REFLECTION OF PHONONS IN 4He BY NaF

The angular distribution after reflection at a cleaved NaF-liquid He interface of a collimated phonon beam incident from the helium at a grazing angle of incidence of ̂ 10° has been measured. The results suggest the existence of interface states which interact with only a part of the incident phonon spectrum. In the light of experiments which have been aarried out on the problem of phonon transmission across solid He interfaces(Wyatt, Sherlock et al./l,2/, it is clear that for phonons which are 4 emitted by a solid into liquid He there are two separate transmission channels : one which conserves the parallel component of phonon wavevector, and one which does not.It has been suggested that interface states are responsible for the scattering of phonons, thereby creating this latter channel of transmission, and that it is this channel which enhances the Kapitza conductance above the acoustic mismatch value of the Khalatnikov model /3/. Further investigation into the nature of these interface states is therefore necessary, and this work was concerned with their effect on the reflection 4 of phonons at a liquid He solid interface. NaF was chosen for the solid as it can be cleaved to form an essentially flat surface on an atomic scale and also because most of the transmission work cited above was carried out with this material. For this work phonons were reflected from the helium side of the interface as helium is in many ways an ideal transport medium for phonons, being isotropic and supporting only one propagating phonon mode. The experimental arrangement is shown schematically in figure 1. The incident phonon beam in the helium was produced and collimated by a wire heater(5mm x 15 ym dia., 50 Q resistance) and collimating slits, and in this preliminary experiment the angular variation of the scattered phonon intensity was measured with a graphite film bolome2 ter (1mm ).