Vacuum ultraviolet photolysis of supersonic free jets of SiH₄

We have measured the relative abundance of neutral photofragments in pulsed supersonic free jets of SiH₄ excited in the wavelength range 115-170 nm using synchrotron radiation and quadropole mass spectrometry (QMS). Using a sub-ionization QMS threshold electron-impact energy of 11 V and taking the difference between VUV irradiated and non-irradiated SiH₄ jet pulses, the relative abundance of the neutral species Si, SiH, SiH₂ and SiH₃ was determined. The neutral Si signal exhibited a threshold wavelength of ≈ 135 nm, reaching its maximum value at ≈ 115 nm, the short wavelength limit of the current measurements. The SiH signal rose rapidly to a peak at ≈ 158 nm, falling off rapidly to ≈ 145 nm. An additional shoulder on the SiH signal was observed at ≈ 138 nm that gradually diminished for Ω < 125 nm. The SiH₂ and SiH₃ radicals exhibited a maximum near 145 nm with the SiH₂ peak exhibiting a secondary maximum at ≈128 nm.