Photoionization mass spectrometric studies of the transient species Si2Hn (n=2-5)

The transient species Si2H5, Si2H4, Si2H3, and Si2H2 have been prepared for the first time, by reactions of F atoms with Si2H6. The species are generated in situ and studied by photoionization mass spectrometry. The adiabatic ionization potentials are (in eV) Si2H5, 7.60±0.05; Si2H4, 8.09±0.03; Si2H3, <7.59; and Si2H2, 8.20+0.01−0.02. In addition, two fragment appearance potential curves were measured, Si2H+3 (Si2H5) and Si2H+2 (Si2H4). By combining these ionization potentials with appearance potentials obtained here and from Si2H6, the following heats of formation (kcal/mol at 0 K) have been inferred: Si2H5, <63.3 (59.2); Si2H4, 67.9±0.9; Si2H3 (∼96); Si2H2, <100.7 (90.2–95.6). The quantities in parentheses are probable values, but less well defined. Structures and structural changes resulting from photoionization are discussed. In particular, Si2H2 and its cation are very likely cyclic (doubly bridged). Heats of formation of cations are also obtained, from which the proton affinities of Si2H2 (199±3 kcal/mol) and Si2H4 (199.9 kcal/mol) are deduced. The structural and energetic differences between corresponding silicon and carbon hydrides are discussed in detail.