Singular vibronic interaction in liquids: manifestation in the optical spectrum of impurity atoms in superfluid helium

The zero phonon line (ZPL) and its phonon-roton wing have been studied both experimentally and theoretically in the optical spectrum of the inner shell transition in the Dy atom in superfluid helium. It is shown that the linear vibronic interaction of impurity atom with long-wave acoustic phonons in the liquid phase is singularly enhanced. As a result, the ZPL of both the superfluid and normal components of liquid helium has a finite width. The temperature dependence of the spectrum is a consequence of the redistribution of the superfluid and normal components of the liquid helium and the temperature dependence of the spectrum of its normal component. Our calculations of the ZPL and its phonon-rotor wing are consistent with the experiment.