Study of infrared emission spectroscopy for the B1Δg–A1Πu and B′ 1Σg+–A1Πu systems of C2

Thirteen bands for the B1Δg–A1Πu system and eleven bands for the B′1Σg+–A1Πu system of C2 were identified in the Fourier transform infrared emission spectra of hydrocarbon discharges. The B′1Σg+ v = 4 and the B1Δg 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 B1Δg–A1Πu system except for a small perturbation in the B1Δg v = 6 level. The calculated rovibrational term energies up to B1Δg v = 12 showed that the level crossing between the B1Δg and d3Πg states is responsible for many of the prominent perturbations in the Swan system observed previously. Nineteen forbidden transitions of the B1Δg–a3Πu transition were identified and the off-diagonal spin-orbit interaction constant AdB between d3Πg and B1Δg was derived as 8.3(1) cm−1. For the B′1Σg+–A1Πu system, only individual band analyses for each vibrational level in the B′1Σg+ state could be done satisfactorily and Dunham parameters obtained from these effective parameters showed that the anharmonic vibrational constant ωexe is anomalously small (nearly zero). Inspection of the RKR (Rydberg-Klein-Rees) potential curves for the B′1Σg+ and X1Σg+ 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.