A quasiclassical trajectory study of final state distributions in collisions of fast H(D) atoms with HF(DF)

Abstract

This paper presents a detailed theoretical study of the reactive and nonreactive final vibrational state distributions obtained in collisions of translationally hot H atoms with HF (and isotopic counterparts D+HF, H+DF, and D+DF). The potential surface used is surface No. 5 of Brown, Steckler, Schwenke, Truhlar, and Garrett, and it is characterized by a high barrier (1.9 eV) to F atom transfer. Cross sections and other dynamical information were generated using the quasiclassical trajectory (QCT) method, and we also did classical infinite-order-sudden (CIOS) calculations to characterize vibrational excitation mechanisms. Perhaps our most important results refer to the nonreactive final state distributions, where we find that collision of H with the F atom end of HF gives a broad vibrational distribution spread over many states while collision with the H atom end of HF gives a narrow distribution in which v′=1 is the only significant excited product. For D+HF, only the first collision mechanism is important, while for H+HF, H+DF, and D+DF, the second mechanism makes the dominant contribution to v′=1, and the first mechanism is the major contributor to v′>1. This leads to nonreactive vibrational distributions for H+HF, H+DF, and D+HF in which v′=1 is much larger relative to v′>1 than in D+HF. Comparison of these results with experiment for H+HF and D+HF indicates excellent agreement. Reactive distributions are also studied, and we find that the variation of these distributions with isotope can be explained in terms of a Franck–Condon overlap model. Comparison of the reactive final state distribution for D+HF with experiment indicates excellent agreement. Rotational excitation is examined for both reactive and nonreactive collisions, and we find that while the nonreactive rotational excitation is sensitive to which end of the molecule is struck, the reactive rotational distribution is controlled by kinematic propensities.