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 ^4He and downarrow H atoms off ^4 He surfaces, as well as the scattering of Ps of metal surfaces, we review the two existing theories for surfaces 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.