Sound velocity measurements of quench-condensed noble-gas films

We have developed a new technique to study the sound velocity in quench-condensed noble-gas films. In this method, we excite the phonons with a laser pulse and derive the speed of sound by tracing the phonon thickness resonance emission out of the film with a super-conducting tunneling junction as phonon detector. Measurements were performed on Ne, Ar, Kr, and Xe, condensed onto a 1.3 K Si-crystal under UHV-conditions. We found that all quench-condensed films have a longitudinal speed of sound comparable to that of the corresponding noble-gas crystal. However, the values of the transverse speed of sound are reduced by a factor of two in comparison with the bulk. The implications of these results on the structure of the condensates will be discussed. (C) 2000 Elsevier Science B.V. All rights reserved.