### Session U19: Focus Session: Ultrafast Dynamics using X-rays and Electrons I

Chair: Dwayne Miller, University of Toronto
 Thursday, March 8, 2007 8:00AM - 8:36AM U19.00001: Ultrafast X-ray Science at SLAC and LCLS Invited Speaker: Philip Bucksbaum Hard x-rays (E greater than 1 keV) can probe the structure of matter on the length scale of a chemical bond. Ultrafast lasers (t less than 1 ps) can capture the quantum dynamics of single vibration in a crystal lattice or in a molecule, and they have also been used to view the transient molecular-scale transformations of chemical reactions. Until recently, only laser-induced plasma radiation was capable of capturing these ultrafast dynamics and also viewing them on the scale of a single chemical bond. The recent Sub-Picosecond Pulse Source experiment at SLAC was the first instrument based on synchrotron radiation from an undulator that could do both. During its two-year run, its 8 keV, 80 fs x-ray pulses were the brightest ultrafast x-rays in the world. This is just the beginning. The planned X-ray free electron laser at SLAC (LCLS) will generate focused x-ray fields as strong as atomic binding fields, comparable to today' highest intensity lasers. These new tools are creating some special opportunities for new science, and also some challenges. I will discuss these, and present recent progress in ultrafast x-ray sources and science. Thursday, March 8, 2007 8:36AM - 9:12AM U19.00002: Ultrafast lattice dynamics in laser-excited solids probed with femtosecond X-ray diffraction Invited Speaker: Klaus Sokolowski-Tinten Ultrafast pulsed excitation of solids provides a unique way of depositing energy into materials and to create states of strong electronic excitation and high temperature and pressure. With the initial deposition of energy a complex chain of elementary physical processes is triggered which can lead to structural changes on very rapid time-scales, and often along unusual, non-equilibrium pathways. Due to the unique combination of \textit{atomic-scale} spatial and temporal resolution, the recent progress in the development of ultrafast X-ray sources has provided new opportunities for studying such processes. This talk will discuss our recent work on ultrafast time-resolved X-ray diffraction using laser-driven as well as accelerator-based femtosecond X-ray sources. Examples include the non-thermal melting transition in semiconductors, the direct observation of large-amplitude coherent optical phonons, and studies of the energy relaxation in optically excited solids through measurements of the transient \textit{Debye-Waller} effect. Thursday, March 8, 2007 9:12AM - 9:24AM U19.00003: Non-Thermal Liquid Formation Dynamics Studied with Ultrafast Diffuse X-Ray Scattering Kelly Gaffney , Christian Blome , Simon Engemann , David Fritz , Patrick Hillyard , Jorgen Larsson , Aaron Lindenberg , Matthieu Nicoul , David Reis , Klaus Sokolowski-Tinten , Jerry Hastings The ultrafast melting dynamics of a laser excited semiconductor crystal have been studied with femtosecond x-ray scattering. We have used diffuse x-ray scattering to determine that a liquid structure appears within 2 ps of laser excitation. This structure preserves the density of the crystal, and can be well fit with a hard sphere structure factor, unlike equilibrium liquid InSb. At a delay time of 100 ps, an under-dense liquid structure forms with large amplitude scattering at intermediate momentum transfer. A concurrent rise in small angle scattering intensity suggests that voids form in this under-dense liquid. Cooling and contraction leads to the formation of a dense liquid structure on the ns time scale distinct from that of the equilibrium liquid InSb. The equilibrium liquid structure does not appear until delay times of 20 ns and longer. Thursday, March 8, 2007 9:24AM - 9:36AM U19.00004: Laser-induced phonon-phonon interaction in bismuth Martin Garcia , Eeuwe Zijlstra We demonstrate that the coupling between laser-induced coherent phonons in bismuth leads to the appearance of mixing signals in the isotropic reflectivity. As a consequence modes that cannot usually be detected by means of the isotropic reflectivity show up. We further demonstrate that this interaction is strongly dependent on the laser fluence and is for that reason only observable when sufficiently intense laser pulses are used. In addition, we demonstrate that the coupling between phonons of the same symmetry leads to the appearance of higher harmonics in the isotropic reflectivity. Thursday, March 8, 2007 9:36AM - 9:48AM U19.00005: Magnetic field effects on ultrafast lattice compression dynamics of Si(111) crystal when excited by linearly-polarized femtosecond laser pulses Koji Hatanaka , Hideho Odaka , Kimitoshi Ono , Hiroshi Fukumura Time-resolved X-ray diffraction measurements of Si (111) single crystal are performed when excited by linearly-polarized femtosecond laser pulses (780 nm, 260 fs, negatively-chirped, 1 kHz) under a magnetic field (0.47 T). Laser fluence on the sample surface is 40 mJ/cm$^{2}$, which is enough lower than the ablation threshold at 200 mJ/cm$^{2}$. Probing X-ray pulses of iron characteristic X-ray lines at 0.193604 and 0.193998 nm are generated by focusing femtosecond laser pulses onto audio-cassette tapes in air. Linearly-polarized femtosecond laser pulse irradiation onto Si(111) crystal surface induces transient lattice compression in the picosecond time range, which is confirmed by transient angle shift of X-ray diffraction to higher angles. Little difference of compression dynamics is observed when the laser polarization is changed from p to s-pol. without a magnetic field. On the other hand, under a magnetic field, the lattice compression dynamics changes when the