, 14682 (2008)
time-resolved spectroscopy
▪ fast initial dynamics▪ slow relaxation (~100 ps)
Tisdale et al.
time-resolved spectroscopy
▪ fast initial dynamics▪ slow relaxation (~100 ps)
Tisdale et al.
Tisdale et al., JPC C 112, 14682 (2008)
summary
▪ introduction: elementary excitations in solids
▪ charge carrier dynamics in metals and semiconductors
▪ molecule dynamics at surfaces
Further reading:P.M. Echenique et al., Surf. Sci. Rep. 52, 219 (2004)
M. Weinelt et al., Prog. Surf. Sci. 82, 224 (2007)
outline
▪ introduction: elementary excitations in solids
▪ charge carrier dynamics in metals and semiconductors
▪ molecule dynamics at surfaces
temporal evolution from reactants to products:
▪ dynamics of the transition state▪ goal: microscopic understanding of elementary
steps and dynamics of energy transfer
▪ femtosecond laser pulses for direct observationin time domain
▪ key concept: dynamics on Born-Oppenheimer potential energy surfaces
▪ non-adiabatic coupling between different electronic states
timescales of chemical reactions
▪ breakdown of the BO approximation near (avoided) crossings
surface reactions and heterogeneous catalysis
heterogeneous catalysis:▪ reduced reaction barrier at surfaces▪ prominent examples:
• automotive catalyst• ammonia synthesis
rate-limiting step:dissociative adsorption
ABgas
Aads Bads ABgas DE
Aads+Bads
dissociative desorption
ABgas
Aads BadsDEAads+Bads
EaABgas
goal: microscopic understanding of surface reactions
chemical reactions typically occurin the electronic ground state
role of electronic excitations?
femtochemistry at metal surfaces
ultrafast energy redistribution after optical excitation of a metal surface
fs-laser pulse
Frischkorn & Wolf, Chem. Rev. 106, 4207 (2006)
▪ optically thin monolayer▪ substrate-mediated excitations
dominate
coupling to adsorbate vibrations induces reaction
reminder: fs-laser excitation in metals
(assuming rapid decay of coherence)
dephasing/ optical absorption
elementary processes:electronic polarization
cooling by e-ph coupling, heat transportelectron thermalization
transient electron distribution functionfs-laser excitation ofelectrons in a metal
t = 0
Lisowski et al., Appl. Phys. A 78, 165 (2004)
coupling to adsorbate motions
t = 0
Desorption Induced byElectronic Transitions
DIET -
Desorption Induced byMultiple Electronic Transitions
DIMET -
* MGR model (Menzel, Gomer, Redhead)
*
fs-laser excitation ofelectrons in a metal
reaction mechanism
phonon-mediatedelectron-mediated
How to distinguish?
substrate
adsorbateE
reaction coordinate
phonons Edes
kBTph
2-pulse correlation
FWHM ~ 10 ps
FWHM ~ 1 ps
phonon-mediated: slow responseelectron-mediated: fast response
pulse-pulse delay Dt = 0 fs
How to distinguish electron and phonon mediated mechanism?
= 5 ps= 2 ps= 1 ps= 0.5 ps
Tel and Tph for 2 pulse excitation with time delay ∆t
experimental setup II
“Feulner cup“ detector
80 mm
O = 33 mm/
Ru(001)
QMSionisationvolume
H2
Ti:sapphire oscillator + amplifier 4 mJ 800 nm, 110 fs, 20-400 Hz,
2 equally intensepulses
Ru(001)
QMS
H2/D2
CCD
pulse profilediagnostics
/
UHV
(1x1) saturation coverage H/Ru(001)
fs-laser activationY = A <F>n
kBT
thermal activationY = A exp (-Ea/kBT)
H+H recombinative desorption
Coupling rates: τel τph τel-ph-1 -1 -1
., ,
met
al
τph
τel-ph Tph
τel
elTheat diffusion
adsT
ads-E /k Ta bRate ~ e
Two-Temperature Model
Coupled heat baths: el ph ads T , T , T
Brandbyge et al., PRB 52, 6042 (1995)
coupled heat baths with Tel, Tph, Tads
first s
hot y
ield [
a.u.]
