Quantum chemical study and experimental observation of a new band system of C2, e 3Πg-c 3Σu+

A new band system of C-2, e (3)Pi(g)-c (3)Sigma(+)(u) was studied by ab initio quantum chemical and experimental methods. The calculations were carried out at the multireference configuration interaction level of theory with Davidson's correction using aug-cc-pV6Z basis set and include core and core-valence correlation as well as relativistic corrections computed with aug-cc-pCVQZ and cc-pVQZ bases, respectively. The vibrational energies and rotational constants of the upper e (3)Pi(g) state were calculated from the computed ab initio potential energy curve. The ab initio results indicate that the electronic transition moment of the e (3)Pi(g)-c (3)Sigma(+)(u) system is approximately one-half that of the Fox-Herzberg e (3)Pi(g)-a (3)Pi(u) system. Franck-Condon factors were calculated for both systems and used to guide experiments aimed at discovering the e (3)Pi(g)-c (3)Sigma(+)(u) system. The e (3)Pi(g)(v(')=4)-c (3)Sigma(+)(u)(v(')=3) band of jet-cooled C-2 was successfully observed by laser-induced fluorescence spectroscopy by monitoring the ensuing e (3)Pi(g)-a (3)Pi(u) emission.