Reactions of homonuclear diatomic ions with metal surfaces. I. Model for X+2 beam–surface reactions in the low kinetic energy-near threshold region

A simple quantum mechanical model has been developed to describe chemical reactions of low energy Ek (≲ 30 eV) beams of diatomic homonuclear cations X+2 with metal surfaces M to produce binary compounds MαXβ. The overall reaction is simulated by four elementary steps: (1) neutralization of the incoming X+2 by resonance and/or Auger electron transfer from M; (2) impact dissociation of X2;(3) de-excitation and thermalization of X; and (4) chemical reaction between X and M. The neutralization process is treated by a simplified quantum mechanical procedure involving resonance and Auger transition probabilities and Franck–Condon and Hönl–London factors for X+2 transitions to the ground and excited electronic states and rovibronic levels of X2. The probability of dissociation from a given rovibronic level is modeled through use of a function developed for decomposition of gas phase polyatomic molecules. For the slow ions treated herein, the model considers that de-excitation of nascent N atoms at the surface is fast and that thermalization occurs in the outer surface layers. The chemical reaction proper is governed by the reaction cross section σr between X and M. Model calculations using free-electron and modified free-electron bands for the metal and various manifolds of electronic and vibronic states for X+2 and X2 are presented in order to illustrate the sensitivity of the model to individual steps in the mechanism.