This research project, supported by the NSF Theoretical and Computational Chemistry Program, is a study of probe molecules in hydrogen and helium clusters. Low lying phonon states of the cluster and the interaction of these states with rovibrational states of a probe molecule will be treated by quantum Monte Carlo theory. Computed quantities include lifetime broadening, shifts and splittings in infrared and Raman spectra of probe molecules such as sulfurhexafluoride or diatomic iodine. By comparing theory with experimental measurements, the host-host and host-impurity couplings can be determined and used to study other energy dissipation processes in quantum fluids. A second major component of this project treats high energy (>1 keV) electron scattering from clusters to study ionization and elastic scattering cross sections. It has long been known that liquids (quantum liquids) composed of very light atoms or molecules, particularly helium atoms or hydrogen molecules, exhibit unusual properties such as superfluidity. The principal goal of this project is to test mathematical and theoretical methods in the theory of these phenomena by computing experimentally accessible spectral effects on small quantum clusters containing probe molecules. The most immediate application of this fundamental knowledge is to the theory of spacelab experiments involving superfluid clusters, and to the development of improved cryogenic rocket fuels.
