Knowledge Bank

The lowest energy singlet-triplet transition of $NO2-$ has been studied in emission and absorption in single crystals of $NaNo2$ at $4.2\circ K$. The space symmetry of the lowest triplet state at $18959.5cm-1$ is shown to be $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$. The spin-state sub-structure of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B4$ level has been studied using the Zeeman effect, and it is shown that the main portion of the transition $(f\cong 8\times 10-8)(2B1\leftarrow 1A1)$ arises because of the electric-dipole activity of the $|T2>$ spin state. The observed spectrum is therefore mainly of type $B2\leftarrow A1$ which is consistant with the polarization measurements. A small fraction of the intensity is induced by $ab2$ vibration $(\nu 31=1170cm-1)$ 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 calculation of the spin-orbit matrix elements which suggest that a low energy $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2(\pi 2-\pi 3)$ state is spin-orbit coupled to the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$ state. The absorption and emission Franck-Condon factors for the prominent angle bending mode $(\nu 2\prime =644;\nu ""2=828cm-1)$ give an indication that the ONO angle opens by about $14\circ$ on excitation to the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$ state. The fact that the $NO2-$ ion emits from the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$ state following excitation into the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1$ state is discussed with emphasis on the nature of radiationless transitions is solids.