Symmetry restrictions in diatom/diatom reactions. II. Nonmass-dependent isotope effects in the formation of O+4

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Abstract
Nonmass‐dependent isotopic enrichment of 17O and 18O has been observed mass spectrometrically in O+4 ions produced by termolecular association reactions of O+2 and O2, where the O+2 was generated by electron ionization. The enhancement is strongly dependent on the energy of the ionizing electrons, decreasing from a near tenfold enhancement at threshold for O+2 production to no enhancement above 40 eV. Additionally, O+2 generated near threshold were found to be significantly less efficient in producing O+4 than O+2 ions generated at higher energies. A permutation‐inversion, symmetry analysis of the termolecular association reaction suggests that the results can be understood in terms of a symmetry restriction on the O+2(2Πg) rotational states which can efficiently access the electronic ground state of O+4 upon collisions with O2. The restriction is rooted in the Pauli principle and vanishes when the O+2 ion is isotopically heteronuclear. The possible relevance of the nonmass‐dependent isotope enhancement in O+4 to similar enhancements found in stratospheric and laboratory‐produced ozone is discussed.

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