High resolution slit-jet infrared spectroscopy of ethynyl radical: 2Π-2Σ+ vibronic bands with sub-Doppler resolution.

High resolution infrared spectra for four (2)Π-(2)Σ(+) bands of jet-cooled ethynyl radical (i.e., C(2)H) in the gas phase are reported. The combination of (i) slit-jet cooling (T(rot) ≈ 12 K) and (ii) sub-Doppler resolution (≈ 60 MHz) permits satellite branches in each (2)Π-(2)Σ(+) band to be observed and resolved for the first time as well as help clarify a systematic parity misassignment from previous studies. The observed lines in each band are least squares fit to a Hamiltonian model containing rotational, spin-rotational, spin-orbit, and lambda-doubling contributions for the (2)Π state, from which we report revised excited state constants and band origins for the observed bands. Three of the four bands fit extremely well within a conventional (2)Π model (i.e., σ < 20 MHz), while one band exhibits a local perturbation due to an avoided crossing with a near resonant dark state. Vibronic assignments are given for the observed bands, with the dark state clearly identified as a highly excited stretch and bending overtone level X̃ (1,2(2),0) by comparison with high level ab initio efforts.