We report the results of a high-resolution laser-spectroscopy study of dysprosium atoms injected into superfluid ${}^4\mathrm{He}$. 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.

• Received 25 August 2017

DOI:https://doi.org/10.1103/PhysRevB.97.094504