Alignment, vibrational, and steric effects in unimolecular and bimolecular systems: Towards an understanding of chemical reactions in quantum-state detail.

摘要

This thesis describes a series of experimental and theoretical projects designed to explore the dynamics of molecules and molecular collision systems in quantum-state resolved detail. First, the unimolecular dynamics of HOD are explored via photoacoustic spectroscopy of the 3νOH and 4νOH overtone bands. Analysis of the overtone series provides the vibrational dependence of rotational constants as well as evidence that the transition dipole moment vector tilts away from the OH bond with increasing excitation, inconsistent with simple local-mode, bond-dipole model predictions but in excellent agreement with full 3D quantum variational calculations. Second, the generation of radical clusters from the photolysis of Ar-H 2S and Ar2-H2S is investigated via quasiclassical trajectory calculations, providing a comparison with earlier experiments in supersonic jet expansions. The calculations confirm the overall efficiency of radical cluster formation as well as significant intracluster excitation; clusters with excess rotational energy above the dissociation limit are found to be trapped by an angular momentum barrier that prevents dissociation. Third, quasiclassical trajectory calculations on experimentally determined intermolecular potentials for He-O2, He-CO, and He-CO2 simulate the collisional formation of rotationally aligned molecular distributions in a supersonic expansion. These calculations verify a preference for j perpendicular to the expansion axis, with asymptotic alignment increasing monotonically with j. This alignment reflects comparable contributions from elastic and inelastic collisions; in addition, molecules with j aligned parallel to the expansion axis have faster average velocities than molecules with j perpendicular. Fourth, the role of intramolecular vibrational redistribution on the timescale of a reactive encounter is explored with a reduced-dimensionality model of Cl + H2O → HCl + OH, with isotopic variations. The vibrational eigenstates of H2O/HOD/D2O are calculated versus Cl-water separation, generating adiabatic potential energy curves and nonadiabatic coupling terms for time-dependent dynamics calculations. For the vibrational eigenstates, the near-resonance of the H2O symmetric and asymmetric stretches rotates the symmetric/asymmetric stretch towards/away from the Cl atom as it approaches in the vibrationally adiabatic limit. For Cl+HOD, vibrational excitation in the OH mode enhances HCl + OD reaction probability by more than an order of magnitude more than excitation in OD.