Knowledge Bank

Recently we have observed OODR transitions terminating to the $B~1A1$ state of $SiH2$ and $SiD2$. The OODR spectra observed indicate the quasi-linear behavior in the $B~$ state based on the following features. Even or odd-$v2$ levels exclusively appear in the OODR via an intermediate $Ka$=odd or even rotational level. This even/odd-$v2$ progression is a typical pattern in the case of bent-linear transition. That is, $SiH2$ acts as a linear molecule in the $B~$ state and its bending vibration is doubly degenerate. Thus there exist a vibrational angular momentum $\ell$. The intensity pattern comes from a selection rule of a c-type transition; $\Delta (\ell -Ka)=\pm 1$. Bending excited levels exhibit negative $g22$-values. This is an indication of the double minimum potential. If an electronic state of interest was doubly degenerate, there would be a possibility of Renner-Teller effect. However, the $B~$ state correlates to $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma -$ state in the linear configuration. Thus, this double minimum potential originates from the quasi-linearity. The height of the barrier to the linearity was calculated to be about $200cm-1.a$ It was confirmed that there are only $\ell (Ka)=0$ rotational levels in the $v2=0$ level. This means that the $v2=0$ level is located above the barrier to linearity. It is very probable considering the very low barrier height. In addition, the absolute value of the $g22$ constant rapidly decreases as the $v2$-value increases. These observations strongly support the quasi-linearity of the $B~$ state. In the presentation, results of the vibrational analysis on the quasi-linearity will be discussed.