Jet spectroscopy and excited state dynamics of SiH2 and SiD2

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Abstract
Silylene radicals, SiH2 and SiD2, are generated in a supersonic free jet by ArF laser (193 nm) photolysis of phenylsilane and phenylsilane-α-d3, respectively. LIF excitation and dispersed fluorescence spectra are measured for the ν2 vibronic bands of the à 1B1−X̃ 1A1 transition. The heterogeneous predissociation to Si (3P)+H2 is proposed from the anomalous rotational structure in the excitation spectra; the rotational lines of the r(1) subbranch (K′a=0←K■a=1) have stronger intensity than those of the r(0) subbranch (K′a=1←K■a=0), though the latter is expected to be stronger due to the low temperature Boltzmann distribution in the jet. The time-resolved excitation spectra demonstrate shorter lifetime of K′a=1 rovibronic levels in the à 1B1 state. The heterogeneous predissociation is interpreted with the second order perturbation: à 1B1 –(a-type Coriolis)→X̃ 1A1 -(spin–orbit)→ã 3B1→Si(3P)+H2. It is demonstrated experimentally that there is a potential barrier associated with the dissociation path of ã 3B1→Si(3P)+H2, the height of which is estimated to be 1540–2160 cm−1 from the bottom of the à 1B1 state. The electronic transition moment of the à 1B1–X̃ 1A1 transition is estimated to be ‖μe‖2=0.26e2a20 from the Einstein equation for spontaneous emission using measured fluorescence lifetimes for single rovibronic levels with Ka=0 and calculated Franck–Condon factors. The onset of a second predissociation channel, à 1B1→Si(1D)+H2, at the (0,7,0) vibronic level of SiH2 à 1B1 is manifested as a sharp decrease in the observed fluorescence lifetime for the v′2=7, J′=0 level relative to that predicted for a pure radiative lifetime.

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