Quantum Sticking, Scattering, and Transmission ofH4eatoms from SuperfluidH

Abstract

We develop a microscopic theory of the scattering, transmission, and sticking of 4 He atoms impinging on a superfluid 4 He slab at near normal incidence, and inelastic neutron scattering from the slab. The theory includes coupling between different modes and allows for inelastic processes. We find a number of essential aspects that must be observed in a physically meaningful and reliable theory of atom transmission and scattering; all are connected with multiparticle scattering, particularly the possibility of energy loss. These processes are (a) the coupling to low-lying (surface) excitations (ripplons/third sound) which is manifested in a finite imaginary part of the self energy, and (b) the reduction of the strength of the excitation in the maxon/roton region. The dynamics of liquid4 He films and the bulk fluid near its free surface continues to be of considerable interest. Experimental information is available about the scattering of helium atoms from helium surfaces and films [1][2][3][4][5]; from the dynamics of localized excitations within the fluid, including excitation scattering from the surface and quantum evaporation [5][6][7][8] ; and from inelastic neutron scattering at grazing angles from adsorbed films [9][10][11][12]. Moreover, information about the condensate fraction in helium may be obtainable directly from elastic transmission of 4 He atoms through superfluid 4 He slabs [13]. The fact that helium slabs have been made in the laboratory [14] leads to the prospect that the dynamic probes previously applied to adsorbed films and bulk surfaces of helium can now be applied to slabs. The slabs should produce simpler and thus easier to interpret results than the adsorbed films.We report here on the results of a manifestly microscopic theoretical analysis of the dynamics of such a slab at zero temperature. We find that atom scattering processes are dominated by multi-particle events, particularly the coupling to ripplons. This is in qualitative agreement with the conclusions of Edwards and collaborators [1][2][3][4] in their results for the related helium atom scattering from the free surface of bulk helium. Our results not only provide insight into the transmission and sticking of a helium beam that was not previously available from theory or experiment, it also provides detailed predictions for the anticipated experiments on slabs.The theoretical method used here has been successfully applied to the bulk [15,16], to adsorbed films [17,18] and to droplets [19]; in the latter two it was successful in making detailed and correct predictions about surface states (ripplons and third sound) which also play a very important role in the slab geometry. The theory is adapted here to the slab geometry, including scattering states. We outline our theory and present some illuminating results waves for at non-normal incidence which also provide insights into quantum evaporation.We consider the ground state of a slab of superfluid 4 He of particle number 1.5Å−2 , corresponding to a thickness of approximately 80Å,...

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“…Among their most recent results are the measurements of the evaporation probability for R + and R − rotons. This is particularly interesting since the calculation of this probability has been a challenge for several groups of theorists [7,25,31,[39][40][41][42][43].…”

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