Vj. Barclay et al., DYNAMICS OF SURFACE-ALIGNED PHOTOCHEMISTRY (THEORY) .4. H-ATOM REACTIONS IN THE HBR(AD) LIF(001)+H-NU SYSTEM/, Journal of physical chemistry, 97(48), 1993, pp. 12541-12552
This paper reports a classical trajectory study of surface-aligned pho
tochemistry in HBr(ad)/(LiF(001) using the detailed interaction potent
ial developed in an earlier study (Polanyi et al. J. Chem. Phys. 1991,
94, 978. Exchange reaction, H + H'Br --> HBr + H', and the minor path
way of abstraction reaction, H + H'Br --> HH' + Br, were incorporated
in the trajectories by means of a London-Eyring-Polanyi-Sato function.
The energy of the reactant H was, in general, E(xs) = 2.6 eV. Marked
changes in angular and energy distributions of the scattered H and H'
were found with increasing coverage in the range 0.2-1.0 ML (monolayer
). This was related to altered dynamics, especially to the increasing
importance of collisions with coadsorbate at higher coverage resulting
in energy loss and in scattering nearer to the surface normal (termed
''channeling''). It was predicted that at low coverages the products
of exchange reaction induced by polarized light would show an angular
anisotropy that mirrored the collision geometries: bent or collinear.
Reaction probability was found to increase with coverage, saturating a
t approximately 0.5 ML. This saturation was found to be due to a marke
d 2D ''surface-aligned caging'' effect (SAC). Reducing the surface tem
perature increased the reaction probability at low coverage due to inc
reased alignment. By contrast, reducing the surface temperature at hig
her coverage (greater-than-or-equal-to 0.5 ML) decreased the reaction
probability due to more pronounced SAC. Exchange reaction exhibited a
higher threshold energy in the adsorbed state as compared with gas, du
e to the less than optimal surface-aligned collision geometry in the p
resent system.