MEAN LIFETIME MEASUREMENTS OF HEH2-SIGMA) ISOTOPES( (2P)

The HeH2+ molecular ion decays by an electronic dipole transition from its bound first excited state (2p sigma) to the repulsive ground stat e (1s sigma). We have calculated the mean lifetimes of the vibrational states of a few HeH2+ isotopes and found a large isotopic effect, in particular for highly excited vibrational states, i.e., states with th e same nu have different decay rates. The measured decay curves of (He H2+)-He-4, (HeD2+)-He-3, and (HeD2+)-He-4 ( i.e., the number of HeH2molecular ions as a function of their flight time from the target cell where they were formed), in contrast, are similar to each other. The lack of a measurable isotopic effect is related to the HeH2+ creation mechanism. In the charge-stripping collisions, a distribution of vibra tional states is populated by vertical transitions and is thus centere d around roughly the same vibrational energy and not around the same q uantum number nu. The mean lifetimes of the different isotopes as a fu nction of their energy are surprisingly similar to each other, therefo re washing out the isotopic effect.