Electronic, Vibrational, and Zeeman Spectra of Triplet NO2

The lowest-energy singlet–triplet transition of NO2− has been studied in emission and absorption in single crystals of NaNO2 at 4.2°K. The space symmetry of the lowest triplet state at 18 959 cm−1 is shown to be B13. This assignment is in agreement with molecular orbital predictions. The spin-state substructure of the B13 level has been studied using the Zeeman effect, and it is shown that the main portion of the transition (f ∼ 8 × 10−8) (3B1←1A1) arises because of the electric-dipole activity of the |Tz〉 spin state. The observed spectrum is therefore mainly of type B2←A1, which is consistent with the polarization measurements. A small fraction of the intensity is induced by a b2 vibration (ν3′ = 1170 cm−1), and for that portion the |Ty〉 spin states of each vibronic level carry the electric-dipole intensity, possibly by virtue of a vibronic spin–orbit coupling mechanism. The assignments are confirmed by calculations of the spin–orbit matrix elements which suggest that a low-energy B21(π2→π3*) state is spin–orbit coupled to the B13 state. The absorption and emission Franck–Condon factors for the prominent angle bending mode (ν2′ = 644; ν2″ = 828 cm−1) gave an indication that the ONO angle opens by ∼15° on excitation to the B13 state. The fact that the NO2− emits only very weakly from the B13 state following excitation into the B11 state, while the emission from the B13 state is ∼ 7 times stronger when excitation is to the A21 state, is discussed in some detail, with special emphasis on the nature of radiationless transitions in solids.