THE SURFACE BOUNDARY-CONDITIONS FOR QUANTUM EVAPORATION IN HE-4

Quantum evaporation of 4He atoms by phonons and rotons in liquid 4He is measured for a wide range of conditions. The results confirm that an atom is evaporated in a single-excitation to single-atom process and that the boundary conditions of conservation of energy and parallel momentum are obeyed. Excitations in the liquid are generated by pulse heating a thin metal film heater, and the evaporated atoms are detected by the energy yielded on condensation at the surface of a superconducting transition edge bolometer. The heater and bolometer can be rotated about a common axis in the liquid surface, and the collimation of the excitations and atoms into beams allows the angles of incidence and evaporation to be defined. It is first shown that the input power and pulse length must be carefully chosen to produce beams of phonons and rotons that are fully ballistic. The time of flight from the heater to the bolometer is measured, and the angular distribution of the evaporated atoms is determined for different angles of incidence. In particular, the wavevector dependence of the roton to atom pulse shape can be seen. The authors see no sign of Pitaevskii roton decay over the long liquid path lengths involved ( approximately 6 mm), and there is no indication that ripplons are created in the evaporation process.