Low-Temperature Chemisorption. I. Flash Desorption of Nitrogen

Abstract

Flash desorption(desorption by continuous temperature displacement) is applied to a study of the low‐temperature interactions of N2 with an initially clean tungstensurface. Molecular nitrogen is found to dissociate into atoms on adsorption even at 115°K, at a rate which diminishes with increasing temperature, but is initially independent of surface concentration. Formation of this atomically bound β nitrogen is hindered at low temperatures (T∼115°K) by competitive growth of an additional state γ, in which nitrogen, at ∼2½ times the concentration in the β state, is bound as molecules with an energy of 9 kcal mole—1, resulting in a total surface concentration of 600×1012 molecules cm—2. The population in this γ state depends sensitively upon the arrangement of atoms in the β state. Preadsorption at T∼300°K equalizes the populations in β and γ; annealing at T∼1000°K at impingement rates of 7×1015 molecules cm—2 min—1, further lowers n γ/n β and brings about rearrangement of the tungstensurface as well, without appreciable change in the adatom concentration. A third state α is formed at temperatures up to 400°K, with a binding energy of ∼20 kcal mole—1. Initially its rate of formation, just as that of γ at low temperatures, is dictated by the concentration of the atomically held β state. At 300°K and above the α concentration passes through a maximum, then diminishes; at low temperatures, it remains constant, achieving a maximum value 1/30 that of the γ state.