Photoionisation mass-spectrometric study of fragmentation of SiBr4 and GeBr4 in the range 400–1220 Å

Abstract

The non-radiative decay channels of the valence electronic states of SiBr⁺₄ and GeBr⁺₄ have been studied in the range 1220–400 Å(10–31 eV) by photoionisation mass spectrometry. Ion-yield curves for the parent ions and for MBr⁺₃, MBr⁺₂, MBr⁺, M⁺ and Br⁺(M = Si, Ge) have been obtained, as well as the relative photoionisation branching ratios. The appearance thresholds for SiBr⁺₃ and GeBr⁺₃ occur at 11.31 and 10.97 eV, respectively. They lie within the Franck–Condon region of the ground state of SiBr⁺₄ and GeBr⁺₄, and are at the thermodynamic thresholds for SiBr⁺₃+ Br and GeBr⁺₃+ Br. The smaller fragment ions have appearance thresholds which relate to energies of excited electronic states of SiBr⁺₄, and GeBr⁺₄, and not to the lower-lying thermodynamic energy of the fragment ion. The results are discussed with reference to our earlier work on radiative decay from excited states of SiBr⁺₄ and GeBr⁺₄(J. Chem. Soc., Faraday Trans., 1990, 86, 2021). We have obtained a new value for the ionisation potential of SiBr₃ of 7.6 ± 0.4 eV, and we suggest that the previously accepted value for SiBr₂(12 ± 1 eV) is ca. 3.5 eV too high.