The Ballik-Ramsay, Mulliken, Deslandres-d'Azambuja and Phillips systems in C 2: a theoretical study of their electronic transition moments

Abstract

An ab initio MRD CI study of the ground and excited states of the C 2 molecule is reported which employs a large DZP AO basis. Special emphasis is given to the computation of intensity distributions (lifetimes) associated with the Ballik-Ramsay (b 3Σ g --a 3Π u), Mulliken (D 1Σ u +-X 1Σ g -). Deslandres-d'Azambuja (DA. C 1Π g-A 1Π u) and Phillips (A 1Π u-X 1Σ g -) band systems known from its molecular spectrum. Good agreement is found between calculation and experiment for bond lengths, vibrational frequencies and T c values characterizing the pertinent C 2 electronic states. The intensity computed for the Ballik-Ramsay transition is also in good accord with that found in shock-tube measurements by Cooper and Nicholls (CN), a result which is consistent with experience obtained in studying other triplet-triplet transitions of C 2 such as the Swan and Fox-Herzberg bands. For the corresponding singlet-singlet transitions there is notably larger variance between the presently calculated lifetimes and intensities and those inferred by experiment, whereby in these cases different measurements are often found to disagree with one another. In the case of the Mulliken system the calculated ∫ value agrees well with an early measurement of Smith but is larger by a factor of three than that obtained in the more recent CN shock-tube experiments. The DA computed intensity is also roughly three times greater than measured by CN, but in the case of the Phillips bands the corresponding comparison between calculations and experiment is better, with the computed lifetimes of the 1Π u upper state as a function of ν′ lying between the respective measured data of CN and Hubrich et al. A possible source of the above discrepancies is suggested to involve cascading effects from high-lying singlet species into the upper states of the Mulliken. DA and Phillips systems, especially since analogously competing allowed transitions are notably lacking in the C 2 triplet manifold for which the agreement between calculated and measured lifetimes and intensities is generally good.