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−*.