Experimental identification of the high-lying 4³Δ_g state of C₂ in the vacuum ultraviolet region

Spectroscopic study of C₂, an important homonuclear diatomic molecule, has attracted considerable attention over the past 200 years owing to its significant applications in both the comprehension of chemical bond theory and astronomical observation. Despite substantial achievements in experimental and theoretical identifications of the low-lying electronic transitions of C₂ in the UV, visible and infrared regions, originating either from its ground singlet <i>X</i>¹Σ+g or triplet <i>a</i>³Π_u state, further spectroscopic investigation in the vacuum ultraviolet (VUV) region is essential, particularly for identifying its high-lying excited states in the energy region from its dissociation to ionization threshold. In this study, we reported the identification of a new high-lying excited state of C₂ originating from the <i>a</i>³Π_u state through the 1VUV + 1'UV resonance-enhanced multiphoton ionization scheme. A total of 13 absorption bands of type ³Δ_g-³Π_u in the VUV photon energy range of 75 500-82 400 cm^-1 were observed, and the upper electronic state was identified as 4³Δ_g of C₂. The term value was determined to be 83 135.1 cm^-1, indicating the highest experimentally identified electronic state of C₂ thus far. The spectroscopic parameters were accurately determined, implying that the 4³Δ_g state might be a valence state instead of a Rydberg-type electronic state. The current spectroscopic study should guide future theoretical investigations on the high-lying electronic structure of C₂.