Laser spectroscopy of phonons and rotons in superfluid helium doped with Dy atoms

We report the results of a high-resolution laser-spectroscopy study of dysprosium atoms injected into superfluid He-4. A special attention is paid to the transitions between the inner 4f and 5d electronic shells of Dy. The characteristic gap is observed between the zero-phonon line and the phonon wing in the experimental excitation spectrum that arises due to the peculiar structure of the phonon-roton spectrum of superfluid He. This observation resolves the longstanding discrepancy between the studies of bulk superfluid He and He nanodroplets.