The a ¹Δ electronic state is the second lowest electronic state, almost 8000 cm^–1 above ground. It is metastable, fairly long-lived. Transitions in a discharge environment can be fairly strong.
The data have been summarized in
(1) B. J. Drouin, S. Yu, C. E. Miller, H. S. P. Müller, F. Lewen, S. Brünken, and H. Habara, 2010, J. Quant. Spectrosc. Radiat. Transfer 111, 1167.
This work provides new data between 0.84 and 1.95 THz. Additional data were taken from
(2) K. W. Hillig, C. C. W. Chiu, W. G. Read, and E. A. Cohen, 1985, J. Mol. Spectrosc. 109, 205.
Additional data were used with respect to the first entry from Feb. 2010. These were contributed by
(3) B. J. Drouin and H. S. P. Müller, 2010, unpublished.
For multiply measured transitions differing in uncertainties by less than a factor of 2 weighted averaged have been derived.
Predictions should be viewed with caution above ~3.5 THz.
The partition function was calculated as if the lowest rotational level were at an energy of zero. The state is basically unpopulated at thermal equilibrium.
Intensities were calculated based on g values obtained by magnetic resonance by
(4) T. A. Miller, 1971, J. Chem. Phys., 54, 330.