In this project supported under the Optical Science and Engineering initiative in the Experimental Physical Chemistry Program, Peter Rentzepis of the University of California at Irvine will use picosecond laser based x-ray and electron sources to probe the structures of transients in the liquid and solid phases. Specific problems to be addressed include the mechanism of lattice distortion in metals and semiconductors and studies of energy transfer from hot electrons to the lattice. Time resolved electron diffraction of gas phase species will be used to investigate photodissociation and isomerization processes. These studies will make it possible to identify structures of ultrafast intermediates and metastable ground state species by directly observing bond and angle changes. These techniques will have application in areas such as protein and enzyme kinetics, molecular electronic devices, and the physics of thermal transport in crystal lattices. X-ray and electron diffraction studies have provided a wealth of information concerning molecular structure. Professor Rentzepis will use ultrafast x-ray and electron pulses to probe, in real time, the structural changes caused by optical excitation. Basic phenomena such as melting, crystallization, heat transport, and the transient structures involved in primary events in chemical and biological systems will be studied by means of this new technique.