ROTATIONAL SPECTROSCOPY OF THIOFORMALDEHYDE, H$_2$CS, IN ITS FOUR LOWEST EXCITED VIBRATIONAL, CORIOLIS-COUPLED STATES

Author Institution: I. Physikalisches Institut, Universitat zu Koln, 50937 Koln, Germany; Department of Physics, The Ohio State University, Columbus, OH 43210, USARotational spectra of vibrationally excited states of H$_2$CS below 1500 cm$^{-1}$ have been obtained by long-path absorption spectroscopy between 120 and currently 670 GHz. The highest $K_a$ accessed range from 9 to 12 and correspond to combined rotational and vibrational energies of almost 2400 cm$^{-1}$. The $v_4 = 1$ (990.185 cm$^{-1}$) and $v_6 = 1$ (991.019 cm$^{-1}$) states are almost degenerate and undergo very strong Coriolis interaction ($\zeta_{46}^a = 0.5148$) which causes $K_a > 0$ to be mixed completely. The $v_3 = 1$ state is at 1059.204 cm$^{-1}$ and the first order Coriolis terms are smaller ($\zeta_{36}^c = 0.2743$ and $\zeta_{34}^b = 0.0574$) so that perturbations are more local. Finally, the $v_2 = 1$ state (1455.496 cm$^{-1}$) is comparatively far away from the three lower states, but there is a large coupling term with $v_4 = 1$: $\zeta_{24}^b = 0.8555$. Because of the large energy difference, only higher order effects are pronounced in this case. The initial assignments were greatly facilitated by previous infrared studies on $\nu_4$, $\nu_6$, and $\nu_3$} {\bf 84} (1980) 531$-$558.}} {\bf 88} (1981) 402$-$419.} as well as on $\nu_2$.} {\bf 159} (1993) 340$-$349.} The results from these as well as previous and present pure rotational studies on the ground vibrational state were considered in the global fit. The very satifactory fit required a modest number of vibrational corrections for the four vibrationally excited states and a comparatively large number of Coriolis interaction term