THE (A)OVER-TILDE-(1)A(2) EXCITED-STATE ROTATIONAL G-TENSOR OF THIOFORMALDEHYDE, H2CS

The first-order Zeeman effects of a series of microwave-optical double resonance (MODR) transitions of thioformaldehyde originating in rotat ional levels of the 4(1) A(1)A(2) excited state and ending in highly e xcited rovibronic levels of the X(1)A(1) ground state (X) have been i nvestigated in fluorescence under high resolution. The analysis of sev eral unperturbed and weakly perturbed spectra resulted in the g-tensor elements of the excited state (moments in units of the nuclear magnet on): g(aa)(A) = +4.280(27), (g(bb)(A) + g(cc)(A))= -0.1123(67) (fit I) or g(aa)(A) = +4.281 (27), g(bb)(A) = -0.101(46), g(cc)(A) = -0.013(4 4) (fit II). Special care was taken in determining the absolute signs of the g-values. There is a positive electronic contribution to the g- value around the a-axis, g(aa)(A(e)) = +3.281(27), or equivalently a n egative paramagnetic susceptibility contribution or high-frequency Van Vleck term of xi(aa)(A(p)) = -0.655(5) MHz T-2, results which resist a classical explanation. A value of \[A(1)A(2)\L(a)\X(1)A(1)]\ = 1.200 +8(-0.125)(+0.005) h was deduced for the matrix element of the electro nic angular-momentum component along the a-axis, connecting the first two electronic singlet states of thioformaldehyde. In addition, it is shown that the Zeeman effect can be used to determine whether the thir d level in a double resonance scheme lies lower or higher than the las er-excited fluorescing state, which enables one to establish the corre ct energy term scheme for the X levels.