Mean lifetime measurements of ${\mathrm{HeH}}^{2+}$(2pσ) isotopes

The ${\mathrm{HeH}}^{2+}$ molecular ion decays by an electronic dipole transition from its bound first excited state (2pσ) to the repulsive ground state (1sσ). We have calculated the mean lifetimes of the vibrational states of a few ${\mathrm{HeH}}^{2+}$ isotopes and found a large isotopic effect, in particular for highly excited vibrational states, i.e., states with the same v have different decay rates. The measured decay curves of ${}_{}{}^4{}_{}{}^{}\mathrm{HeH}_{}^{2+}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}\mathrm{HeD}_{}^{2+}{}_{}{}^{}$, and ${}_{}{}^4{}_{}{}^{}\mathrm{HeD}_{}^{2+}{}_{}{}^{}$ ( i.e., the number of ${\mathrm{HeH}}^{2+}$ molecular ions as a function of their flight time from the target cell where they were formed), in contrast, are similar to each other. The lack of a measurable isotopic effect is related to the ${\mathrm{HeH}}^{2+}$ creation mechanism. In the charge-stripping collisions, a distribution of vibrational states is populated by vertical transitions and is thus centered around roughly the same vibrational energy and not around the same quantum number v. The mean lifetimes of the different isotopes as a function of their energy are surprisingly similar to each other, therefore washing out the isotopic effect. © 1996 The American Physical Society.