Equilibrium and Non-equilibrium Effects in Adsorption-Desorption Kinetics: Influences of Interaction Dynamics, Reaction Kinetics, and Statistical Mechanics of the Adlayer

Abstract

The various meanings of non-equilibrium in our context (irreversibility of experimental situation; deviation from equilibrium within the layer; non- equilibration in energy and/or momentum transfer during the surface collision, leading to non-equipartition among the various degrees of freedom) are clarified first. If quasi-equi1ibrium persists (no influence of irreversibility; internal equilibrium of the layer), then the expression for desorption rates can be split into an equilibrium factor and a dynamic factor; the latter can be identified with the sticking coefficient and contains the non-equipartition effects. It can then be expected that, depending on circumstances, one or the other will govern the rate behaviour. Examples of experimental results from physisorption as well as chemisorption systems are discussed which show that effects of the statistical mechanics of the adsorbate layer, the internal equilibration of surface layers, and of the microscopic dynamics of the gas-surface interaction can be clearly seen in many cases, even though they can mix and interfere as well. Thermodynamic influences become most obvious if detailed coverage dependences of desorption rate parameters are investigated which can then be used to derive the statistics of the layer. Dynamic effects can be most easily seen in state-resolved beam experiments, but are also obvious in more classical experiments, in particular of sticking coefficients. Some remarks about experimental difficulties are made in an appendix.