PHOTOIONIZATION MASS-SPECTROMETRIC STUDY OF SI2H6

The adiabatic I.P. of Si2H6 obtained by a photoionization mass spectrometric study at two temperatures is 9.74 +/- 0.02 eV. The first fragment, Si2H4+, initially appears with a shallow slope at less-than-or-equal-to 10.04 +/- 0.02 eV, and with a much steeper slope at less-than-or-equal-to 10.81 + 0.02 eV. It is argued that the initial onset corresponds to formation of H2SiSiH2+, while the steeper onset is attributed to formation of H3SiSiH+. The second fragment, Si2H5, has an appearance potential of less-than-or-equal-to 11.59 +/- 0.02 eV (11.41 +/- 0.03 is a probable value). Successive decomposition leads to Si2H2+ (from Si2H4+) and Si2H3+ (from Si2H5+). The photoion yield curve for Si2H3+ also displays shallow and steep onsets. Upper limits for the appearance potentials can be readily extracted, but the true thermochemical onsets are less well defined. Heats of formation (or upper limits) are presented for each of these species. For Si2H6+, Si2H5+, and Si2H4+, the experimental values are in good agreement with recent ab initio calculations. For the daughter species, the experimental values exceed the calculated ones, as expected.