Laser absorption spectroscopy of the d3Π(g) ← c3Σ(u)+ transition of C2.

High-resolution laser absorption spectroscopy using concentration modulation has been applied to record the near-infrared spectrum of C2 in an AC hollow-cathode discharge of acetylene/helium mixtures. The (2,1) and (1,0) vibronic bands of the d3Π(g)– c3Σ(u)+ system have been observed in the spectral region between 12,010 and 12,540 cm(–1). While the analysis of the (1,0) band was straightforward, the (2,1) band was found to be perturbed. Using the effective Hamiltonians for 3Π and 3Σ+ states, molecular constants for the vibrational levels of the c3Σ(u)+ state were retrieved in least-squares fits of the observed spectral lines. The experimental conditions, detailed analysis of the perturbations, and the determined molecular constants are reported.