Rotational analysis of the 6480 Å absorption of NO2

Abstract

About 300 rotational transitions in the 6480 Å region in the visible absorption of NO2 gas have been assigned by laser-excited fluorescence. A majority of the stronger transitions belong to the K = 0 and K = 1 subbands of a vibronic band, T0 = 15 434.9 cm−1, whose upper state is predominantly an a1, vibrational level of the electronic 2B2 basis state. Other groups of relatively weaker lines form (i) a severely-perturbed K = 2 subband attributed to the same series as the K = 0 and 1 subbands; (ii) a well-developed though relatively weak K = 3 subband assigned to a hybrid level of mainly 2A1, character, the transition to this level being induced by vibronic coupling; and (iii) a K = 6 subband assigned to a parent (i.e., 2B2 basis) vibrational level different from that identified at 15 435 cm−1. The spectrum in this region abundantly illustrates the irregularities, 'extra' lines, resonance crossings, and erratic spin splitting now recognized as widespread in the NO2 visible absorption. Rotational constants are not well defined and vary considerably from one subband to another: large pseudo-centrifugal distortion constants are attributed to higher-order effects of the vibronic coupling. Franck–Condon analysis of the intensity distribution in fluorescence leads to a tentative vibrational assignment of 1 60 for the stale at 15 435 cm−1.