Isotopic effect of the mean lifetimes of theNeAr2+doubly charged rare-gas dimer

Abstract

It has been suggested recently by Chen et al. [Phys. Rev. A 49, 3472 (1994)] that the measured long-lived NeAr + formed in fast NeAr+ + Ar charge-stripping collisions is mostly in its 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 Ne Ar + and Ne Ar + 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 + 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 + are given.PACS number (s): 31.50.+w, 34.50.Gb, 33.15. Ta, 39.10.+j The first long-lived state of the NeAr + molecular ion was recently discovered in charge-stripping collisions of 900-KeV NeAr+ projectiles with Ar atoms, and its mean lifetime was determined to be 275+ 25 ns by Ben-Itzhak, Gertner, and Rosner [1].Following this measurement Koch, Frenking, and Gobbi [2] have performed ab initio calculations of the electronic ground state of NeAr + and found a rather deep local minimum. They have suggested that this long-lived molecular ion is formed in this metastable ground state. In a very recent paper, Chen et al. [3] have reported phase-shift calculations of the energies and resonance widths of the vibrational states bound in this potential well. The calculated tunneling decay rates differ by more than three orders of magnitude from one vibrational state to the other.Only the v = 12 vibrational state has a mean lifetime that is close enough to the experimental value, even though it is still underestimating the experimental value by about a factor of 3. This small discrepancy was attributed to an inaccuracy in the potential energy curve of the order of a few meV. It was concluded by Chen et al. that the long-lived NeAr + molecular ion was formed in the v = 12 vibrational state of the electronic ground state. Such highly excited vibrational states are expected to be populated by vertical transitions, because the two molecular ions involved have significantly different equilibrium internuclear distances.One of the parameters to which the transition rates are most sensitive in a tunneling process is the reduced mass of the decaying system. Although the electronic potential energy curves do not depend on the masses of the atoms in the molecule, the energies of the vibrational states, which in turn determine the mean lifetime, do. This dependence of the Present address: