ELECTRONIC, VIBRATIONAL AND ZEEMAN SPECTRA OF TRIPLET $NO^{-}_{2}$

Author Institution: University of Pennsylvania, Laboratory for Research on Molecular StructureThe lowest energy singlet-triplet transition of $NO^{-}_{2}$ has been studied in emission and absorption in single crystals of $NaNo_{2}$ at $4.2^{\circ}K$. The space symmetry of the lowest triplet state at $18959.5 cm^{-1}$ is shown to be $^{2}B_{1}$. The spin-state sub-structure of the $^{3}B_{4}$ level has been studied using the Zeeman effect, and it is shown that the main portion of the transition $(f \cong 8 \times 10^{-8}) (^{2}B_{1} \leftarrow ^{1}A_{1})$ arises because of the electric-dipole activity of the $|T_{2}>$ spin state. The observed spectrum is therefore mainly of type $B_{2}\leftarrow A_{1}$ which is consistant with the polarization measurements. A small fraction of the intensity is induced by $a b_{2}$ vibration $(\nu^{1}_{3}=1170 cm^{-1})$ and for that portion the $|T_{y}>$ 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 calculation of the spin-orbit matrix elements which suggest that a low energy $^{1}B_{2} (\pi_{2}-\pi_{3})$ state is spin-orbit coupled to the $^{3}B_{1}$ state. The absorption and emission Franck-Condon factors for the prominent angle bending mode $(\nu^{\prime}_{2}=644; \nu""_{2}=828 cm^{-1})$ give an indication that the ONO angle opens by about $14^{\circ}$ on excitation to the $^{3}B_{1}$ state. The fact that the $NO^{-}_{2}$ ion emits from the $^{2}B_{1}$ state following excitation into the $^{1}B_{1}$ state is discussed with emphasis on the nature of radiationless transitions is solids