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The objective of this program was to develop an instrument for characterization of nanosecond and picosecond time evolution of surface properties. This instrument was developed based on the technique of picosecond time-resolved reflection high-energy electron diffraction (RHEED). The basic idea of this technique is the utilization of 150 ps laser pulses to generate electron pulses by the photoelectric effect. The photogenerated electrons are accelerated, focused, and collimated using electron optics. Only a very small fraction of the laser pulse energy is needed to generate the fast (10-30 kV) well-collimated electron pulses; thus, most of the laser energy is available for sample irradiation. (eg)

The objective of this program was to develop an instrument for characterization of nanosecond and picosecond time evolution of surface properties. This instrument was developed based on the technique of picosecond time-resolved reflection high-energy electron diffraction (RHEED). The basic idea of this technique is the utilization of 150 ps laser pulses to generate electron pulses by the photoelectric effect. The photogenerated electrons are accelerated, focused, and collimated using electron optics. Only a very small fraction of the laser pulse energy is needed to generate the fast (10-30 kV) well-collimated electron pulses; thus, most of the laser energy is available for sample irradiation. (eg)

This report describes the progress that has been made over the last years toward the generation of ultrashort electron pulses, and their application to time resolved electron diffraction. We have generated electron pulses of picosecond duration using an ultrafast laser system, tested a number of photocathode materials, built an apparatus to measure electron pulse durations in the femtosecond domain, and designed and built an electron diffractometer for time resolved electron diffraction. We demonstrated that the diffractometer, which uses a novel one dimensional detection scheme, is well capable of determining atomic distances in molecules to better than on hundredth of an Angstrom. A sophisticated noise suppression system maintains a signal to noise ratio sufficient for pump probe experiments. Further developments include the design of a reflectron electron gun for femtosecond electron pulses, and a two dimensional electron diffraction detector. Finally, significant progress has been made in the interfacing of adaptive optics to a learning algorithm, and the generation of spectrally tunable ultrashort laser pulses at 200 nm.

It is the goal of this research to construct an instrument for Time-Resolved Electron Diffraction (TRED) studies of the structures of short-lived, energetic molecular states and of the dynamics of primary dissociation processes found in molecules of energetic materials. The TRED experiments will be performed by focusing a pulsed ArF excimer laser onto the cold photocathode of a high voltage electron gun, where electron pulses will be generated by field-assisted photoelectric emission. These electron pulses will be scattered off the gas-phase molecules in an effusive molecular beam after some fraction of the beam population has been excited by a pulse from a XeCl excimer-pumped dye laser. Electron diffraction intensities will be recorded using the real-time gas electron diffraction (GED) technology developed at the University of Arkansas. By synchronizing the dye laser excitation pulse and the diagnostic electron pulse, intensity data can be obtained from molecules which all have the same age relative to the time of excitation. Thus, time-resolved observations of excited molecular states and of transient reaction products should be possible.

The diffraction profiles and density correlation functions are calculated for transient atomic configurations generated in molecular dynamics simulations of a 20 nm Au film irradiated with 200 fs laser pulses of different intensity. The results of the calculations provide an opportunity to directly relate the detailed information on the atomic-level structural rearrangements available from the simulations to the diffraction spectra measured in time-resolved x-ray and electron diffraction experiments. Three processes are found to be responsible for the evolution of the diffraction profiles. During the first several picoseconds after the laser excitation, the decrease of the intensity of the diffraction peaks is largely due to the increasing amplitude of thermal atomic vibrations and can be well described by the Debye-Waller factor. The effect of thermoelastic deformation of the film prior to melting is reflected in shifts and splittings of the diffraction peaks, providing an opportunity for experimental probing of the ultrafast deformations. Finally, the onset of the melting process results in complete disappearance of the crystalline diffraction peaks. The homogeneous nucleation of a large number of liquid regions throughout the film is found to be more effective in reducing long-range correlations in atomic positions and diminishing the diffraction peaks as compared to the heterogeneous melting by melting front propagation. For the same fraction of atoms retaining the local crystalline environment, the diffraction peaks are more pronounced in heterogeneous melting. A detailed analysis of the real space correlations in atomic positions is also performed and the atomic-level picture behind the experimentally observed fast disappearance of the correlation peak corresponding to the second nearest neighbors in the fcc lattice during the laser heating and melting processes is revealed.

It is the goal of this research to perform gas electron diffraction (GED) studies, time-resolved and not time resolved, of the photochemical and thermal decomposition of RDX and trinitro-azetidine. (Proposal 7MAR88 to 15AUG90; no-cost extension granted to 15AUG91. The two compounds are highly explosive. Therefore, the GED experiments are not routine and feasibility of the project had to be demonstrated first by recording the ground state data of the two systems. Two specific problems were identified and discussed in the original proposal: Can effective vapor densities of RDX or TNAZ be achieved which are sufficient for GED studies? After flash heating or laser excitation, can important intermediates of decomposition be identified in the product mixtures?

We investigate the order parameter dynamics of the stripe-ordered nickelate, La$_{1.75}$Sr$_{0.25}$NiO$_4$, using time-resolved resonant X-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer re-orientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the non-equilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom.

Coherent phonons (CP) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, non-thermal switching in phase-change materials such as Ge$_{2}$Sb$_{2}$Te$_{5}$(GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100 ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal, however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone implying the presence of transient non-equilibrium electron heating in addition to temperature driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase-transitions in GST.

A method of computer simulation of Time-Resolved X-ray Diffraction (TRXD) in asymmetric Laue (transmission) geometry with an arbitrary propagating strain perpendicular to the crystal surface is presented. We present two case studies for possible strain generation by short-pulse laser irradiation: (i) a thermoelastic-like analytic model; (ii) a numerical model including effects of electron-hole diffusion, Auger recombination, deformation potential and thermal diffusion. A comparison with recent experimental results is also presented.

Using a quantum electrodynamic framework, we calculate the off-resonant scattering of a broad-band X-ray pulse from a sample initially prepared in an arbitrary superposition of electronic states. The signal consists of single-particle (incoherent) and two-particle (coherent) contributions that carry different particle form factors that involve different material transitions. Single-molecule experiments involving incoherent scattering are more influenced by inelastic processes compared to bulk measurements. The conditions under which the technique directly measures charge densities (and can be considered as diffraction) as opposed to correlation functions of the charge-density are specified. The results are illustrated with time- and wavevector-resolved signals from a single amino acid molecule (cysteine) following an impulsive excitation by a stimulated X-ray Raman process resonant with the sulfur K-edge. Our theory and simulations can guide future experimental studies on the structures of nano-particles and proteins.