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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