Negative Ion Formation in Selected Hexafluoride Molecules

Abstract

Negative ion formation in gaseous SeF₆, TeF₆, MoF₆, ReF₆, and UF₆ has been studied as a function of incident electron energy in the region from 0 to ∼10 eV. Negative hexafluoride ions formed by direct electron attachment were observed only in the cases of MoF₆ and ReF₆. SeF₆⁻, TeF₆⁻, and UF₆⁻ were produced by charge exchange with thermal SF₆^−* and rates for these reactions and for the reaction UF₅⁻ + UF₆→UF₆⁻ + UF₅ were measured and are reported here. Electrons having energies close to zero are known to attach to SF₆ with a cross section approaching the maximum for s‐wave capture. The excited negative ion SF₆^−* formed in this way has a lifetime of 26 μsec against autodetachment of the electron. An argument based on molecular orbital theory is given for the existence of a Jahn–Teller effect in SF₆⁻. This could provide a means for the coupling between electronic and nuclear motion needed to trap the low‐energy electron. Selenium and tellurium lie directly under sulfur in the same column of the periodic table, but the large spin‐orbit splittings expected in SeF₆ and TeF₆ should dominate any Jahn–Teller effect in these otherwise similar molecules. The suggestion that a Jahn–Teller effect provides the coupling mechanism between electronic and nuclear motion is partially supported by the observation that ReF₆ (large Jahn–Teller distortion) strongly captures slow electrons to form ReF₆^−*.