Scattering of He-3 Atoms from He-4 Surfaces

TL;DR

This paper develops a microscopic theory for impurity atom scattering from inhomogeneous quantum liquids like He-4, analyzing elastic and inelastic effects, with a focus on He-3 scattering from He-4 films and the role of inelastic processes.

Contribution

It introduces a first-principles, dynamic theory of impurity scattering in quantum liquids, emphasizing inelastic effects and impurity resonances, which were not thoroughly addressed before.

Findings

Inelastic processes dominate reflection probabilities.

Impurity resonances cause direction-dependent scattering.

Quantum reflection depends on substrate and impurity states.

Abstract

We develop a first principles, microscopic theory of impurity atom scattering from inhomogeneous quantum liquids such as adsorbed films, slabs, or clusters of He-4. The theory is built upon a quantitative, microscopic description of the ground state of both the host liquid as well as the impurity atom. Dynamic effects are treated by allowing all ground-state correlation functions to be time-dependent. Our description includes both the elastic and inelastic coupling of impurity motion to the excitations of the host liquid. As a specific example, we study the scattering of He-3 atoms from adsorbed He-4 films. We examine the dependence of ``quantum reflection'' on the substrate, and the consequences of impurity bound states, resonances, and background excitations for scattering properties. A thorough analysis of the theoretical approach and the physical circumstances point towards the essential role played by inelastic processes which determine almost exclusively the reflection probabilities. The coupling to impurity resonances within the film leads to a visible dependence of the reflection coefficient on the direction of the impinging particle.