INTERNAL FORCE FIELD DETERMINATION OF ~C1 B2 STATE of SO2

The internal force field of ~{C}$^1$B$_2$ state of chem{SO_2} is determined up to quartic terms. The fit incorporates observed vibrational energy levels of both S$^{16}$O$_2$ and S$^{18}$O$_2$ below 3000 wn, as well as rotational information of both isotopologues. With inclusion of nine recently observed B$_2$ symmetry levels of S$^{16}$O$_2$ in the fit, the double-well potential in asymmetric stretching coordinate can be better characterized. By inspecting the wavefunctions, as well as the basis state distribution of the eigenvectors, we are able to give vibrational assignments to majority of states in this energy region, based on Kellman's semiclassical study on fermi-resonant systems. Our analysis calls into question the validity of previous assignments of several vibrational levels. In addition, the force field allows us to calculate Coriolis matrix elements between vibrational bands, and the calculated values agree well with experimentally derived values. In particular, it predicts and explains why the experimental values are always much smaller than numbers predicted based on a naive harmonic picture. Our work is a first step towards a more complete understanding of the ~{C} state potential energy surface near the equilibrium geometry and it is relevant to the question of how vibronic coupling between ~{C}$^1$B$_2$ state and higher lying $A_1$ state(s) gives rise to unequal S-O bond length