Photoionization and imaging spectroscopy of rubidium atoms attached to helium nanodroplets.

Highly excited states of rubidium (Rb) atoms attached to helium (He) nanodroplets are studied by two-photon ionization spectroscopy in combination with electron and ion imaging. We find high yields of RbHe and RbHe(2) exciplexes when exciting to the 4D and 6P bands but not at the 6S band, in accord with a direct formation of exciplexes in binding RbHe pair potentials. Photoion spectra and angular distributions are in good agreement with a pseudodiatomic model for the RbHe(N) complex. Repulsive interactions in the excited states entail fast dissociation followed by ionization of free Rb atoms as well as of RbHe and RbHe(2) exciplexes.