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

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.