QUANTUM-THEORY OF SURFACE-ADSORPTION

We review the progress in the study of quantum coherence effects observed in surface physics at very low temperatures. After a brief discussion of experiments on the scattering of He-4 and down H atoms off He-4 surfaces, as well as the scattering of Ps off metal surfaces, we review the two existing theories for surface adsorption, and their application to the experiments, We demonstrate that the two theories - based respectively on perturbation theory and on time-dependent Hartree theory - are in fundamental conflict, Finally, we describe a recent coherent-state theory of quantum adsorption which appears to resolve the conflict. The theory finds that as a function of the particle-surface coupling constant, there is a very rapid crossover - potentially a phase transition - from quantum to classical behavior. Similar singular behavior is known from the study of related phenomena such as quantum tunneling in the presence of dissipation.