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arXiv:physics/0010069

Transition from reflection to sticking in ultracold atom-surface scattering

Areez Mody, John M. Doyle, Eric J. Heller

Published Oct 27, 2000• 0 citations
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Abstract

In paper I (physics/0010068) entitled "The No-sticking Effect in Ultra-cold Collisions", we showed that under very general circumstances, atoms approaching a surface will not stick as its incoming energy approaches zero. This is true of either warm or cold surfaces. Here we explore the transition region from non sticking to sticking as energy is increased. The key to understanding the transition region is the WKB approximation and the nature of its breakdown. Simple rules for understanding the rollover to higher energy, post-threshold behavior, including analytical formulae for some asymptotic forms of the attractive potential are presented. We discuss a practical example of atom-surface pair in various substrate geometries. We also discuss the case of low energy scattering from clusters.

DOI
10.48550/arxiv.physics/0010069

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In paper I (physics/0010068) entitled "The No-sticking Effect in Ultra-cold Collisions", we showed that under very general circumstances, atoms approaching a surface will not stick as its incoming energy approaches zero. This is true of either warm or cold surfaces. Here we explore the transition region from non sticking to sticking as energy is increased. The key to understanding the transition region is the WKB approximation and the nature of its breakdown. Simple rules for understanding the rollover to higher energy, post-threshold behavior, including analytical formulae for some asymptotic forms of the attractive potential are presented. We discuss a practical example of atom-surface pair in various substrate geometries. We also discuss the case of low energy scattering from clusters.
We provide a general and nonperturbative theoretical basis for quantum reflection 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 emerges that the physical reason for not sticking is that the many internal degrees 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 behavior, when sticking prevails at higher incident energies. Studying the WKB wave functions of the atom provides a quick understanding of our results even in the ultracold regime where WKB is not applicable. Explicit examples are examined in detail and we predict the temperatures required to reach the various regimes.
The role of shape resonances and many-body effects on universal quantum sticking of ultracold atoms onto solid surfaces is examined analytically and computationally using an exactly solvable representation of the Dyson equation. We derive the self-energy renormalization of the transition amplitude between an ultracold scattering atom and the bound states on the surface in order to elucidate the role of virtual phonon exchanges in the limiting behavior of the sticking probability. We demonstrate that, to first order in the interactions for finite ranged atom–surface potentials, virtual phonons can only rescale the strength of the atom–surface coupling and do not rescale the range of the coupling. Thus, universal sticking behavior at ultralow energies is to be expected for all finite ranged potentials. We demonstrate that the onset of the universal sticking behavior depends greatly on the position of the shape resonance of the renormalized potential and for sufficiently low energy shape resonances, deviations from the universal s(E)∝√E can occur near these energies. We believe that this accounts for many of the low energy sticking trends observed in the scattering of submillikelvin H atoms from superfluid 4He films.
We provide the theoretical basis for understanding the phenomenon in which an ultra cold atom incident on a possibly warm target will not stick, even in the large nnn limit where nnn is the number of internal degrees of freedom of the target. Our treatment is non-perturbative in which the full many-body problem is viewed as a scattering event purely within the context of scattering theory. The question of sticking is then simply and naturally identified with the formation of a long lived resonance. One crucial physical insight that emerges is that the many internal degrees of freedom serve to decohere the incident one body wavefunction, thus upsetting the delicate interference process necessary to form a resonance in the first place. This is the physical reason for not sticking.
The zero energy scattering limit of inelastic molecule–surface scattering is studied within the context of a multiphonon expansion of the molecule–bath wave function. By assuming that at low scattering energies the expansion may be truncated at first order in the phonon operators, we derived a closed form solution to the Lippmann Schwinger equation for the scattering wave function which includes a nonlocal and energy dependent self-energy term which correctly incorporates virtual phonon transitions in the elastic channel. The closure relation results from the use of a discrete spectral (ℒ2) form of the inelastic channel Greens functions. We compute the zero energy limit of these wave functions and discuss the trapping and reflection of cold atoms from ultracold surfaces. Our results indicate that for realistic atom surface interactions the low energy limit of the sticking coefficient, s, can deviate markedly from the expected s∝E1/2 behavior and is shown to approach a constant nonzero limiting value. This trend is consistent with recent experimental work involving the sticking of spin polarized hydrogen atoms on liquid He films.
In low energy atom-surface scattering, it is possible for the atom to be reflected in a region of attractive potential with no classical turning point. This phenomenon has come to be known as quantum reflection and it can reduce the sticking probability of atoms to surfaces, as well be used for atom trapping. We simulate the quantum reflection process in a one-dimensional model with a slow-moving atom moving in a Morse potential in the presence of an applied laser field. We show that in the case of laser-assisted quantum reflection, the laser field imparts additional momentum and kinetic energy to the atom. This results in a decreased distance of closest approach between the atom and surface. Our results show that the distance of closest approach and can be controlled through the timing and intensity of the laser pulse, which may result in enhanced sticking probability and/or reduced quantum reflection probability.
We study scattering of ultracold atoms by absorbing surfaces. Loss of flux through inelastic reactions and adsorption is described in an unambiguous and model-independent way by incoming boundary conditions in the semiclassical region near the surface. The near-threshold behaviour of the scattering amplitude is determined by a few parameters of the potential tail beyond the semiclassical region. Investigation of quantum reflection and scattering by flat and spherical surfaces shows that the curvature of the surface strongly influences the range to which the scattering amplitudes are sensitive in the atom-surface interaction.