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 vibronically 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 triplet state Hamiltonian that allowed for slight variations in the geometries of the individual spin components. The results show that the triplet state of selenoformaldehyde involves case (ab) coupling, in which two of the spin 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 triplet 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 components to lower energy. (C) 2000 American Institute of Physics. [S0021-9606(00)01105-3].