Dj. Clouthier et al., SUB-DOPPLER SPECTROSCOPY OF THIOFORMALDEHYDE - EXCITED-STATE PERTURBATIONS AND EVIDENCE FOR ROTATION-INDUCED VIBRATIONAL MIXING IN THE GROUND-STATE, The Journal of chemical physics, 101(9), 1994, pp. 7300-7310
High-resolution intracavity dye laser spectroscopy has been used to ob
tain sub-Doppler spectra of transitions to 350 rotational levels in th
e 4(0)(1) band of the ($) over tilde A (1)A(2)-($) over tilde X (1)A(1
) electronic transition of thioformaldehyde. Ground state combination
differences from the sub-Doppler spectra, combined with microwave and
infrared data, have been used to improve the ground state rotational a
nd centrifugal distortion constants of H2CS. The upper state shows a r
emarkable number of perturbations. The largest of these are caused by
nearby triplet levels, with matrix elements of 0.05-0.15 cm(-1). A par
ticularly clear singlet-triplet avoided crossing in K-a' = 7 has been
shown to be caused by interaction with the F-1 component of the 3(1)6(
2) vibrational level of the ($) over tilde a (3)A(2) state. At least 5
3% of the S-1 levels show evidence of very small perturbations by high
rovibronic levels of the ground state. The number of such perturbatio
ns is small at low J, but increases rapidly beyond J=5 such that 40%-8
0% of the observed S-1 levels of any given J are perturbed by ground s
tate levels. Model calculations show that the density and J dependence
of the number of perturbed levels can be explained if there is extens
ive rotation-induced mixing of the vibrational levels in the ground st
ate.