DISTINGUISHING THE DIRECT AND INDIRECT PRODUCTS OF A GAS-SURFACE REACTION

It has long been postulated that gas-surface chemical reactions can oc cur by means of two distinct mechanisms: direct reaction on a single g as-surface encounter or reaction between two adsorbed species. It is s hown here that these mechanisms have distinct dynamical signatures, as illustrated by the reaction of hydrogen with chlorine on gold(111). T he direct reaction product leaves the surface with a high kinetic ener gy in a narrow angular distribution that displays a ''memory'' of the direction and energy of the incident hydrogen atom. The indirect react ion product has a near-thermal energy distribution and an angular dist ribution that is close to that of a cosine function.