Perturbation-facilitated detection of the first quintet-quintet band in C

The first high-spin transition in C (1 Π - 1 Π) is observed by perturbation-facilitated optical-optical double resonance spectroscopy. The experiment is performed by applying unfolded two-color resonant four-wave mixing. C radicals in the initial a Π, v = 5 state are produced by using a discharge source in a molecular beam environment. The final quintet state is excited via intermediate "gateway" states exhibiting both substantial triplet and quintet character due to a perturbation between the 1 Π, v = 0 and the d Π, v = 6 states. Fifty seven rotational transitions in the P, Q, and R branches of all spin sub-states are measured and yield accurate molecular constants of the newly found upper level 1 Π. In addition, satellite transitions (ΔJ ΔN) are observed and allow an accurate determination of the spin-orbit constant. The results are compared with high-level ab initio computations at the multi-reference configuration interaction level of theory. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only.