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 ${}^3{B}_1$ . This assignment is in agreement with molecular orbital predictions. The spin‐state substructure of the ${}^3{B}_1$ level has been studied using the Zeeman effect, and it is shown that the main portion of the transition ${\text{(f\hspace{0.17em}∼\hspace{0.17em}8\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−8}}{\mathrm{)\; (}}^3{B}_1{\leftarrow }^1{A}_1)$ arises because of the electric‐dipole activity of the ${\mathrm{|T}}_z〉$ spin state. The observed spectrum is therefore mainly of type $B_2{\mathrm{\leftarrow A}}_1$ , which is consistent with the polarization measurements. A small fraction of the intensity is induced by a $b_2$ vibration ${\mathrm{(\nu }}_3\text{′\hspace{0.17em}=\hspace{0.17em}1170}{\mathrm{cm}}^{\text{−1}})$ , and for that portion the ${\mathrm{|T}}_y〉$ 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 ${}^1{B}_2{\mathrm{(\pi }}_2{\mathrm{\to \pi }}_3\mathrm{*)}$ state is spin–orbit coupled to the ${}^3{B}_1$ state. The absorption and emission Franck–Condon factors for the prominent angle bending mode ${\mathrm{(\nu }}_2{\text{′\hspace{0.17em}=\hspace{0.17em}644; ν}}_2\text{″\hspace{0.17em}=\hspace{0.17em}828}{\mathrm{cm}}^{\text{−1}})$ gave an indication that the ONO angle opens by ∼15° on excitation to the ${}^3{B}_1$ state. The fact that the NO₂⁻ emits only very weakly from the ${}^3{B}_1$ state following excitation into the ${}^1{B}_1$ state, while the emission from the ${}^3{B}_1$ state is ∼ 7 times stronger when excitation is to the ${}^1{A}_2$ state, is discussed in some detail, with special emphasis on the nature of radiationless transitions in solids.