ISOTOPIC EFFECT OF THE MEAN LIFETIMES OF THE NEAR2+ DOUBLY-CHARGED RARE-GAS DIMER

It has been suggested recently by Chen et al. [Phys. Rev. A 49, 3472 (1994)] that the measured long-lived NeAr2+ formed in fast NeAr+ + Ar charge-stripping collisions is mostly in its upsilon = 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 (NeAr2+)-Ne-20-Ar-40 and (NeAr2+)-Ne-22-Ar-40 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 NeAr2+ 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 NeAr2+ are given.