FIRST OBSERVATION OF THE $\tilde{B}^{1} A_{1}$ STATE OF $SiH_{2}$ AND $SiD_{2}$ RADICALS BY OODR SPECTROSCOPY

$^{a}$Y. Yamaguchi, T. J. Van Huis, C. D. Sherrill, H. F. Schaefer III Theor, Chem. Acc. 97 341 (1997).Author Institution: Department of Chemistry, Graduate School of Science, Tohoku UniversitySilylene $(SiH_{2})$ is a silicon analogue of methylene $(CH_{2})$. The number of spectroscopic studies on $SiH_{2}$ is much smaller compared with that on $CH_{2}$ is small. Until now, only the $\tilde{X}^{1} A_{1}, \tilde{A}^{1} B_{1}$, and $\tilde{a}^{3} B_{1}$ states have been experimentally investigated. Recently, energetics and equilibrium structure of the next low-lying $\tilde{B}^{1} A_{1}$ state was studied by high-level calculations by Yamaguchi $et al.^{a}$ However, a corresponding electronic state has not yet been observed. In the course of our SEP spectroscopic study on highly excited vibrational levels of $SiH_{2}$, we have identified several bands that can be assigned as transitions to the $\tilde{B}$ state for the first time. When a $J = 0$ rotational level of the $\tilde{A}$ state was used as an intermediate level of the OODR measurement, several vibronic bands of $1100 - 1200 cm^{-1}$ interval were observed in the energy region of $28000 - 30100 cm^{-1}$ above the $\tilde{X}$ state. Based on a rotational selection rule and a predicted bending vibrational frequency, we assigned these bands as an odd-$v_{2}$ progression. This means that the $SiH_{2}$ in the $\tilde{B}$ state behaves as a linear molecule. To confirm our assignment, it is necessary to observe OODR spectra via a $K_{a} = 1$ rotational level. To avoid a difficulty in measuring desired OODR transitions due to a predissociation in the $\tilde{A}$ state of $SiH_{2}$, an OODR spectroscopy of $SiD_{2}$ was also carried out. We have succeeded in measuring the OODR transitions to the $\tilde{B}$ state of $SiD_{2}$ and determined the value of $T_{0}$ of $SiD_{2}$ to be $27214.11 cm^{-1}$. Our observation on $SiD_{2}$ confirmed the quasi-linear behavior in the $\tilde{B}$ state. Details of our observation will be presented in the paper