Precursor and Nonequilibrium Diffusion
Many researchers postulate for the processes of the trapping-desorption type the existence of the precursor — some intermediate preadsorbed state on the surface. The study of the role of surface diffusion in adsorption and catalysis is important. The notion about precursor and its role in chemical adsorption goes back to the pioneer work of I. Langmuir, and in catalysis to the work of S. Z. Roginsky and Ya. B. Zeldovitsch. There are two main reasons for the nature of the precursor: the precursor is a physisorbed state of molecule and the precursor is in non-equilibrium, in the vibrationally excited state. A large number of studies have been devoted to the theory of the precursor. D. J. Doren and J. C. Tully carried out trajectory calculations on the basis of a complicated model for nondissociative adsorption involving multidimensional potential surfaces, surface vibrations, interaction between different molecular and solid modes, etc.