Spectroscopy of the C2molecule: Valence and Rydberg states in the 7–10 eV region. An ab initio study

Abstract

The potential curves of selected valence and Rydberg states of C2lying in the 7–10 eV region are reported for the first time. The states studied, using MRCI wave funtions, include (2–5)3,1Πu, (1–3)3Σ+g, (2–5)3Σ–g, (1–3)3Δg, and relevant quintet states. The f3Σ–g, g3Δg, and F1Πustates observed in absorption from 8.88 to 9.25 eV by Herzberg et al. have originally been assumed to be πu® 3s Rydberg states, with respect to a3Πu(13Πu) for the triplets and to X1Σg+for the singlet. Our calculations partially support such assignments: f3Σg–corresponding to 33Σg–has mixed valence σuσgπu3πgand Rydberg σu2σgπu23s character, g3Δgcorresponding to 23Δghas a valence σuσgπu3πgstructure, while F1Πu(21Πu) is confirmed to be a σu2πu33s state. The calculated f00-values 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 13Σg+(σuσgπu3πg) state at Te= 7.57 eV lies much higher than expected by experimentalists. PACS Nos.: 31.20T, 31.50, 33.20N, 33.70.PACS Nos.: 31.20T, 31.50, 33.20N, 33.70.