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

As one of the most important diatomic molecules in the universe, the spectroscopic characterizations of C₂ have attracted wide attention in various fields, such as interstellar chemistry, planetary atmospheric chemistry, and combustion. In recent years, a systematic spectroscopic study of C₂ 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 C₂ from the lower a³Π_u state are experimentally observed in the VUV photon energy range 72000-81000 cm^-1, and 6 new upper vibronic levels of ³Δ_g symmetry are identified, which are assigned as the v' = 0-5 vibrational levels of the 3³Δ_g state of C₂. The term energy T of the 3³Δ_g state is determined to be in the range of 78425-78475 cm^-1 (9.724-9.730 eV) with respect to the ground X¹Σ_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 3³Δ_g state are observed, which is tentatively attributed to the strong perturbations between the 3³Δ_g and 2³Δ_g states, as previously predicted by theory.