Spectroscopic Study of a New Electronic Band System 3³Δ_g-a³Π_u of C₂

Abstract

As one of the most important diatomic molecules in the universe, the spectroscopic characterizations of C 2 have attracted wide attention in various fields, such as interstellar chemistry, planetary atmospheric chemistry, and combustion. In recent years, a systematic spectroscopic study of C 2 in the vacuum ultraviolet (VUV) region has been carried out in our laboratory by using the (1VUV+1′UV) resonance-enhanced multiphoton ionization method based on the combination of a tunable VUV laser source and a time-of-flight mass spectrometer. Two new electronic transition band systems have been reported, following the pioneering work of Herzberg and co-workers in 1969. In the current study, a total of 18 vibronic transition bands of C 2 from the lower a 3 Π u state are experimentally observed in the VUV photon energy range 72000−81000 cm −1 , and 6 new upper vibronic levels of 3 Δ g symmetry are identified, which are assigned as the v′ = 0− 5 vibrational levels of the 3 3 Δ g state of C 2 . The term energy T e of the 3 3 Δ g state is determined to be in the range of 78425−78475 cm −1 (9.724−9.730 eV) with respect to the ground X 1 Σ g + state, and the molecular constants such as vibrational and rotational constants are also determined, which are in reasonable agreement with those predicted by high-level ab initio theoretical calculations. Irregular vibrational energy level spacings in the 3 3 Δ g state are observed, which is tentatively attributed to the strong perturbations between the 3 3 Δ g and 2 3 Δ g states, as previously predicted by theory.