Mean lifetime measurements and calculations of long-lived HeNe2+ isotopes

The mean lifetimes of the He-3 Ne-20(2+) and He-3 Ne-22(2+) isotopes of the HeNe2+ molecular ion have been measured to be 55 +/- 10 ns and 72 +/- 14 ns, respectively. These mean lifetimes are similar to each other and about a factor of 3 smaller than the previously reported mean lifetime of the most abundant He-4 Ne-20(2+) isotope. The small differences between the mean lifetimes of the different isotopes suggest that the long-lived HeNe2+ decays mainly by dipole electronic transitions to lower dissociating states. We have performed multireference configuration interaction (MRCI) calculations of the HeNe2+ ground state and many excited electronic states. The electronic ground state is found to be metastable and deep enough to sustain a single vibrational state in contrast to previous calculations. However, its calculated mean lifetime is similar to 11 ps which is too short to be detected in our experiments. Excited states associated with the He + Ne2+ dissociation limits are bound by the long-range interaction between Ne2+ and polarized He. Similarly, the lowest state associated with a He2+ + Ne dissociation limit is also bound by long-range interaction Due to an avoided crossing, the singlet state correlating with the first excited He+ + Ne+ limit is metastable. These states decay mainly by dipole electronic transitions to lower dissociating states. Decay rate calculations suggest five possible electronic states which are in agreement with the measured values.