STI Round Table, June 22-24, 2004Thomas Möller, Technical University Berlin

• Interaction of intense X-ray pulses with clusters• Imaging of clusters and nanocrystals

Mechanisms of photoabsorption

Coulomb explosion

• Parameter list

Clusters Structure and Dynamics studied with X-ray FEL Pulses

Issues and Questions

Clusters as new materialsSize dependent properties• Optical properties• Catalytic activity• Phase transitions• Magnetic properties • Photochemical processes• Dissociation and exlosion dynamics

geometric structure positions of the individual atomsSo far: mass spectroscopy,

spectroscopy, (TEM)X-ray diffraction

Clusters in intense X-ray pulses

• Coulomb explosion of clusters(T. Ditmire et al.)

• More than one charge per atom, absorption of many photons (non-linear processes)

• Time scale of electron emission and of ion motion

• Testing theoretical models

Focussing of 3*1012 photons into a spot of ~ 100 nm diameterup to1020 W/cm2, ionisation yield close to one ( λ=0,1 nm)

Xenon clusters, 2500 atoms

2.5*1013 Watt/cm2 , λ= 97nm

non-linear process

complete desintegration forpower density > 1011 W/cm2

very efficient absorption

charge states higherthan in an atomic beam

2*1010 Watt/cm2

H. Wabnitz et. al, Nature 420, 482 (2002)

Xe+Xe++Xe6+

Time of flight [ns]

Multi-photon ionisation with the VUV-FEL

200 400 600 800

2*1010

Xe+

8*1010

Zäh

lrate

(w

illk.

Ein

heite

n)

6*1011

Xe2+

6*1012

3

54

PFEL

=2.5*1013

W/cm2

876

Ionisation dynamics at 96 nm wavelength

Full scale 200 fs Full scale 1200 fs

• Charge states of cluster atoms higher than that of isolated atoms

• cluster explosion starts after 20 fs

• part of the cluster remains neutral on a time scale of several hundred fs

C. Siedschlag, J. M. Rost,

PRL, in press, Xe80

MD simulation for Xe80

Recent theoretical work

L-shell ionization of argon atoms and clustersU. Saalmann, J.M. Rost, PRL, 143401 (2002)

Photon energy350 eV

power density 3*1014- 3*1018 W/cm2

absorbed energy peratom in clusters reducedcompared to isolated atom:

delocalized electrons, reduced Auger rates

Time evolution of exploding clusters

Z. Jurek, G. Faigel, M.TegzeEur. Phys. J. D 29, 217(2004)1500 atom carbon cluster, λ= 0,1nm100 nm focus, 5*1012 Photons, 50 fs pulse

Cluster inside a large He droplet:Delayed explosion?

• geometry, positions of atoms

• surface reconstruction(semiconductor clusters)

• atomic segratation

• study of isomerization (Si clusters)

• cluster melting

• surface chemistry, positions of ligands

• catalytic properties

Structure of size selected clusters

Imaging of cluster ions with X-rays

time resolved studies with fs laser

Similar to biomoleculeimaging

Scattering of clusters with some translation symmetry

• fcc- gold cluster with 9261 atoms

• Strong scattering, high Z atoms

• Some translational symmetry

Bragg type peaks

E. Weckert

Cluster imaging with the XFEL

one cluster ion in 100nm x100nm x 0.1mmcorresponds to 1012 cluster ions/ cm3

strong focussing of the ion beam and/or many photon pulses

Parameter list and summary

1012 Photons / pulsepuls length < 50 fsFocussing optics, spot size 100 nm

Wavelength 0.1-5 nm, < 0. 3nm for imaging10 Hz rate, pulse trains with many pulsesfs-optical laser synchronized with the XFEL1000x1000 pixel detector

Completely new scientific opportunities: geometric structure and dynamic processes in free clusters and nanocrystals can be directly investigated in real space and in real time Soft X-ray cluster collaboration: K.H, Meiwes-Broer, G. Ganteför, W. Eberhardt, T. Möller, E. Rühl, W. Wurth, H. Schmidt-Böcking

Charge distribution in exploding clusters

C. Siedschlage, J. M. Rost,PRL, in press, Xe80 neutral core inside the cluster

Cluster inside a large He droplet:Delayed ionisation?

XPS, XANES, element specific

investigation of catalytic processes on clusters e.g. CO2 on Ptn

Hexagonal silicon cage around a tungsten atom

Hiura et al PRL 86, 1733 (2001)

Determination of Electronic and Geometric Structure of Clusters: Interior Sites and

Adsorbates

• Pulse length 30-200 fs• Wavelength 80-120 nm, 60 nm – 6 nm starting 2005• Gigawatt peak power• 0.1 –1mJ pulse energy, 1013 Photons/shot• Fully coherent beam• Power density ~ 1014Watt/cm2

Multi-photon processes Pump probe experiments

more than 100 times higher peak brilliancethan any other source at this wavelength

