Detachment of metastable helium molecules from helium nanodroplets

Metastable helium a(3)Sigma(u)(+) molecules are produced in helium nanodroplets by electron bombardment, and become attached to their surface. Photo induced electronic transitions release them, and they are subsequently detected when they eject secondary electrons upon reaching a nearby surface. We report here the first high resolution spectroscopic study of this system. It reveals a highly non-thermal rotational state distribution with N = 1 and 11 less than or equal to N less than or equal to 29 populated but with the intermediate N states missing. Further studies of the bombarding electron energy appearance threshold for the high rotational states suggest that their precursor is the atomic 2(3)P state.