Velocity-Map-Imaging Spectroscopy of Rubidium Atoms Attached to Helium Nanodroplets

Velocity-map-imaging measurements both of ions and electrons on gas-phase rubidium (Rb) are compared to results obtained with atoms attached to the surface of helium nanodroplets. At excitation wavelengths close to the 5s-6p transition (~420 nm), gas-phase Rb atoms emit only electrons from the 6p state. In contrast, Rb atoms attached to helium droplets emit electrons also from the lower-lying atomic levels 5p and 4d. These states cannot be populated via spontaneous relaxation from the 6p level as the nanosecond laser pulse is much shorter than the natural lifetime of the 6p state. Upon desorption of the Rb atoms off the droplets, electrons are released from the sharp atomic energy levels. The Photoion spectra and angular distributions are in good agreement with a pseudodiatomic model for the RbHeN complex. Repulsive interactions in the excited states entail fast dissociation followed by ionization of free Rb atoms as well as of RbHe and RbHe2 exciplexes.