| The measurement of the elastic reflection coefficient for ('4)He atoms incident on the surface of liquid ('4)He are described. The elastic reflection coefficient, R(k,(theta)), was measured by varying the speed, v = (H/2PI)k/m, of the incident atoms and the angle of incidence, (theta). The range of the wavevector, k, was 0.2 A('-1) to 0.7 A('-1). The angle of incidence was varied from 53(DEGREES) to 87(DEGREES) with respect to the normal to the liquid ('4)He surface.;The elastic reflection coefficient was found to be a function of the perpendicular component of the momentum. The range of the perpendicular component of the wavevector, k(,z), was extended to 0.01 A('-1). The results for k(,z) between 0.05 A('-1) to 0.4 A('-1) are in agreement with the measurements of Edwards et al. (1975a). The reflection rises to 20% for k(,z) equal to 0.01 A('-1). This agrees with the variational theory of Edwards and Fatouros (1978) in which the trial wave function is unsymmetrized and the incident ('4)He atom scatters from a one-dimensional effective potential. The elastic scattering, for k(,z) small, is due to scattering from the long-range Van der Waals interaction between the incident atom and the liquid. The scattering appears to be essentially elastic and there is no evidence for any significant inelastic scattering. Our apparatus is however relatively insensitive to diffuse inelastic scattering.;A heat pulse applied to a graphite film resistor generates a beam of ('4)He atoms. The power of the atoms condensing on the surface of a graphite film resistor, used as a bolometer, was measured for the direct beam and for a beam reflected off the liquid surface. A comparison of the reflected and the direct power allows the computation of the reflection coefficient as a function of the time of flight. |
