Vibronic analysis of fluorescence spectrum of NO2 D̃ 2B2(0,0,0) in the region of 250–550 nm

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Abstract
The dispersed fluorescence spectrum of NO2 D̃ 2B2(0,0,0) was measured and analyzed in the spectral range of 250–550 nm. The strong fluorescence bands in 250–350 nm correspond to D̃ 2B2(0,0,0)→X̃ 2A1(n1=0–9, n2=0–5, n3=0) with a Franck–Condon maximum at n1=4 and n2=0. The weak and broad bands in 350–410 nm are built on a progression of bending frequency, 710 cm−1. The lower state responsible for this fluorescence was interpreted as admixture evB2 levels generated by a vibronic coupling between a1-vibrational levels on à 2B2 and highly excited b2 levels on X̃ 2A1. The medium-intensity bands in 410–550 nm were assigned to D̃ 2B2(0,0,0)→C̃ 2A2(n1=0–2, n2=0–5, n3=0–2) with a Franck–Condon maximum at n1=0, n2=2, and n3=0. The vibrational frequencies of C̃ 2A2 are 1010 cm−1 for symmetric stretch (ω1), 740 cm−1 for bending (ω2), and 250 cm−1 for antisymmetric stretch (ω3). The simple Franck–Condon calculation for D̃ 2B2(0,0,0)→C̃ 2A2(n1,n2,n3) gives the approximate geometry of the C̃ 2A2 state as r(N–O)∼134 pm and θ∼108°. The partial rotational structure of C̃ 2A2(0,0,0) was analyzed using an optical–optical double resonance measurement, which confirms the A2 vibronic symmetry. The origin of NO2 C̃ 2A2 (T0) was determined to be around 16 234 cm−1.

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