laser is p-polarized which is vertical to the magnetic field vector. These results may be assigned to photo-carrier formation and energy-band distortion. Thursday, March 8, 2007 9:48AM - 10:00AM U19.00006: X-ray studies of acoustic vibrations from semiconductor superlattices. Mariano Trigo , Yu-Miin Sheu , David Reis , Roberto Merlin , Matthew Reason , Rachel Goldman We present ultrafast X-ray studies of acoustic phonons transmitted from a GaAs/AlAs superlattice. An ultrafast laser pulse impulsively excites coherent acoustic waves in the superlattice which subsequently transmit into the GaAs substrate. A short x-ray pulse can be used to probe the wave packet traveling in the bulk material, without the need of a transducer such as a second SL by detecting sidebands of Bragg diffraction. Unlike optical probes, the short wavelength of the x-rays allows momentum resolved detection over a wide range of wavevectors. This method should in principle be able to detect the whole spectrum of the generated excitations. Furthermore, the coherent part of the excitation is followed by a much slower thermal diffusion which, as we will show, can also be studied by time resolved x-ray scattering. [1] R. Merlin, Solid State Comm. 102, 207 (1997). [2] D. A. Reis et al., Phys. Rev. Lett. 86, 3072 (2001) Thursday, March 8, 2007 10:00AM - 10:12AM U19.00007: Ultrafast X-Ray Diffraction Study of Potential Energy Surface Evolution in InSb Under Intense Laser Excitation Patrick Hillyard , Kelly Gaffney , Aaron Lindenberg , Simon Engemann , David Reis , Aniruddha Deb , Drew Meyer , Jerome Hastings Ultrafast time-resolved x-ray diffraction has been used to directly monitor atomic disordering in InSb as a function of carrier density. The carrier dependent curvature of the potential energy surface has been determined from the time evolution of the atomic structure. Three regimes have been identified. At low carrier densities, atomic disordering occurs via a thermal mechanism with an exponential time constant determined by the electron-phonon coupling constant. Upon increasing excited carriers to roughly 5{\%} of the valence band electron population, a sharp transition is observed and the predominant disordering mechanism is inertial motion on a softened potential energy surface with a Gaussian time constant of $\sim$400 fs. For a carrier density above $\sim$20{\%}, accelerated atomic motion on an inverted potential energy surface is observed. This inverted regime was previously predicted by theory but had been unobserved until now. Thursday, March 8, 2007 10:12AM - 10:24AM U19.00008: Transient Lattice Deformation in Laser-Irradiated Semiconductor Studied by Picosecond Time-Resolved X-ray Diffraction Kazutaka Nakamura , Hiroaki KIshimura , Yoichiro Hironaka , Ken-ichi Kondo , Toshiyuki Atoh The transient lattice behavior of Si(111) single crystal under 300-ps laser irradiation has been studied by using picosecond time-resolved X-ray diffraction. When the laser is irradiated, the rocking curves of the laser-irradiated Si(111) have a higher-angle-shifted component due to lattice compression by laser ablation. The maximum lattice strain is estimated at 5.6 \%, which is larger than the Hugoniot elastic limit for Si (111). After 1000 ps, a broadening of the main peak is observed. In addition, the rocking curve of the recovered sample is clearly broader than that of a pristine sample. Reciprocal space mapping for the recovered sample shows that the lattice spacing of the recovered sample does not change from that of the pristine sample, whereas lattice planes are misoriented. The results of the time-resolved measurement and the assessment of the recovered sample indicate that mosaic blocks with inclined orientations are induced by laer-driven elastic compression and the subsequent pressure release within 1000 ps, rather than plastic deformation. Thursday, March 8, 2007 10:24AM - 10:36AM U19.00009: ABSTRACT WITHDRAWN Thursday, March 8, 2007 10:36AM - 10:48AM U19.00010: Ultrafast Photoinduced Solid State Phase Transitions Probed by Femtosecond Electron Diffraction R. Ernstorfer , M. Harb , C.T. Hebeisen , T. Dartigalongue , R.E. Jordan , G. Sciaini , R.J.D. Miller Femtosecond Electron Diffraction harbors great potential for providing atomic resolution of structural changes as they occur, essentially watching atoms move in real time. It combines temporal resolution on the hundreds of femtoseconds scale -- a time scale typically only accessible by time-resolved optical spectroscopy -- with real-space structural information on the atomic scale. We applied this technique to study the structural response of thin free-standing metal and semiconductor [1] films upon ultrafast electronic photo-excitation within a wide range of excitation levels. These studies distinguish the different mechanisms, thermal vs. non-thermal, of energy transfer from electronic to vibrational degrees of freedom resulting in different melting mechanisms for both classes of materials. In addition, we discuss a technique we recently established to determine the duration of the electron pulses by using the ponderomotive force of an intense femtosecond laser pulse to sequentially scatter parts of the electron pulse and found the electron pulse duration to be about 400 fs [2]. [1] M. Harb \textit{et.al.}, \textit{J. Phys. Chem. B}, in print. [2] C.T. Hebeisen \textit{et al.}, \textit{Opt. Lett} \textbf{31}$,$ 3517 (2006). Thursday, March 8, 2007 10:48AM - 11:00AM U19.00011: Ultrafast Time-resolved Electron Diffraction with Megavolt Electron Beams Jerome Hastings , Fedor Rudakov , David Dowell , John Schmerge , Stephen Gierman , Peter Weber An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. Details of the measurements and applications will be discussed