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arXiv:cond-mat/9612042 · Journal of Physics Condensed Matter

Quantum evaporation from superfluid helium at normal incidence

F. Dalfovo, M. Guilleumas, A. Lastri, Лев П. Питаевский, S. Stringari

Published Jun 16, 1997• 6 citations
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Abstract

We study the scattering of atoms, rotons and phonons at the free surface of at normal incidence and calculate the evaporation, condensation and reflection probabilities. Assuming elastic one-to-one processes and using general properties of the scattering matrix, such as unitarity and time reversal, we argue that all nonzero probabilities can be written in terms of a single energy-dependent parameter. Quantitative predictions are obtained using linearized time-dependent density functional theory.

DOI
10.1088/0953-8984/9/24/004

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We study the scattering of atoms, rotons and phonons at the free surface of at normal incidence and calculate the evaporation, condensation and reflection probabilities. Assuming elastic one-to-one processes and using general properties of the scattering matrix, such as unitarity and time reversal, we argue that all nonzero probabilities can be written in terms of a single energy-dependent parameter. Quantitative predictions are obtained using linearized time-dependent density functional theory.
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We have shown that rotons as well as phonons can evaporate 4He^{4}\mathrm{He}4He atoms in a single-quantum process. Measurements of the time of flight and the angular distribution of the evaporated atoms clearly distinguish between evaporation by phonons and rotons. The results indicate that energy and the parallel component of momentum are conserved at the free liquid surface.
We report variational Monte Carlo calculations which give amplitudes and phases of the reflected and transmitted components of states representing scattering of helium atoms normally incident on a superfluid 4He{}^{4}\mathrm{He}4He slab. The wave function describes a previously postulated condensate mediated process [J. W. Halley et al., Phys. Rev. Lett. 71, 2429 (1993)] and the results are consistent with uncertainty principle arguments suggesting that the transmission time for thin (but macroscopic) samples will be independent of slab thickness.
A beam of ${\mathit{R}}^{\mathrm{\ensuremath{-}}}$ rotons, i.e., rotons which have oppositely directed group velocity and momentum, is reated by condensing an atom beam onto the surface of liquid 4He^{4}\mathrm{He}4He. ${\mathit{R}}^{\mathrm{\ensuremath{-}}}$ rotons are identified by quantum evaporation and their time of flight. Measured signal shapes agree well with those modeled. Signal sizes are compared with those from atoms specularly reflected from the He surface, from which it is estimated that the quantum efficiency for the condensation or evaporation process is high and of the order of 0.5 for angles of incidence where mainly rotons are created. ${\mathit{R}}^{\mathrm{\ensuremath{-}}}$ rotons can specularly reflect from a solid-liquid He interface with high probability, if the interface is particularly good.
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