The photoionization spectra of effusive and supersonic molecular beams of monosilane

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Abstract
Effusive and supersonic molecular beams of monosilane have been ionized by photons from the Berlin synchrotron light source (BESSY). Photoionization efficiency curves have been taken as a function of wavelength (50 to 120 nm) for a number of fragment ions. The main products observed are SiH+3 and SiH+2 . No appreciable SiH+4 could be detected and less than 5% of the signal was due to lower fragments (SiH+, Si+). The threshold energies Ethr, the standard enthalpy of formation ΔH○f,g of the fragment ions SiH+2 and SiH+3 and the ionization potentials Iz of the corresponding radicals have been determined to be SiH+2: Ethr=11.67±0.04 eV ΔH○f,g=276.3±0.9 kcal/mol, Iz(SiH2)=9.47±0.03 eV. SiH+3: Ethr=12.23±0.02 eV, ΔHf,g=237.1±0.6 kcal/mol, Iz(SiH3)=8.32±0.07 eV. Discrete structure in the photoionization curves is observed in the range of Ethr=15.8–17.6 eV and interpreted as vibrational progressions of a highly excited autoionizing state of SiH4 which lies 15.8 eV above the ground state. Vibrational constants have been found to be 1790±80 and 680±50 cm−1 which correspond to the ground state constants 2187 and 711 cm−1 of the ν1 and the ν2 modes of monosilane. The broadening of some lines in the progression is interpreted as being caused by a transition into a dissociative excited neutral state with lifetimes as short as 3×10−14 s for v=3 of the ν2 mode. Additional ions (predominantly Si2H+4, Si2H+6, Si2H+7) have been observed when photoionizing a supersonic molecular beam of monosilane. These are shown to stem from the ionization and subsequent fragmentation of dimers and possibly higher multimeres. Threshold values have been determined to be Si2H+4: 11.58±0.02 eV; Si2H+6: 11.4 eV; and Si2H+7: 11.4 eV.

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