Experimental and theoretical investigation of the vibrational band structure of the 1 5Πu - 1 5Πg high-spin system of C2.

Vibrational levels of the recently observed high-spin transition (1 ⁵Π_u - 1 ⁵&3928;_g) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear doubleresonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d ³Π_g, 3 = 8 and 1 ⁵Π_g, 3 = 3 states results in accurate molecular constants for the 3 = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d ³Π_g, 3 = 8 are improved. The first excited vibrational state of 1 ⁵Π_u is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d ³Π_g, 3 = 6 ~ 1 ⁵Π_g, 3 = 0 system. The rotationally resolved spectra yield 11 transitions to 1 ⁵Π_u, 3 = 1 that include four spinsubstates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, 3 = 1 and 2 of the lower quintet state (1 ⁵Π_g) are measured by performing perturbation-assisted double-resonant excitation to the 1 ⁵Π_u, 3 = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings. [ABSTRACT FROM AUTHOR]