15min:

Y. MURAMOTO, H. ISHIKAWA AND N. MIKAMI, Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.

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. \par Even or odd-v₂ levels exclusively appear in the OODR via an intermediate K_a=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; \mit (\ell - K_a ) = \pm 1. \par 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 ^{1 -} 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 200 cm^-1. It was confirmed that there are only \ell(K_a)=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. \par 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.