Ionization Thresholds of Alkali Metal Atoms on Helium Droplets

Superfluid helium droplets (HeN) have attracted strong interest as cold hosts for the investigation of weakly bound molecules. Whereas the host–dopant interaction is weak for neutral molecules, ion impurities may be surrounded by frozen shells of polarized helium atoms. An extreme example of the different behavior is given by alkali metal impurities that stay at the surface of the droplet but immerse into the droplet as cations releasing a considerable amount of binding energy. We report measurements of the photoionization efficiency for the rubidium-HeN and cesium-HeN systems and find that the ionization threshold is lowered compared with the free atoms. The corresponding energy shift increases when going from heavy to light alkali metals and from small to large helium droplets. Both effects can be explained by the difference in polarization energies associated with submerged alkali metal cations. The findings agree qualitatively well with recent calculations of helium snowball formation around alkali metal cations.