Sufficient Conditions for Quantum Reflection with Real Gas-Surface Interaction Potentials

Abstract

In complete contradiction to intuition based on classical mechanics, an atom striking a solid surface at extremely low velocity does not stick. Understanding of this non-sticking emerges only through quantum theory, which predicts the sticking probability S(k) of the atom to vanish linearly with decreasing incident wave vector k for sufficiently small k [1]. Accordingly this non-sticking of ultra-slow atoms has come to be known as “quantum reflection.” Considerable theoretical effort [1]-[5] has been devoted to the phenomenon over the last two decades. Measurements of the sticking probability of H-atoms scattered from a surface of liquid helium at extremely low k have been reported [6], the only accredited empirical observation of quantum reflection to date. By criteria developed below those measurements did indeed broach the quantum reflection regime. Even more satisfying would be an explicit verification of the phenomenon as might be provided by molecular beam scattering, for example by monitoring elastic helium atom scattering from a single crystal solid surface while microscopically roughening the surface through ion bombardment. This would destroy coherent elastic scattering at normal k but should have no effect in the quantum reflection regime. There may be practical uses of quantum reflection as well: If energy exchange between the surface and the atom is truly absent, ultra-cold atoms can be stored (and even evaporatively cooled) by placing them in a simple container of any temperature under UHV conditions. The sole constraint is that the atoms be slow enough to quantum reflect from the wall material.