The dynamics of the H+D₂O→OD+HD reaction at 2.5 eV: experiment and theory

Abstract

The title reaction has been studied both experimentally and computationally at a mean collision energy of 2.48 eV. OD quantum state populations, rotational alignment parameters, rovibrational quantum state-resolved center-of-mass angular scattering distributions and HD co-product internal energy release distributions have been determined, along with OD quantum state averaged energy disposals. The experiments employ pulsed laserphotolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the radical products. The OD angular scattering distributions show a preference for scattering in the forward direction, and are quite different from those observed previously at the lower collision energy of 1.4 eV. So too are the kinetic energy release distributions, which reveal that the HD co-products are born significantly more internally excited at 2.48 eV than at 1.4 eV. The HD internal energy distributions obtained from analysis of the Doppler resolved profiles are in reasonable accord with that derived from the direct HD population measurements performed by Zare and co-workers [J. Chem. Phys. 98, 4636 (1993)] at collision energies around 2.7 eV. The data are compared in detail with the results of new quasi-classical trajectory(QCT) calculations employing two alternative potential energy surfaces (PESs), as well as with the results from previous QCT studies of the title reaction by other workers. Refinements to the most recent of the PESs employed here, that developed using the iterative methods of Collins and Zhang and co-workers [J. Chem. Phys. 115, 174 (2001)], are also described. The theoretical results obtained using this refined PES agree very well with many of the experimental observables, and the surface appears to be a significant improvement on those previously developed. However, even with this new PES, the QCT calculations at 2.48 eV overestimate the internal excitation of the HD products.