Folie 1

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Sub 20-fs examination of the optically pumped
amorphous silicon with keV absorption spectroscopy
E. Seres1, 2, C. Spielmann1 1 Physikalisches
Institut EP1, Universität Würzburg, Am Hubland,
97074 Würzburg, Germany 2Institut für Photonik,
Technische Universität Wien, Gusshausstrasse
27/387, 1040 Wien, Austria
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Motivation molecular (atomic) movie
Create a movie of ultrafast phase transitions,
chemical reactions including the transition
state(s), studying coupling between electronic
and nuclear motion
Molecular (atomic) movie
Atomic structural resolution (Å) x-ray
spectroscopy
Atomic temporal resolution (fs) ultrafast x-rays
X-ray diffraction X-Ray absorption spectroscopy
(XAS) element specific electronic
structure specific geometric structure
specific for all samples
Ultrafast x-ray source ultrashort x-ray
pulses continuum or line radiation up
to several keV spatially coherent
synchronized excitation
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High harmonic generation (HHG)
Laser radiation is up-converted into x-ray
radiation in a spatially coherent beam
atoms
Femtosecond laser pulse
X-ray detector
Harmonics have a well defined temporal
structure X-rays emitted in a burst of
attosecond pulses Burst length ltlt laser pulse
duration
X-rays / Laser field
Harmonics have a well defined spatial
structure X-rays emitted in a single spatially
coherent beam Efficient coupling to
experiment Efficient focusing
beam profile 1.5mm ? 1mm 60 cm from source
double slit image
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keV harmonic generation
Why is it so difficult to generate keV
harmonics? Single atom photon energy
proportional to laser intensity But HHG is a
coherent process, signal increases only over
coherence length (lt 1µm for few 100 eVs)
For efficient generation phase-matching
Predicted nonadiabatic self-phase-matching
(NSPM) using intense few cycle laser pulses
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X-ray absorption spectroscopy
Absorption µ
Absorption µ
XANES
EXAFS
Photon energy eV
Photon energy eV

• XANES
• X-ray absorption near edge
• structure depends on
• chemical surrounding
• molecular orbits
• band structure
• density of state

EXAFS Extended X-ray absorption fine structure
depends on local atomic coordinates chemical
states
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EXAFS theory
Photon hn ionizes atom Outgoing photoelectron
wave scatters on neighboring atoms Interference
between outgoing and scattered wave Electron wave
number k E0 absorption edge energy
Photon hn
Normalized x-ray absorption coefficient c(k) at
K-edge or L1 edge with j different next neighbors
at distance rj
Photoelectron phase shift
EXAFS coefficient at L2,3 edge with j different
next neighbors
Teo and Lee, J Am. Chem. Soc.101, 2815 1979
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Static EXAFS _at_ Si L-edge
absorption Si atom µAt (E)
static distance
measured absorption µ(E)
Fourier- transform
EXAFS signal c(E) (? µ -µAt )
Atom distance Å
Photon energy eV
Calculated static atomic separation r
2.37Å Literature amorphous silicon 2.36 Å
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Static EXAFS _at_ Si K-edge
Transmitted Intensity Calculated filter
transmission
Distance can be calculated correctly HH signal
up to 3 keV
E. Seres et al APL 89, 181919, 2006
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Time-resolved x-ray absorption spectroscopy

• Following structural changes in Si after
  excitation with intense laser pulses
• Optical pump sub-20fs laser pulses _at_ 800nm, pump
  fluence well below damage threshold ( typ. few
  mJ/cm2)
• X-ray probe HH continuum generated with sub-20fs
  laser pulses
• Spectroscopy Probing x-ray absorption fine
  structure (EXAFS) above the L (100eV) and K
  (1800eV) edge

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Time-resolved EXAFS _at_ L edge
Delay (fs)
Change atom distance
EXAFS evaluation
Photon energy (eV)
2D Fourier- transform
Two dominant oscillations at high frequency ? 16
THz low frequency ? 3.7 THz First observation
of collective motion of atoms with time-resolved
EXAFS with sub-20fs resolution
Intensity (a.u.)
Frequency (THz)
E. Seres et al APL 91, 121919, 2007
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Time resolved EXAFS _at_ Si K edge
Difference spectra recorded above Si Kedge (1.8
keV)
EXAFS evaluation
2D Fourier transform
DR/R0 ()
Delay (fs)
Frequency (THz)
Oscillatory motion after excitation high
frequency ? 15.5 THz Measurement for different
pump energies
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Amplitude and frequency vs. pump energy
Dominant high frequency component (THz)
Average atom separation DRrms (a.u.)
Pump energy (µJ)

• Time resolved EXAFS (above L and K edge)
  evaluation oscillatory motion of the atom
  distance
• Frequency nearly independent from pump energy
• Amplitude increases (linearly) with pump energy
  (number of excited carriers)

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Coherent phonons
Stampfli Bennemann (PRB 49 7299 1994) hot
carriers launch coherent phonons in covalently
bound crystals
TA -phonon
LO -phonon
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Coherent phonons in Si
Experimental verification (IR probe) A. L.
Dobryakov et al. Observation of coherent phonon
states in porous silicon films JETP Lett. 71,
298 (2000) M. Hase et al. The birth of a
quasiparticle in silicon observed in
time- frequency space Nature 42, 51 (2003)
IR probe Integration of the properties in a
large volume average atom distance only optical
phonons
X-ray probe Direct measurement of the
interatomic distance all phonons
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Molecular spectroscopy
Spectroscopic methods to follow the motions of
atoms in a molecule
Optical spectroscopy
Theory
Atomic motion
X-ray spectroscopy
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Photoisomerization of azobenzene
Nitrogen
hn 2. 5eV
Ahlrichs Uni Karlsruhe
cis-azobenzene
trans-azobenzene
Bond length dNN 1.427 Å dCN 1.428 Å
Bond length dNN 1.25 Å dCN 1.45 Å
Structural change on a sub-picosecond time scale
Tiago, et al, J. Chem. Phys. 122 094311 2005
Probe EXAFS with XUV continuum near K-edge of N
( 402 eV)
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Conclusions and outlook
Laser-generated ultrafast x-rays important tool
for recording molecular real time movies
Static and time resolved x-ray absorption
spectroscopy in the keV range with sub-20fs
resolution
Collective atomic motion in Si probed with EXAFS
Studying structural dynamics of randomly oriented
samples e.g. molecules in gas phase or liquid