Properties of the VUV-FEL

amplitude of free e-

ion core

• Field modulates the atomic potential at visible laser frequency

• Outer e- has time to tunnel or overcome the barrier:2Up > Ip where Up ~ Iλ2

• Field modulates the atomic potential at soft x-ray laser frequency

• e- do not have time to tunnel free• multi photon process and innershell

electrons are important

laser-atom process at I ~ 1014 W/cm2 , ponderomotive energy 10-100 eV

VUV FEL laser-atom process at I ~ 1014 W/cm2, ponderomotive energy 10-100 meV

P. Bucksbaum et al

Interaction of Intense Soft X-rays with Matter

• which multi-photon processes are observed

• cross sections (surface, bulk)

• which ions are prepared (charge state, electronically excited states)

• mechanism of absorption and

ionisation

Idea of the cluster experiment:Interaction of intense soft x-rays with matter

Experiments with IR light:T. Ditmire et al.M. Lezius et al.W. Castleman et al.J. Tiggesbäumker, Meiwes-Broer et al.

H. Wabnitz J. Schulz P. Gürtler, W. Laasch T. Laarmann, A. SwiderskiK. Haeften

0 100 200 300 400 500 600 700 800

0,0

0,2

0,4

average size of clustersN=300

7+6+

8+

5+

4+

Xe++

Xe3+

Xe+

inte

nsity

[arb

. uni

ts]

time of flight [ns]σ = 50 Mbarn (single photon ionisation, 100 photons within σ)

IpXe = 12.1 eVEphot= 12.8 eV

2*1013W/cm2

Single shot mass spectrum: Xe cluster

200 400 600 800

Xe+

inte

nsity

time of flight [ns]

atom

67

Xe3+

8

Xe++

N~2-20

5

4+

N~80

4 23876 5 1

N~30000• single shot spectra

• multiply charged ions from clusters

• singly charged atomsH. Wabnitz et al, Nature 420, 482(2002)

1*1013 W/cm2

Thoretical work for 350 eV

U. Saalmann, J.M. Rost,PRL 89, 143401(2002) predicts reduced ionisation in clusters

Time of flight mass spectra of Xeatoms and clusters

IpXe = 12.1 eVEphot= 12.8 eV

average absorption/atom

E = 550 eV ~ 43 photons/atom

average charge state

q=3

Absorption and average charge state

• energy dependent transmission

Coulomb explosion simulated in 3D

• sensitivity of channelplate εz

εz ~ (Etot)1/23x1013 W/cm2

average cross section

σ = 70 Mbarn_

0.2 0.4 0.60

200

400

8+7+

6+5+ Xe+

Xe4+Xe3+ Xe2+gemessen

Zähl

rate

(will

k. E

inh.

)Zä

hlra

te (w

illk.

Ein

h.)

0

200

400 Xe+

rückwärts

Xe2+

rückwärtssimuliert

simulation

experiment

Time of flight (µs)

backwardbackward

1 2 3 4 5 6 70

500

1000

1500

2000

2500

ejec

tion

ener

gy [e

V]

ion charge state

xenon clustersN=2000 • Quadratic dependence on charge

• Coulomb explosion

• Up to 3 keV kinetic energy

• Each atom in the cluster absorbs ~ 40 photons

Kinetic energy of the ejected ions

• What is the ionisation mechanism?

• Which process allows the absorption of up to 40 photons/peratom?

• How can we explain the high charge states?

Questions

VcklJ r( )

r

200 100 0 100 200

3000

2500

2000

1500

1000

500

500

1000

N =5000, Q=5000 R=4 nm

1600 eVare needed to remove the last of 5000 electrons

1014 Watt/cm2

ponderomotive energy 100 meV

qiver radius 0.008 nm

Ionization energy Ip of Q electrons in a cluster ~ Q2

Ip = 4.1*106 eV (Xe5000) for Q=5000

Field ionization of atoms at thecluster surface by the Coulomb-field

Multi-photon versus field ionization

Infrared light (800 nm) 1016 W/cm2

directed electron emission in the polarization plane

→ field ionization

VUV (98 nm) 1016 W/cm2

isotropic electron emission

→ photon assistet thermionicelectron emission

VUV (98 nm) 1014 W/cm2

σ = 0.5 Mbarn for Xe13 for q =1 experimentσ = 1.2 Mbarn for Xe147 70 Mbarnσ = 9 Mbarn for Xe147 for q =3less than 5 photons/atom ~ 40 photons/atom

Classical simulation of electron motion

78 electrons in Xe13

wavelength 97 nm

Electron trajectories

J. Schulz et al., Nucl Instr. Meth, in press

nmr

rr

BU

rC

rBZ

U

ele

ion

02.00

20

2

6

=

+=

+−=

thermionic electronemission

Recent theoretical work

)/exp()}exp(][{1

)( Dii rriZir

rV λα −−−+−=

Atomic potential:

Inverse bremsstrahlung with atomic potentialRobin Santra and Chris H. Green (Boulder),

PRL accepted

30 photons/atom

In good agreementwith experiment

(43 photons)

