SCATTERING OF HE-3 ATOMS FROM HE-4 SURFACES

We develop a first-principles, microscopic theory of impurity atom sca ttering from inhomogeneous quantum liquids such as adsorbed films, sla bs, or clusters of He-4. The theory is built upon a quantitative, micr oscopic description of the ground state of both the host liquid as wel l as the impurity atom. Dynamic effects are treated by allowing all gr ound-state correlation functions to be time dependent. Our description includes both the elastic and inelastic coupling of impurity motion t o 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 consequ ences of impurity bound states, resonances, and background excitations for scattering properties. A thorough analysis of the theoretical app roach and the physical circumstances point towards the essential role played by inelastic processes which determine almost exclusively the r eflection probabilities. The coupling to impurity resonances within th e film leads to a visible dependence of the reflection coefficient on the direction of the impinging particle.