THE ROLE OF SURFACE CORRUGATION IN DIRECT TRANSLATIONALLY ACTIVATED DISSOCIATIVE ADSORPTION

Recent experimental results concerning reactive scattering in the hype rthermal kinetic energy regime can be described by energy scaling rela tionships E(i) cos(n) theta(i), where n<2, and E(i) and theta(i) are t he incident kinetic energy and incident angle, respectively. Such powe r law scaling arguments are empirical, the results of which cannot eas ily be related to the fundamental parameters that describe the gas-sur face interaction. We present a detailed and thorough analysis where th e role of surface corrugation in determining the coupling between inci dent kinetic energy and incident angle in these translationally activa ted systems is considered explicitly. The key features of the analysis involve the assumption that the kinetic energy directed along the loc al surface normal (E(perpendicular to)) controls the reaction probabil ity (S-R), and that by averaging this quantity over the unit cell, one obtains the appropriate energy scaling relationship. The major advant age associated with the proposed analysis is that one need not assume a functional form concerning how the reaction probability depends on k inetic energy, i.e., S-R(E(perpendicular to)). Our analysis demonstrat es that in the absence of shadowing, a single ''universal'' scaling fu nction exists E(i) theta(theta(i)), which is given by the expression t heta(theta(i))=(1-Delta)cos(2) theta(i)+3 Delta sin(2) theta(i), where Delta is a corrugation parameter (0 less than or equal to Delta less than or equal to 1) and only in-plane corrugation has been considered. Shadowing plays an important role at sufficiently large corrugation a mplitudes and/or sufficiently large angles of incidence. Specifically, it leads to more complex scaling functions, which depend on the shape of the surface corrugation, for which several examples have been cons idered. Both local minima and local maxima can be observed for theta(t heta(i)) as a function of incident angle. Two factors can introduce er rors in the analysis, namely, the presence of nonlinearities, and the effects of nonuniform surface reactivity, and illustrative examples ar e considered. The model accounts well for recent experimental results concerning the dissociation of silanes on silicon surfaces, and alkane s on a corrugated platinum surface. It is probable that other systems involving reactive scattering in the hyperthermal kinetic energy regim e may also be described well employing this analysis.