Knowledge Bank

Silylene $(SiH2)$ is a silicon analogue of methylene $(CH2)$. The number of spectroscopic studies on $SiH2$ is much smaller compared with that on $CH2$ is small. Until now, only the $X~1A1,A~1B1$, and $a~3B1$ states have been experimentally investigated. Recently, energetics and equilibrium structure of the next low-lying $B~1A1$ state was studied by high-level calculations by Yamaguchi $etal.a$ However, a corresponding electronic state has not yet been observed. In the course of our SEP spectroscopic study on highly excited vibrational levels of $SiH2$, we have identified several bands that can be assigned as transitions to the $B~$ state for the first time. When a $J=0$ rotational level of the $A~$ state was used as an intermediate level of the OODR measurement, several vibronic bands of $1100-1200cm-1$ interval were observed in the energy region of $28000-30100cm-1$ above the $X~$ state. Based on a rotational selection rule and a predicted bending vibrational frequency, we assigned these bands as an odd-$v2$ progression. This means that the $SiH2$ in the $B~$ state behaves as a linear molecule. To confirm our assignment, it is necessary to observe OODR spectra via a $Ka=1$ rotational level. To avoid a difficulty in measuring desired OODR transitions due to a predissociation in the $A~$ state of $SiH2$, an OODR spectroscopy of $SiD2$ was also carried out. We have succeeded in measuring the OODR transitions to the $B~$ state of $SiD2$ and determined the value of $T0$ of $SiD2$ to be $27214.11cm-1$. Our observation on $SiD2$ confirmed the quasi-linear behavior in the $B~$ state. Details of our observation will be presented in the paper.