Evaporation from superfluid helium

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Abstract
We calculate the spectral distribution of atoms evaporating from superfluid helium, with the help of a theory, derived from the exact treatment of tunneling, which is free from the transfer-Hamiltonian approximation. In the absence of a detailed microscopic model for He II, the current cannot be computed completely. However, it is found that the spectrum must exhibit a singularity at the energy of the roton minimum: It is shown that, contrary to earlier predictions, this is of the "cusp" or "rounded wedge" type, due to the shape of the transmission-coefficient singularity at the threshold of the roton channel.