Dynamics of surface-aligned photochemistry (theory). I. Trajectory study of H→+BrH′(ad)

Abstract

The displacement reaction H + BrH′(ad) → HBr(g) + H′ resulting from the photoinduced interaction of adsorbed HBr molecules on a LiF(001) substrate has been modeled by a quasiclassical trajectory (QCT) calculation. This process constitutes "surface-aligned photoreaction" (PRXN) in which atomic H→ photoejected from HBr(ad) approaches an adjacent HBr(ad) with a restricted range of impact parameters b in either linear or bent (90°) configurations. The surface alignment (whether linear or bent) was found to eliminate the reaction pathway to yield H2 product, of major importance in the gas. Product HBr energy and angular distributions, as well as excitation functions Pr (ET), were obtained for each geometry at initial relative translational energies ET spanning 8-40 kcal mol-1. These distribution functions were compared to the gas. For both PRXN variants, linear or bent, reaction occurred with high probability, Pr (ET) rising sharply in each case from threshold to unity within a narrow range of ET. By contrast, Pr (ET) for the gas was broader and smaller. Product (HBr) vibrational excitation for PRXN exceeded that for gas-phase reaction; linear PRXN showed the greatest enhancement. Product rotational distributions were substantially narrower than for the gas-phase reaction, with P(J′) for bent PRXN exhibiting bimodality attributable to the occurrence of reaction with either positive or negative impact parameters. The computed HBr angular distributions from PRXN were markedly narrower than for the gas-phase reaction, and were specific to the reagent configuration (linear or bent). The existence of a reference plane in PRXN (the crystal surface) governing the plane of reaction, has the consequence that product angular distributions with respect to each polar angle Pr(θs′) and P r(φs′), embody information concerning the reaction dynamics. It is evident that PRXN by restricting reagent parameters renders product attributes more informative. © 1988 American Institute of Physics.