OPTICAL OPTICAL DOUBLE-RESONANCE OF NO2 IN THE REGION OF 612-614 NM - THE ROLE OF 2A1 VIBRONIC LEVELS AS DARK AND PERTURBING STATE

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. © 1990 American Institute of Physics.