Detachment spectroscopy of metastable excitations in helium nanodroplets

Metastable states of helium nanodroplets in a beam are produced by electron impact and after several milliseconds are probed by a transverse light beam. Absorption of infrared photons by the excited droplets leads to the release of metastable triplet He{sup *}{sub 2}molecules. These are subsequently detected when they eject electrons upon reaching a surface. By this method we can study the rotational structure of the absorption lines and infer the rotational state distribution of the molecules while in contact with the clusters. The results are rather surprising, and suggest that only the high J states and the J=1 state of the molecule are populated.