Two-phonon Raman scattering of hcp solid helium-4

Abstract

Raman spectra of hcp solid 4He have been measured at a temperature of 0.71 K and at a molar volume of 20.98 cm 3/mol. The spectra measured with an instrumental resolution of 1.1 cm -1 show the feature: a sharp one-phonon peak is observed at 7.39±0.06cm -1, the two-phonon spectrum appears in the energy region about 10–45 cm -1, and a broad continuous tail extends to energy shifts higher than the two-phonon region. A newly found fact is that four peaks have been resolved in two-phonon spectra at 17.1, 20.9, 25.2, and about 30 cm -1. The two-phonon spectral structure has good qualitative similarity to that calculated by theoretical works.