MECHANISM, CONFIGURATIONAL DEGENERACY, AND MASS-ACTION IN TRANSITION-STATE RATE-FUNCTIONS FOR ADSORPTION DESORPTION REACTIONS

A discussion of classical transition-state theory for adsorption and desorption rates is presented, the purpose of which is to demonstrate through several examples how the reaction mechanism and the configurational degeneracy of the adsorbed layer conspire to determine mass actions in rate functions and the relationship between preexponential factors and molecular degrees of freedom. An approximate cell model for the degeneracy of ordered layers is introduced, and the efficacy of the model is illustrated through analyses of several representative systems: e.g., CO on Ni(110) at low coverages, where independent-molecule lattice gas behavior is identified; CO on Ni(110) at high coverages, where desorption converts the overlayer from (2x1) to c(4x2) symmetry; and H-2 on (2x1)-Si(100), where the rate of recombinative desorption displays first-order kinetics due to site pairing. Finally, the principles of these analyses are used to develop a model that describes the behavior of the hexagonal compression structure that forms for CO on Ru(001) at high coverages.