Energy Transfer between a Solid and Helium by Excited Atoms

Abstract

In some recent experiments^1,2,3 it has been shown that the reflectivity of a crystal surface, for phonons incident on it from within the crystal, is sensitive to very thin layers of helium on the crystal surface. The reflectivity drops by ∿ 25% with only ∿ 3 atomic layers of helium and does not decrease further when the He thickness is increased to bulk values. This loss of energy on reflection must be accompanied by a transfer of energy through the interface into the helium. These experiments strongly suggest that the first few layers of He are all important in this energy transfer and they are of central importance to the Kapitza conductance problem. We suggested that single atom excitations were more likely than collective excitations in the thin ⁴He film and we envisaged that acoustic energy excited He atoms out of bound states and the atoms transported the energy away as kinetic and potential energy². The other hypothesis was that energy is converted into an interface excitation which is then reconverted into low frequency bulk phonons³.