Thermal broadening of the zero-phonon line in superfluid helium

Based on the two-fluid model of helium II, a theory is proposed to explain the recently observed temperature dependence of the zero-phonon line (ZPL) in the optical spectra of Dy atoms in superfluid helium. According to this theory, the main reason for the observed dependence is the temperature redistribution of the normal and superfluid components of the liquid helium. It is also found that due to the Archimedes' principle, the linear vibronic interaction with long-wave phonons in the liquid phase is singularly enhanced. This enhancement, in turn, leads to a finite broadening of the ZPL of the superfluid component and a linear temperature dependence of the ZPL width of the normal component. The proposed theory allows us to explain the experimental results.