Symmetry-dependent isotopic fractionation in the formation of He₂⁺

The termolecular association He⁺ + 2He → He₂⁺ + He has been studied at low pressure using helium enriched in ³He. The formation rate of the ⁷He₂⁺ ion is observed to be equivalently enhanced above statistical levels relative to that of either ⁶He₂⁺ or ⁸He₂⁺. This non-mass-dependent kinetic isotope effect can be understood in terms of a new symmetry correlation scheme developed in this laboratory (J. Chem. Phys. 1992, 96, 4387) which indicates that only 50% of He⁺/He collisions can access the He₂⁺ ground electronic state when the He isotopes are identical whereas 100% of the collisions can do so when the He isotopes are different. The results further indicate that stabilization of the (He₂⁺)* complex by collisions with He occurs primarily without atom exchange, suggesting an important role for large impact parameter, rotational state changing collisions.