Optical Spectroscopic Determination of the Zero-Field Splitting in Vibronic Levels of the Triplet State of Nitrite

We have used conventional high resolution optical spectroscopic methods to determine the principal values of the fine structure tensor for the lowest triplet state (B31 at 18 959 cm−1) of NO2− in Na NO2 single crystals. From the intensities in the Zeeman spectrum we find D − E = 0.86 ± 0.04 cm−1, D + E = 0.40 ± 0.04 cm−1, where the z axis is taken as the twofold axis and E = (Z − X) / 2. From the measurement of line positions in the high-field Zeeman spectrum we find D − E = 0.84 ± 0.05 cm−1, D + E = 0.41 ± 0.05 cm−1. The values of D and E calculated for one-center spin–spin interaction are D − E = 0.42 cm−1 and D + E = 0.24 cm−1. The calculated ordering of the spin states is the same as that observed, but we suggest that there is significant second-order spin–orbit energy associated with the real splittings. Roughly the same splittings are seen for different vibronic levels of B31 associated with ν2′.