MEAN LIFETIME OF THE BOUND 2P-SIGMA STATE OF HEH2+

Recently, experimental evidence for the existence of a bound state of the HeH2+ molecular ion was reported by Ben-Itzhak, Gertner, Heber, an d Rosner [Phys. Rev. Lett. 71, 1347 (1993)]. This three-body system wa s predicted by Bates and Carson to have bound vibrational states in th e 2psigma electronic state despite the strong repulsion potential betw een the two nuclei. These 2psigma vibrational states decay via an elec tronic transition to the 1ssigma repulsive ground state with a mean li fetime of about 1 nsec. We have used the ''fragment-fragment'' coincid ence method to measure the mean lifetime of the HeH2+ molecular ions w hich dissociate in flight before reaching the detector. The yield of t hese molecular ions was measured as a function of the distance between the target cell and the analyzer exit. From this measurement a mean l ifetime of 3.9 +/- 0.4 nsec was determined. The 2psigma --> 1ssimga tr ansition rates for all vibrational states have been calculated within the Born-Oppenheimer approximation using available oscillator strength s. It is shown that the transition rates decrease with increasing vibr ational energy because of the ''stretch'' of the molecular ion. The di stribution of 2psigma vibrational states populated by vertical transit ions from stripping the HeH+ electronic ground state was evaluated, an d the resulting decay curve compares well with the measured one.