DOUBLY SHAPED D-2 EXCITATION-SPECTRA OF CS AND RB ATOMS IN SUPERFLUID-HELIUM DUE TO A QUADRUPOLE BUBBLE SURFACE OSCILLATION

Our recent experiments show that the D-2 excitation spectra of Cs and Rb atoms in superfluid helium have doubly shaped profiles. Such line s hapes are beyond the description of a simplified spherical atomic bubb le model which explains properly general features of optical spectra o f neutral atoms in liquid helium. This paper gives a theoretical expla nation of the doubly shaped profiles, with a model that the bubble str ucture of surrounding helium is not spherical, but is deformed instant aneously by a quadrupole oscillation. The calculated energy level of t he P-3/2 State of the deformed atomic bubble is split into two branche s, giving the D-2 excitation line shape consisting of two components w ith different peak intensities and widths. The obtained line shape agr ees qualitatively with the observed one, which enables us to confirm t he importance of the dynamic Jahn-Teller effect due to a quadrupole os cillation of the atomic bubble surface induced by zero-point fluctuati ons.