skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Ultrafast XAFS Measurements on Laser Excited Ge Films

Abstract

Laser-pump/x-ray-probe measurements were made on Ge films with nominal time-delay steps of 18 ps. The response of the Ge lattice to the excitation of a high efficiency 200 fs pulse laser operating at 800 nm wavelength and at the Advanced Photon Source (APS) ring frequency of 272 kHz is probed by x-ray absorption fine structure (XAFS) measurements which determine the time dependences of the local structure. Initially the lattice response is dominated by a large increase of the RMS nearest neighbor bond disorder that decays within a single delay step. The next nearest neighbor RMS disorder shows an increase delayed by {approx} 30 ps, consistent with only optical modes dominating the initial lattice response and other phonons being excited more slowly. The different rate of excitation of optical than the rest of phonons require a different mechanism for exciting the optical phonons. Because the laser excites electrons from binding to anti-binding states, a Franck-Condon like effect is suggested as the cause of this initial response, instead of the standard hot electron-hole coupling to phonons.

Authors:
;  [1];  [2]
  1. (UWASH)
  2. (
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
USDOE
OSTI Identifier:
1009010
Resource Type:
Conference
Resource Relation:
Conference: 13th International Conference on X-ray Absorption Fine Structure-XAFS13;9-14 July 2006;Stanford, California, USA
Country of Publication:
United States
Language:
ENGLISH
Subject:
43 PARTICLE ACCELERATORS; ABSORPTION; ADVANCED PHOTON SOURCE; EFFICIENCY; ELECTRON-HOLE COUPLING; ELECTRONS; EXCITATION; FINE STRUCTURE; LASERS; OPTICAL MODES; PHONONS; TIME DELAY; TIME DEPENDENCE; WAVELENGTHS

Citation Formats

Stern, E.A., Brewe, D., and APS). Ultrafast XAFS Measurements on Laser Excited Ge Films. United States: N. p., 2007. Web.
Stern, E.A., Brewe, D., & APS). Ultrafast XAFS Measurements on Laser Excited Ge Films. United States.
Stern, E.A., Brewe, D., and APS). Thu . "Ultrafast XAFS Measurements on Laser Excited Ge Films". United States. doi:.
@article{osti_1009010,
title = {Ultrafast XAFS Measurements on Laser Excited Ge Films},
author = {Stern, E.A. and Brewe, D. and APS)},
abstractNote = {Laser-pump/x-ray-probe measurements were made on Ge films with nominal time-delay steps of 18 ps. The response of the Ge lattice to the excitation of a high efficiency 200 fs pulse laser operating at 800 nm wavelength and at the Advanced Photon Source (APS) ring frequency of 272 kHz is probed by x-ray absorption fine structure (XAFS) measurements which determine the time dependences of the local structure. Initially the lattice response is dominated by a large increase of the RMS nearest neighbor bond disorder that decays within a single delay step. The next nearest neighbor RMS disorder shows an increase delayed by {approx} 30 ps, consistent with only optical modes dominating the initial lattice response and other phonons being excited more slowly. The different rate of excitation of optical than the rest of phonons require a different mechanism for exciting the optical phonons. Because the laser excites electrons from binding to anti-binding states, a Franck-Condon like effect is suggested as the cause of this initial response, instead of the standard hot electron-hole coupling to phonons.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Jan 18 00:00:00 EST 2007},
month = {Thu Jan 18 00:00:00 EST 2007}
}

Conference:
Other availability
Please see Document Availability for additional information on obtaining the full-text document. Library patrons may search WorldCat to identify libraries that hold this conference proceeding.

Save / Share:
  • Laser-pump/x-ray-probe measurements were made on Ge films with nominal time-delay steps of 18 ps. The response of the Ge lattice to the excitation of a high efficiency 200 fs pulse laser operating at 800 nm wavelength and at the Advanced Photon Source (APS) ring frequency of 272 kHz is probed by x-ray absorption fine structure (XAFS) measurements which determine the time dependences of the local structure. Initially the lattice response is dominated by a large increase of the RMS nearest neighbor bond disorder that decays within a single delay step. The next nearest neighbor RMS disorder shows an increase delayedmore » by {approx} 30 ps, consistent with only optical modes dominating the initial lattice response and other phonons being excited more slowly. The different rate of excitation of optical than the rest of phonons require a different mechanism for exciting the optical phonons. Because the laser excites electrons from binding to anti-binding states, a Franck-Condon like effect is suggested as the cause of this initial response, instead of the standard hot electron-hole coupling to phonons.« less
  • A facility at PNCCAT in the Advanced Photon Source measures with subnanosecond time resolution both XAFS and diffraction on femtosecond laser excited samples. XAFS measures with relatively high efficiency the time for the laser excitation to couple to the lattice, the sample temperature after reaching thermal equilibrium, any ablation of the sample with time, and, in many cases can distinguish between the amorphous and crystalline states. Preliminary measurements on 200nm thick polycrystal Ge films indicate that the time for transferring the laser excitation to thermal heating of the lattice is less than 2nanoseconds when the initial temperature is 560K.
  • The timing structure and the high photon flux of x-ray pulses from the Advanced Photon Source permit pump-probe techniques widely used in ultrafast laser spectroscopy to be extended into the x-ray regime. The intrinsic time resolution of the experiments is determined by the FWHM of the single pulses from the synchrotron at 70-100 ps. The challenges and the solutions in such experiments will be discussed. Using laser pulse pump, x-ray pulse probe XAFS, several excited state molecular structures in solutions were studied. We will mainly describe molecular structures of the photoexcited metal-to-ligand-charge-transfer state of [CuI(dmp){sub 2}]{sup +}, where dmp ismore » 2,9-dimethyl-1,10-phenanthroline, in toluene and acetonitrile. The experimental results indicated that the copper ion in the thermally equilibrated MLCT state in both solvents had the same oxidation state as the corresponding Cu(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distances 0.04 {angstrom} longer in toluene and 0.04 {angstrom} shorter in acetonitrile than that of the ground state [CuI(dmp){sub 2}]{sup +}. The results further revealed that what distinguishes the MLCT state structures in non-coordinating and coordinating solvents is not the 'exciplex' formation, but the strength of the interactions between the solvent and the Cu(II)* species at the MLCT state. In addition, future direction of time-resolved XAFS will be discussed.« less
  • The generation of coherent optical phonons in a polycrystalline antimony film sample has been investigated using femtosecond electron diffraction method. Phonon vibrations have been induced in the Sb sample by the main harmonic of a femtosecond Ti:Sa laser (╬╗ = 800 nm) and probed by a pulsed ultrashort photoelectron beam synchronized with the pump laser. The diffraction patterns recorded at different times relative to the pump laser pulse display oscillations of electron diffraction intensity corresponding to the frequencies of vibrations of optical phonons: totally symmetric (A{sub 1g}) and twofold degenerate (E{sub g}) phonon modes. The frequencies that correspond to combinationsmore » of these phonon modes in the Sb sample have also been experimentally observed.« less