Phonon Bands Associated with the Inner-Shell Electronic Absorption Lines of Eu Atoms in Bulk Liquid Helium

The phonon bands associated with the inner-shell electronic absorption lines of Eu atoms have been studied in bulk liquid helium in the temperature range between 1.6 and 3.1 K. Profiles of the phonon bands show the remarkable temperature dependence from a broad peak at 1.6 K to a monotonic decrease of the intensity by energy at higher temperatures. Two phonon bands, measured at about 1.9 K for two different lines with different interaction energies with the liquid, are found to show identical profiles, suggesting that the profiles depend only on the temperature. The broad peak at 1.6 K has been interpreted as temperature-broadened roton/maxon peaks, while the monotonically decreasing profiles have not been well understood yet.