THE SINGLET-TRIPLET TRANSITION IN $H_{2}CSe$: CASE (AB) COUPLING IN THE TRIPLET STATE OF A POLYATOMIC MOLECULE

Author Institution: Department of Chemistry, University of Kentucky; Department of Chemistry, University of Wisconsin-Parkside; Department of Chemistry, Brock UniversityHigh resolution laser excitation spectra of the $\tilde{a}^{2}A_{2} - \tilde{X}^{1} A_{1}$ band system of $H_{2}CSe$ will be presented. Rotational analysis shows that the bands are A-type with no evidence of spin-splittings. However the Zeeman effect on these bands is very pronounced in contrast to minimal effects on the corresponding $S_{1}-S_{0}$ lines in the same vicinity. On the basis of ab initio prediction of the zero flied splittings in the triplet state and a rotational analysis of the vibronically induced $4^{1}_{0}$ band, for which all three components have been detected, we have concluded that the triplet state corresponds to case (ab) type 1, in which one of the spin components is far removed form the other two. The triplet state energy level structure of this highly unusual case will be discussed