JET SPECTROSCOPY AND EXCITED-STATE DYNAMICS OF SIH2 AND SID2

Silylene radicals, SiH, and SiD2, are generated in a supersonic free jet by ArF laser (193 nm) photolysis of phenylsilane and phenylsilane-alpha-d3, respectively. LIF excitation and dispersed fluorescence spectra are measured for the v2 vibronic bands of the approximately A B-1(1) -approximately X 1A1 transition. The heterogeneous predissociation to Si (3p) + H-2 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 approximately A B-1(1) state. The heterogeneous predissociation is interpreted with the second order perturbation: approximately A B-1(1)-(a-type Coriolis) --> approximately X 1A1-(spin-orbit) --> approximately a B-3(1) --> Si(3P) + H-2. It is demonstrated experimentally that there is a potential barrier associated with the dissociation path of approximately a B-3(1) --> Si(3P) + H-2 the height of which is estimated to be 1540-2160 cm-1 from the bottom of the approximately A B-1(1) state. The electronic transition moment of the approximately A B-1(1) - approximately X 1A1 transition is estimated to be \mu(e)\2 = 0.26e2a0(2) from the Einstein equation for spontaneous emission using measured fluorescence lifetimes for single rovibronic levels with K(a') = 0 and calculated Franck-Condon factors. The onset of a second predissociation channel, approximately A B-1(1) --> Si(1D) + H-2, at the (0,7,0) vibronic level of SiH2 approximately A B-1(1) is manifested as a sharp decrease in the observed fluorescence lifetime for the upsilon-2' = 7, J' = 0 level relative to that predicted for a pure radiative lifetime.