Experimental proof of the case (ab) coupling hypothesis in the first excited triplet state of selenoformaldehyde (H2C=Se)

A high-resolution laser-induced phosphorescence spectrum of the vibronicall y induced 4(0)(1) band of the (a) over tilde (3)A(2)-(X) over tilde (1)A(1) system of (D2CSe)-Se-80 has been recorded with Doppler-limited resolution. Rotational analysis revealed that transitions to all three spin components occur within the 12 265-12 525 cm(-1) region. The rotational structure of the band was fitted to the experimental precision of measurement using a tr iplet state Hamiltonian that allowed for slight variations in the geometrie s of the individual spin components. The results show that the triplet stat e of selenoformaldehyde involves case (ab) coupling, in which two of the sp in components are close together and the third is ca. 125 cm(-1) higher in energy. This unusual occurrence of large zero-field splittings in the tripl et state of an asymmetric top arises because of the close proximity of the (3)A(1)(pi,pi*) state which substantially displaces two of the spin compone nts to lower energy. (C) 2000 American Institute of Physics. [S0021-9606(00 )01105-3].