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RATIONALE

Sulfur hexafluoride (SF6) is the most frequently used standard for anion mass spectrometry because of its large temperature-independent cross-section for electron attachment. However, the kinetic and thermodynamic properties – the products of the reactions, and the number of negative ion states of the SFn – are presently in dispute.

METHODS

The electron affinities for SFn (n = 1–6) are predicted by assuming dissociation energies and ground-state electron affinities based on literature values. The temperature dependence of the 2012 negative surface ionization mass spectrometer data, and other negative ionization mass spectrometry, electron capture detector, and beam and magnetron data, are analyzed using a kinetic model.

RESULTS

More precise and accurate activation energies for thermal electron attachment and electron affinities of SF6, SF5 and SF4 are reported. The largest experimental electron affinities of SF, SF2 and SF3 are assigned to the ground states. Ionic Morse potentials for multiple states are calculated. A mechanism for the formation of the ionic products observed in negative surface ionization thermal electron attachment to SF6 is presented.

CONCLUSIONS

Negative surface ionization on a hot filament with mass spectrometry is a relatively simple and effective method for determining electron affinities similar to the electron capture detector, magnetron, swarm and beam procedures. A new method of predicting the number of negative ion states from dissociation limits establishes targets for the data analysis. Pseudo one-dimensional anionic Morse potentials illustrate the mechanism for the reaction of thermal electrons with SF6 and the consecutive dissociation pathways. Copyright © 2014 John Wiley & Sons, Ltd.