Mixed quantum-classcial modelChristian Siedschlag and Jan M. Rost

(MPI Dresden), PRL submitted

Barriers pulled down by surrounding charges

Much higher inner ionisationup to Xe7+

Electron can penetrate deeper into ion coreand sees charge up to q=5 instead of q=1

Recent theoretical work on inverse Bremsstrahlung, quantum mechanical approach

Robin Santra, Chris H. Green, (Boulder), PRL submitted

Inverse Bremsstrahlung with an atomic potential

i average charge state, Z nuclear charge

Number of absorbed photonsas a function of time

results with atomic potentialin good agreementwith experiment (43 photons)

atomic correction

---30 photons /atom

Recent theoretical work

Mixed quantum-classcial model (λ=98 nm) Femtosecond dynamics of inner ionizationChristian Siedschlag and Jan M. Rost(MPI Dresden), PRL submitted

Barriers pulled down by surrounding charges

much higher inner ionisationup to Xe7+ in Xe80

Recent theoretical work II

0 100 fs

good agreement with experiment:• high charge states• efficient absorption

Xe+

Xe+Xe+Xe+

Atomic potential versus Coulomb potential

0 1 .10 102 .10 103 .10 104 .10 105 .10 106 .10 107 .10 108 .10 109 .10 101 .10 9100

90

80

70

60

50

40

30

20

10

0

radius [m]

pote

ntia

l ene

rgy

[eV

]

V1 r( )

VCoulomb r( )

r

Electrons can penetrate into the ion core and see a larger chargee.g. up q = 5 instead of q =1

Time dependence of the charge in Xe clusters

Charge state 2 after a 20-30 fs

Strong absorption at the very beginning of the pulse

Gaussian shaped pulse, 100 fs width (FWHM)

Coulomb potentialof the charged cluster

t0 beginning of the pulse t1 maximum t2 end of the pulse

Coulomb explosion of clusters induced by absorption of many photons

Xe+

Xe3+

Xe2+

hω

hω

hω

hω

hω

hω

hω

hω

hω

hω

hω

hω

n=6

n=3

Ionisation mechanism in Xe atoms

direct MPI stepwise MPI

-2.93.94.8

21.0 232.1 3 46.7 459.7 5

32.1 365.2 6111.9 9171.6 14

Xe2+

Xe3+

Xe4+

Xe5+

nIp [eV] nIp [eV] n

experimentstepewisedirect

stepwise multiphoton ionisation

MPI mechanism

hω =12.7 eV

>3.5

H+ N3+

N2+

N2

+

N+ N2

2+

Inte

nsity

[arb

. uni

ts]

3.0-3.5

2.5-3.0

2.0-2.5

1.5-2.0

1.0-1.5

0.5-1.0

50 100 150 200 250 300

0.1-0.5

time of flight [ns]

N2

+

N+ N2

2+

N2+

H+

x=0mm

Inte

nsity

[arb

. uni

ts]

x=1mm

x=3mm

x=5mm

x=8mm

50 100 150 200 250 300

time of flight [ns]

H2O+

x=15 mm

x=2mm

x=4mm

variation of the intensity variation of the spot size

5*1013 W/cm2

1*1011 W/cm2

Photoionisation of nitrogen molecules

Ionistaion of N2 at 98 nm: two photons are needed

Summary

VUV-FEL provides light for first experimenthigh power and short pulses (<100 fs)

multiple ionisation of clusters

• Coulomb explosion (keV ions)• very efficient absorption, 40 photons/ atom at 3*1013 W/cm2

• new theoretical work:§ quantum mechanical model of plasma absorption with atomic potential§ barrier supression produces highly charges ions (Xe7+)

• thermonic electron emission (1-30 eV)

Explosion dynamics and radiation damage of clusters and biomolecules

first and third harmonic of the FEL

• Mie-scattering/diffraction imaging

• real time dynamics

absorption processes:• innershell absorption

• inverse Bremsstrahlung

metal clustersplasma frequency 10-50 eV

Pump-probe with the FEL

1997 UCLA 12 µm *

1999 Argonne 500-375 nm*

2000 BNL 600 *2000 DESY 82-100 nm *2006 Spring8 20 nm2004 DESY 6-60 nm2008 SLAC 0.15- 5 nm2011 DESY 0.1- 6 nm

* lasing

SASE FEL Projects:Towards coherent X-rays

Time structure VUV-FEL

Thom

as Tschentscher, 12/07/2004

Photon beam

frequency : 10 Hzbunch length : 200 fs (Phase II)macro bunch : 7200 bunches

beam divergence : ~ 30 µrad bandwidth : 0,4 %

100 ms t

100 ms

7200 bunches à 1 nC ∆ t = 110 ns

Time structure

Thom

as Tschentscher, 12/07/2004

Thom

as Tschentscher, 12/07/2004

single molecule crystal

With atomic resolution Lysozyme

R. Neutze, J. Haidu et al., Nature 406, 752 (2000)Radiation damageand Coulomb explosion

The TESLA XFEL

Single molecule structure determination with X-rays from a FEL