Doubly shaped D2 excitation spectra of Cs and Rb atoms in superfluid helium due to a quadrupole bubble surface oscillation.

Abstract

Our recent experiments show that the ${D}_{2}$ excitation spectra of Cs and Rb atoms in superfluid helium have doubly shaped profiles. Such line shapes are beyond the description of a simplified spherical atomic bubble model which explains properly general features of optical spectra of neutral atoms in liquid helium. This paper gives a theoretical explanation of the doubly shaped profiles, with a model that the bubble structure of surrounding helium is not spherical, but is deformed instantaneously by a quadrupole oscillation. The calculated energy level of the ${P}_{\frac{3}{2}}$ state of the deformed atomic bubble is split into two branches, giving the ${D}_{2}$ excitation line shape consisting of two components with different peak intensities and widths. The obtained line shape agrees qualitatively with the observed one, which enables us to confirm the importance of the dynamic Jahn-Teller effect due to a quadrupole oscillation of the atomic bubble surface induced by zero-point fluctuations.