PHOTOIONIZATION MASS-SPECTROMETRIC STUDY OF FRAGMENTATION OF SIBR4 AND GEBR4 IN THE RANGE 400-1220-A

The non-radiative decay channels of the valence electronic states of SiBr4+ and GeBr4+ have been studied in the range 1220-400 angstrom (10-31 eV) by photoionisation mass spectrometry. Ion-yield curves for the parent ions and for MBr3+, MBr2+, M+ and Br+ (M = Si, Ge) have been obtained, as well as the relative photoionisation branching ratios. The appearance thresholds for SiBr3+ and GeBr3+ occur at 11.31 and 10.97 eV, respectively. They lie within the Franck-Condon region of the ground state of SiBr4+ and GeBr4+, and are at the thermodynamic thresholds for SiBr3+ + Br and GeBr3+ + Br. The smaller fragment ions have appearance thresholds which relate to energies of excited electronic states of SiBr4+ and GeBr4+, 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 SiBr4+ and GeBr4+ (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.