Theoretical investigation of desorption problems.

The empirical theory of thermal desorption described by simple rate equations has been extended to include the case of coupled rate equations describing interconnected multi-site desorption. The rate equations were also further extended to include the hard sphere nature of adsorbate-adsorbate interactions. Starting from a fundamental statistical mechanical viewpoint it has been demonstrated how adsorbate-adsorbate interactions can be incorporated into the expression for the surface delay time. Thermal desorption data for the system of lead desorbed from. GaAs (100) has been analysed by several different methods and conclusions drawn as to their relative merits. For the lead-GaAs (100) system it has been demonstrated how the adsorbate-adsorbate interaction dominates the adsorbate-substrate interaction. In order to model the thermal desorption spectra, a variable-order desorption process was invoked which described the desorption of lead atoms from lead bubbles formed on the surface during the experiment. A probability distribution function has been derived to describe the motion of a Brownian particle adsorbed on a substrate which is well represented by a phonon model. The probability function is shown to satisfy the Fokker-Planck equation in phase space and from that calculation an analytic expression is derived for the viscosity; a physical parameter always assumed, up to present, to be arbitrary and constant.