Observation of b2 symmetry vibrational levels of the SO2 C ̃ 1B2 state: Vibrational level staggering, Coriolis interactions, and rotation-vibration constants

The C ̃ 1B2 state of SO2 has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. However, low-lying levels with odd quanta of antisymmetric stretch (b2 vibrational symmetry) have not previously been observed because transitions into these levels from the zero-point level of the X ̃ state are vibronically forbidden. We use IR-UV double resonance to observe the b2 vibrational levels of the C ̃ state below 1600 cm−1 of vibrational excitation. This enables a direct characterization of the vibrational level staggering that results from the double-minimum potential. In addition, it allows us to deperturb the strong c-axis Coriolis interactions between levels of a1 and b2 vibrational symmetry and to determine accurately the vibrational dependence of the rotational constants in the distorted C ̃ electronic state.