Forming Rb⁺ snowballs in the center of He nanodroplets

Helium nanodroplets doped with rubidium atoms are ionized by applying a resonant two-step ionization scheme. Subsequent immersion of rubidium ions is observed in time-of-flight mass spectra. While alkali-metal atoms usually desorb from the surface of a helium nanodroplet upon electronic excitation, rubidium in its excited 5²P_1/2 state provides an exception from this rule (Auböck et al., Phys. Rev. Lett., 2008, 101, 35301). In our new experiment, Rb atoms are selectively excited either to the 5²P _1/2 or to the 5²P_3/2 state. From there they are ionized by a laser pulse. Time-of-flight mass spectra of the ionization products reveal that the intermediate population of the 5²P _1/2 state does not only make the ionization process Rb-monomer selective, but also gives rise to a very high yield of Rb⁺-He _N complexes. Ions with masses of up to several thousand amu have been monitored, which can be explained by an immersion of the single Rb ion into the He nanodroplet, where most likely a snowball is formed in the center of the He nanodroplet. As the most stable position for an ion is in the center of a He nanodroplet, our results agree well with theory.