No-sticking effect and quantum reflection in ultracold collisions - art. no. 085418

We provide a general and nonperturbative theoretical basis for quantum refl ection of an ultracold atom incident on a cold or warm surface. Sticking is identified with the formation of a long-lived resonance, from which it eme rges that the physical reason for not sticking is that the many internal de grees of freedom of the target serve to decohere the incident one body wave function, thereby upsetting the delicate interference process necessary to form a resonance. We then explore the transition to the post-threshold beh avior, when sticking prevails at higher incident energies. Studying the WKB wave functions of the atom provides a quick understanding of our results e ven in the ultracold regime where WKB is not applicable. Explicit examples are examined in detail and we predict the temperatures required to reach th e various regimes.