SPIN ORBIT-INDUCED AND SYMMETRY-FORBIDDEN C2A2-X2A1 ABSORPTION OF NO2 USING OPTICAL OPTICAL DOUBLE-RESONANCE SPECTROSCOPY IN THE 588-591-NM RANGE

The visible absorption bands of NO2 in the 588-591-nm range have been proved to contain a vibronic transition to the dark C2A2 state by rotational analysis using the optical-optical double-resonance (OODR) techniques. The sequential two-photon excitation to the 2(2)B2 state via the visible excited state has allowed us to determine the rotational quantum numbers (N'Ka,Kc) of six intermediate rovibronic levels. All the levels are assigned as [N'(v1) = odd, K(a) = 0] and [N'(v2) = even, K(a) = 0] from the first-step (v1) and second-step (v2) excitation, respectively. The intermediate state is verified to be a mixed state of dark C2A2 and light A2B2 coupled by spin-orbit interaction with the selection rules DELTA-N = +/- 1 and DELTA-K(a) = 0. Although the C2A2 state is dark in the v1 transition from X2A1, it becomes a light state in the v2 transition to 2(2)B2. We have succeeded to detect a vibronic level of the C2A2 state lying at T(v) = 16970 cm-1 above X2A1 (0,0,0), possibly C2A2 (0,1,0).