Symmetry-dependent isotopic fractionation in the formation of He2+

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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.

Original languageEnglish
Pages (from-to)34-39
Number of pages6
JournalJournal of physical chemistry
Issue number1
StatePublished - 1993


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