Elementary Excitations of Superfluid Helium Droplets Probed by Ion Spectroscopy

Electronic spectra of molecules in helium droplets reveal spectral features that are related to the elementary excitations of the superfluid helium environment. In order to determine to what extend the interaction strength of the molecule with the helium affects these excitations, the spectrum corresponding to the transition of 2,5-difluorophenol cations in a helium droplets has been recorded. The vibronic resonances reveal a sharp zero-phonon line whose width is largely determined by the rotational band contour, followed by a broad structureless phonon wing at higher frequencies. The splitting between the zero-phonon line and phonon wing is approximately half that found for neutral 2,5-difluorophenol. This difference is attributed to the increased helium density around the ion due to its strong interaction with the helium.