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The photoionization spectra of effusive and supersonic molecular beams of monosilane

The Journal of Chemical Physics, ISSN: 0021-9606, Vol: 83, Issue: 7, Page: 3426-3432
1985
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  • Citations
    30
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      30

Article Description

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 and SiH. No appreciable SiH could be detected and less than 5% of the signal was due to lower fragments (SiH, Si). The threshold energies E, the standard enthalpy of formation ΔH° of the fragment ions SiH and SiH and the ionization potentials I of the corresponding radicals have been determined to be SiH: E = 11.67 ± 0.04 eV,ΔH° = 276.3 ± 0.9 kcal/mol,I (SiH) = 9.47 ± 0.03 eV. SiH : E = 12.23 ± 0.02 eV, ΔH = 237.1 ± 0.6 kcal/mol, I(SiH ) = 8.32 ± 0.07 eV. Discrete structure in the photoionization curves is observed in the range of E = 15.8-17.6 eV and interpreted as vibrational progressions of a highly excited autoionizing state of SiH which lies 15.8 eV above the ground state. Vibrational constants have been found to be 1790 ± 80 and 680 ± 50 cm which correspond to the ground state constants 2187 and 711 cm of the ν and the ν 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 s for v = 3 of the ν mode. Additional ions (predominantly SiH , SiH, Si H) 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 multimers. Threshold values have been determined to be SiH: 11.58 ± 0.02 eV; SiH: 11.4 eV; and SiH: 11.4 eV. © 1985 American Institute of Physics.

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