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 line s of Eu atoms have been studied in bulk: liquid helium in the temperature r ange between 1.6 and 3.1 K. Profiles of the phonon bands show the remarkabl e 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, measu red at about 1.9 K for two different lines with different interaction energ ies with the liquid are found to show identical profiles, suggesting that t he profiles depend only on the temperature. The broad peak at 1.6 K has bee n interpreted as temperature-broadened roton/maxon peaks, while the monoton ically decreasing profiles have not been well understood yet.