Optical–optical double resonance of NO2 in the region of 612–614 nm: The role of 2A1 vibronic levels as dark and perturbing state

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Abstract
An optical–optical double resonance (OODR) technique has been applied to the rotational analysis and vibronic assignment of NO2 absorption band in the region of 612–614 nm. The two step excitation through 2 2B2←2B2←X̃ 2A1 has allowed us to determine rotational quantum numbers (NKa,Kc) for 73 eigenstates with B2 vibronic symmetry, lying at 16 306–16 465 cm−1 above the ground state. Although they are severely perturbed and irregular in the rotational structure and spin doubling, we can classify the rovibronic levels as four stacks; two Ka=0 stacks with subband origins of 16 306.2 and 16 321.0 cm−1, and two Ka=1 stacks with origins of 16 312.5 and 16 326.0 cm−1. A near-prolate asymmetric top approximation is used to obtain the term values and rotational constants. Extraordinary large DN measured for 2B2 vibronic levels can be understood by well-known, strong vibronic coupling between à 2B2 and highly excited vibrational levels of X̃ 2A1. Among a number of perturbations observed, the spin–orbit (and/or orbital–rotation) coupling between ‘‘light’’ 2B2 and ‘‘dark’’ 2A1 vibronic levels is clearly shown for the first time by analyzing intensity patterns of the ν2 -scanned (second step) OODR spectra.