Isotopic effect of the mean lifetimes of the NeAr2+ doubly charged rare-gas dimer.

Abstract

It has been suggested recently by Chen {ital et} {ital al}. [Phys. Rev. A {bold 49}, 3472 (1994)] that the measured long-lived NeAr{sup 2+} formed in fast NeAr{sup +} + Ar charge-stripping collisions is mostly in its {ital v}=12 vibrational state bound to the electronic ground state, and that this molecular ion decays by tunneling through the potential barrier. Such a decay rate is expected to depend strongly on the reduced mass of the molecular ion leading to large isotopic effects. We have measured the mean lifetimes of the {sup 20}Ne{sup 40}Ar{sup 2+} and {sup 22}Ne{sup 40}Ar{sup 2+} isotopes in order to see this isotopic effect. Surprisingly, the mean lifetimes of both isotopes are similar to each other. Thus, it is suggested that the observed NeAr{sup 2+} molecular ions do not decay via a tunneling mechanism, which would indicate that they reside in the metastable electronic ground state. Rather, electronic transitions from bound or metastable excited states into other repulsive states are the origin for the experimentally observed decay. Qualitative estimates for the shapes and ordering of these states in the electronic spectrum of NeAr{sup 2+} are given.