THE EVAPORATION OF ATOMS FROM THE SURFACE OF SUPERFLUID HE-4 BY ROTONS

A beam of rotons is directed toward the liquid surface at normal incid ence. At the surface some rotons will evaporate atoms, with a wave-vec tor dependant probability p(q), The kinetic energy of the evaporated a tom is equal to the difference in energy between the energy of the rot on and the binding energy of a He-4 atom to the surface (7.16 K). The atoms which are evaporated are detected by a bolometer above the surfa ce, The path length of the roton in the liquid, and the atom in the va cuum, are varied by changing the liquid level in the cell, By thus cha nging the liquid level, we change the flight time between heater and b olometer for a given wavevector of roton. We analyse the time-of-fligh t spectra received by the bolometer to reveal the product n(q)p(q) whe re 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 roton s, we can deduce p(q). The resulting p(q) shows an unexpected decrease towards roton minimum at q < 2.1 Angstrom(-1).