Dark state vibronic coupling in the A(2[capital Pi]) ? [X with combining tilde](2[capital Sigma]+) band of ethynyl radical via high resolution infrared absorption spectroscopy

The high resolution infrared spectrum for the A (2[capital Pi]) ? [X with combining tilde] (2[capital Sigma]+) origin band of jet-cooled ethynyl radical (C2H) in the gas phase is reported, which exhibits a strong, parity-specific local perturbation in the upper 2[capital Pi]1/2 state. Based on revised parity assignments of the levels, the perturbing state is unambiguously determined to be 2[capital Sigma]+ symmetry, and thus coupled to the A (2[capital Pi]) state by [capital Delta]K = +/-1 Coriolis interactions. By incorporating [capital Sigma]-[capital Pi] Coriolis coupling into the unperturbed Hamiltonian (containing only rotational, spin-rotational, spin-orbit, and lambda-doubling contributions), we are now able to fit the observed 2[capital Pi]-2[capital Sigma]+ origin band to a sub Doppler experimental uncertainty of 15 MHz (0.0005 cm-1). In addition, the observation of pairs of transitions to mixed states permits determination of the band origin ([small nu]pert) and rotational constant (Bpert) for the "dark" 2[capital Sigma]+ state, which prove to be in remarkably quantitative agreement with full vibronic predictions of Tarroni and Carter as well as UV dispersed fluorescence studies of Hsu et al. This represents an important benchmark in mapping out non-Born-Oppenheimer vibronic interactions and energy level structure in a polyatomic combustion radical system, an understanding of which will be key to modeling chemical reactions in both terrestrial and astronomical environments.