INHOMOGENEOUS BROADENING OF ELECTRONIC-TRANSITIONS IN A LIQUID-HELIUM BUBBLE - THE ROLE OF SHAPE FLUCTUATIONS

Recent experiments of Grimes et al. [Phys. Rev. B 41, 6366 (1990)] and Parshin et al. [JETP, 74, 68 (1992)] demonstrate a substatial broadening in the 1s-1p transition of a single electron trapped in a liquid helium bubble (''bubblonium'') compared to theoretical predictions based on natural radiative linewidth. We show that the larger observed linewidth can be explained by inhomogeneus broadening due to quantum quadrupole fluctuations in the bubble shape. A simple adiabaticity rule for the bubblonium transitions similar to the Franck-Condon principle for molecular transitions is established. Quantitative estimates of the additional inhomogeneous linewidth at T=0 and 2.2 K are provided The full theoretical linewidth, due to inhomogeneous and homogeneous broadening, has a Voigt-profile shape, and accounts for the data reasonably well.