The mechanism of magnetically tuned singlet-triplet avoided crossings in the (A)over-tilde(1)A(2)-(X)over-tilde(1)A(1) 4(0)(1) band of thioformaldehyde H2C=S

Magnetically tuned singlet-triplet perturbations in the 4(1)(A) over tilde( 1)A(2)-2(1)3(1)(a) over tilde(3)A(2) system of thioformaldehyde, Found in o rthorotational states (I = 1, the two hydrogen spins parallel) have been id entified as being caused by vibronic spin-orbit coupling. This perturbation mechanism has been confirmed in several avoided crossings observed in this work for pura stares (I = 0, hydrogen spins antiparallel) which are much s tronger. Parametrization of the theory has led to a quantitative understand ing of the experimental frequency-field relations, and to an accurate predi ction of the rovibrational energies of the triplet stare. This in turn perm itted the detection of about 100 Doppler-limited 2(1)3(1)(a) over tilde(3)A (2)-0(0)(X) over tilde(1)A(1) rovibronic transitions which led into line st ructure states. The combined data was then used to determine a set of rotat ional, fine, and hyperfine tripler-state parameters, the term value T-0(2(1 )3(1)(a) over tilde(3)A(2)) = (16 685.385 +/- 0.002) cm(-1), and the spin-o rbit vibronic singlet-tripler coupling constant, W-ST = (0.0691 +/- 0,0016) cm(-1). A large number of frequency perturbations observed in the crossing s, ranging from 2 to 300 MHz, can be explained with this single parameter. (C) 2000 Academic Press.