Reinvestigation of the Herzberg-Lagerqvist-Malmberg transitions of C2 in vacuum ultraviolet region and the implications for astronomical observations

The dicarbon radical, C2, is one of the most abundant molecules in the universe, and has been widely observed in various energetic environments. Even though numerous experimental and theoretical investigations on C2 have been done during the last two centuries, spectroscopic study of C2 in vacuum ultraviolet (VUV) region has been rare. The only three known absorption band systems in VUV region were identified by Herzberg and co-workers in 1969 by VUV spectrograph, namely the electronic transitions F1Πu(v′)–X1Σg+(v″), f3Σg−(v′)–a3Πu(v″) and g3Δg(v′)–a3Πu(v″) (Herzberg-Lagerqvist-Malmberg transitions). In this study, we employ a two-photon resonance-enhanced four-wave mixing based VUV laser source and a time-of-flight mass spectrometer for reinvestigating the above three electronic transitions of C2 through a resonant (1VUV + 1′UV) photoionization scheme. Besides those vibronic transitions as identified by Herzberg and co-workers, many more absorption bands belonging to the electronic transitions f3Σg−(v′)–a3Πu(v″) and g3Δg(v′)–a3Πu(v″) are observed with their spectroscopic parameters determined. The rather astrophysically important F1Πu state is not observed here by the resonant (1VUV + 1′UV) photoionization scheme, which must be due to its fast predissociation process. Instead, our study shows that the vibronic band g3Δg(v′ = 2)–a3Πu(v″ = 0) exactly overlaps with F1Πu(v′ = 0)–X1Σg+(v″ = 0), which was not realized in previous studies. The potential implications of these findings to astronomical observations are discussed.