Spectroscopy of the C2 molecule: Valence and Rydberg states in the 7-10 eV region. An ab initio study

The potential curves of selected valence and Rydberg states of C2 lying in the 7-10 eV region are reported for the first time. The states studied, using MRCI wave funtions, include (2-5)3,1Pu, (1-3)3S+g, (2-5)3S-g, (1-3)3Dg, and relevant quintet states. The f 3S-g, g3Dg, and F1Pu states observed in absorption from 8.88 to 9.25 eV by Herzberg et al. have originally been assumed to be pu ® 3s Rydberg states, with respect to a 3Pu (13Pu) for the triplets and to X1Sg+ for the singlet. Our calculations partially support such assignments: f 3 Sg- corresponding to 33Sg- has mixed valence susgpu 3pg and Rydberg su 2sgpu 23s character, g 3Dg corresponding to 23 Dg has a valence susgpu 3pg structure, while F 1 Pu(21Pu) is confirmed to be a su 2pu 33s state. The calculated f00values for these bands are 0.027 for f¬ a, 0.051 for g ¬ a, and 0.098 for F ¬ X. The singlet-singlet transition F ¬ X constitutes the strongest absorption band reported so far for C2. Our theoretical value agrees with f00(F¬X) = 0.10 ± 0.01 derived indirectly from interstellar data collected by the Hubble Space Telescope. The 13Sg+ (susgpu 3pg) state at Te = 7.57 eV lies much higher than expected by experimentalists.