Study of infrared emission spectroscopy for the B¹Δg-A¹Πu and B'¹Σg+-A¹Πu systems of C2.

Thirteen bands for the B¹Δ_g-A¹Π_u system and eleven bands for the B'¹Σ_g⁺-A¹Π_u system of C₂ were identified in the Fourier transform infrared emission spectra of hydrocarbon discharges. The B'¹Σ_g⁺ v = 4 and the B¹Δ_g v = 6, 7, and 8 vibrational levels involved in nine bands were studied for the first time. A direct global analysis with Dunham parameters was carried out satisfactorily for the B¹Δ_g-A¹Π_u system except for a small perturbation in the B¹Δ_g v = 6 level. The calculated rovibrational term energies up to B¹Δ_g v = 12 showed that the level crossing between the B¹Δ_g and d³Π_g states is responsible for many of the prominent perturbations in the Swan system observed previously. Nineteen forbidden transitions of the B¹Δ_g-a3?u transition were identified and the off-diagonal spin-orbit interaction constant AdB between d³Π_g and B¹Δ_g was derived as 8.3(1) cm-1. For the B'¹Σ_g⁺-A¹Π_u system, only individual band analyses for each vibrational level in the B'¹Σ_g⁺ state could be done satisfactorily and Dunham parameters obtained from these effective parameters showed that the anharmonic vibrational constant w_ex_e is anomalously small (nearly zero). Inspection of the RKR (Rydberg-Klein-Rees) potential curves for the B'¹Σ_g⁺ and X¹Σ_g⁺ states revealed that an avoided crossing or nearly avoided crossing may occur around 30 000 cm-1, which is responsible for the anomalous molecular constants in these two states. [ABSTRACT FROM AUTHOR]