Measurements of the scattering of 4 He atoms grazing the surface of liquid 4 He show that the reflectivity approaches unity as the perpendicular wave vector tends to zero, in agreement with theoretical predictions.PACS numbers: 79.20. Kz, 67.40.Fd, 68.10.Jy This Letter describes an experiment on the scattering of free 4 He atoms striking the surface of the liquid. The reflection probability (or "reflectivity" R) of the atoms is measured as a function of their momentum Hk and angle of incidence & In contrast with previous experiments 1 ' 2 we have investigated the scattering when the de Broglie wavelength associated with the vertical motion of the atoms 2it/k g is large compared with the characteristic lengths associated with the surface. These lengths are the interatomic spacing, which is roughly the same as the thickness of the surface, and X the range of the van der Waals potential above the liquid, ~a/z 3 .If the mass of the atom is ra, then A is given 1 by 2ma/f. For 4 He, X is 20 A. In the short-wavelength regime, where k e X> 1, the earlier experiments showed that R is very small and that the dominant mode of scattering is absorption of the atom into the liquid.If k e X « 1 the potential at the liquid surface should appear to the atom like a sharp attractive step. As k g -+0 the reflectivity should tend to unity, in agreement with the well-known quantum mechanical result for a step potential, provided that excitation of the liquid, i.e., inelastic processes, can be neglected. The experiment is carried out at low temperatures so that thermal excitation of the surface is unimportant. The experimental result shows that R -*1 as k e -~0 and the detailed dependence of R on k e follows the predictions of a theory [by Edwards and Fatouros 3 (EF)] which neglects inelastic processes completely. The EF theory has previously been used to calculate the binding of 3 He and spin-polarized hydrogen and deuterium to the 4 He surface, as well as to fit earlier data on scattering when k g X > 1. (Reviews of earlier work can be found in Refs. 2 and 4.) The agreement between the experiment and the EF theory suggests that other current theories 5 ' 6 overestimate the effect on R of the excitation of ripplons (quantized capillary waves) due to the van* der Waals attraction between the incoming atom and the liquid. According to Echenique and Pendry 5 the attraction causes the multiple production of ripplons when the atom approaches within a few angstroms of the surface. They calculate R by a Feynman path integral restricted to a special class of trajectories. In Usagawa's theory 6 the same ideas are used but the coupling between the atom and the ripplons is assumed to be weak. This means that only single-ripplon production is considered, and the path-integral method is not needed.In the EF theory 3 the atom moves in an effective potential V eff (z) which is asymptotic to the real potential -a/z 3 at large distances above the liquid surface. The effective potential is derived by minimizing the energy of a variational wave fun...
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