The evaporation of atoms from the surface of superfluid 4He by rotons

A beam of rotons is directed toward the liquid surface at normal incidence. At the surface some rotons will evaporate atoms, with a wave-vector dependant probability p(q). The kinetic energy of the evaporated atom is equal to the difference in energy between the energy of the roton and the binding energy of a 4He atom to the surface (7.16 K). The atoms which are evaporated are detected by a bolometer above the surface. The path length of the roton in the liquid, and the atom in the vacuum, are varied by changing the liquid level in the cell. By thus changing the liquid level, we change the flight time between heater and bolometer for a given wavevector of roton. We analyse the time-of-flight spectra received by the bolometer to reveal the product n(q)p(q) where n(q) is the distribution of rotons. If we assume that n(q) reflects the temperature of the phonons in the heater used to inject the rotons, we can deduce p(q). The resulting p(q) shows an unexpected decrease towards roton minimum at q < 2.1 angstrom -1