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

Thirteen bands for the B-1 Delta(g)-A(1)Pi(u) system and eleven bands for the B'(1)Sigma(+)(g)-A(1)Pi(u) system of C-2 were identified in the Fourier transform infrared emission spectra of hydrocarbon discharges. The B'(1)Sigma(+)(g) v = 4 and the B-1 Delta(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 B-1 Delta(g)-A(1)Pi(u) system except for a small perturbation in the B-1 Delta(g) v = 6 level. The calculated rovibrational term energies up to B-1 Delta(g) v = 12 showed that the level crossing between the B-1 Delta(g) and d(3)Pi(g) states is responsible for many of the prominent perturbations in the Swan system observed previously. Nineteen forbidden transitions of the B-1 Delta(g)-a(3)Pi(u) transition were identified and the off-diagonal spin-orbit interaction constant A(dB) between d(3)Pi(g) and B-1 Delta(g) was derived as 8.3(1) cm(-1). For the B'(1)Sigma(+)(g)-A(1)Pi(u) system, only individual band analyses for each vibrational level in the B'(1)Sigma(+)(g) state could be done satisfactorily and Dunham parameters obtained from these effective parameters showed that the anharmonic vibrational constant omega(e)chi(e) is anomalously small (nearly zero). Inspection of the RKR (Rydberg-Klein-Rees) potential curves for the B'(1)Sigma(+)(g) and X-1 Sigma(+)(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. (C) 2016 AIP Publishing LLC.