pulse-pulse delay [ps]
H2exp. data
0-10 -5 0 5 10
model
FWHM = 1.1 ps
2-pulse correlation
2-pulse correlation
Energy transfer by coupling betweenadsorbate vibration and metal electrons
Electronic friction:
hotelectrons
EFermi
Eleveladsorbate
electronshot
metal
Coupling rate η = ~ 11Melτ
electronic friction model
Struck et al., PRL 77, 4576 (1996)
▪ energy transfer by coupling betweenadsorbate vibration and metal electrons
▪ example: CO desorption from Cu(100)
0
160140120100806040200
0
exp. data
model
power law fit
yield weighted fluence [J/m2]
first s
hot y
ield [
a.u.]
isotoperatio
H2
D2
Y ~ <F>3
yield weighted fluence (J/m2)
isot
ooe
ratio
Y(H
2)/Y
(D2)
2-pulse correlation
origin of isotope effect: DIMET
real-time probing of vibrational dynamics
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
▪ so far: only the (desorbing) reaction products probed→ surface-sensitive non-linear optics (SHG, SFG):
direct look inside the excited adlayer
vibrational sum-frequency generation (SFG)
▪ no background▪ visible detection▪ sensitivity: 0.001 ML▪ short pulses: time resolution▪ surface/interface sensitive
Pot.
Ener
g y
C-O distancev=0
v=1v=2
5 mm + 800 nm = 690 nm
IR
SFGVIS
▪ background▪ IR detection▪ optical linear technique
C-O distance
IRv=0
v=1v=2
Pot.
Ener
g y
IR absorption spectroscopy (FTIR)
CO
C-O stretch
vibrational spectroscopy
▪ no background▪ visible detection▪ sensitivity: 0.001 ML▪ short pulses: time resolution▪ surface/interface sensitive
Pot.
Ener
g y
C-O distancev=0
v=1v=2
5 mm + 800 nm = 690 nm
IR
SFGVIS
vibrational SFG spectroscopy
P E E = χ ω(2) (2)
IR ωVISω ωSFGSFG
time-resolved SFG as molecule-specific probe of surface reactions
second order non-linear optical process
→ surface sensitive method: SFG is symmetry forbidden in isotropic (bulk) media (dipole approximation)
broadband IR vibrational SFG spectroscopy
SFG spectrum ( 3x 3)-CO/Ru(001) √ √
SFG wavelength [nm]690 688 686 684694 692
SFG
inte
nsity
IR
20001950
FWHM9 cm-1
1900 2050IR wavenumber [cm ]-1
2100 2150
200 K
SFG
IR intensity
IR
VIS SFG
IR
VIS
Ru
SFG
IRIR
VIS
VIS
Γn
ΓSFG
01
2
~5 cm-1
~150 cm-1
Richter et al., Opt. Lett. 23, 1594 (1998)
spectrally broad fs-IR pulse + spectrally narrow VIS upconversion pulse
experimental setup III
Ti:sapphire oscillator + amplifier4 mJ, 800 nm , 110 fs, 20-400 Hz,
OPG/OPA (TOPAS), 20-30 J µ2.5-10 mµ
tunableIR pulse
pump pulse
spectrometer
CCD camera
multi-channel detectionRu(001)
QMS
SFGprobe
pulse shaperFWHM 7 cm , 7 J µ-1
narrow bandVIS pulse
SFG
inte
nsity
(a.u
.)
21002050200019501900IR wavenumber (cm-1)
3 ps
23 ps
0.5 ps×8
×8
×8
Pump-Probe delay –4.5 ps–2.0 ps–1.5 ps–1.0 ps–0.5 ps0.5 ps3.0 ps
23.0 ps68.0 ps
168.0 ps
CO
yie
ld (a
.u.)
100shot number
210020001900
T =340 K, F=55 J/m02
CO/Ru(001) optical probe
time-resolved SFG of CO/Ru during desorption
▪ pronounced transient red shift▪ linewidth broadening▪ decrease of intensity
problems:▪ dipole-dipole coupling in the adlayer▪ small concentration of „products“
Bonn et al., PRL 84, 4653 (2000)
calculation:▪ thermal excitation of frustratedtranslation▪ anharmonic coupling to CO stretch mode
high fluence:▪ excitation and coupling
to higher frequency modes
strong redshift indicates: frustrated rotation dominantes
low fluence: good fit
time-resolved SFG of CO/Ru during desorption
Bonn et al., PRL 84, 4653 (2000)
summary
▪ introduction: elementary excitations in solids
▪ charge carrier dynamics in metals and semiconductors
▪ molecule dynamics at surfaces
Further reading:A.M. Wodke et.al., Prog. Surf. Sci. 83, 167 (2008)
M. Bonn et al., J. Phys.: Condens. Matter 17, 201 (2005)