Quantum Evaporation from the Free Surface of Superfluid 4He

TL;DR

This paper investigates atom and roton scattering at the free surface of superfluid 4He using mean field theory, predicting evaporation probabilities and analyzing scattering processes with implications for quantum fluid behavior.

Contribution

It introduces a numerical approach to study scattering and evaporation of excitations at the superfluid surface, providing detailed probability predictions for different processes.

Findings

Evaporation probability for R+ rotons increases rapidly with energy.

Evaporation from R- rotons remains below 0.25 across energies.

Mode-change process dominates near the roton minimum.

Abstract

The scattering of atoms and rotons at the free surface of superfluid 4He is studied in the framework of linearised time dependent mean field theory. The phenomenological Orsay-Trento density functional is used to solve numerically the equations of motion for the elementary excitations in presence of a free surface and to calculate the flux of rotons and atoms in the reflection, condensation, and evaporation processes. The probability associated with each process is evaluated as a function of energy, for incident angles such that only rotons and atoms are involved in the scattering. The evaporation probability for R+ rotons is predicted to increase quite rapidly from zero, near the roton minimum, to 1 as the energy increases. Conversely the evaporation from R- rotons remains smaller than 0.25 for all energies. Close to the energy of the roton minimum the mode-change process is the dominant one. The consistency of the results with general properties of the scattering matrix, such as unitarity and time reversal, is explicitly discussed. The condensation of atoms into bulk excitations is also investigated.