à 2Π←X̃ 2Σ+ infrared electronic transition of C2H

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Abstract
Five bands of the à 2Π←X̃ 2Σ+ lowest electronic transition of C2H have been observed in the region 3600–4200 cm−1 in an argon discharge over polyacetylene using color center laser spectroscopy with magnetic rotation sensitivity enhancement. The band origins are approximately 3692, 3773, 3785, 4011, and 4106 cm−1. Rotational assignments and preliminary fittings of all five bands have been made. Four of the bands have a 2Σ+ lower vibronic state which is undoubtedly the ground vibronic state of the molecule, and different 2Π upper vibronic states which correspond to mixtures of the lowest vibronic state of the à electronic state and three different vibrationally excited levels of the X̃ electronic state with an odd number of quanta in the bending mode and overall 2Π symmetry. These three ground state vibrational levels have been tentatively assigned as (1,1,0), (0,1,2), and (0,5,1). The four 2Π vibronic levels are mixed by vibronic coupling between the X̃ and à electronic states thereby providing electronic oscillator strength for the vibrational transitions. The fifth band is a 2Σ−←2Π hot band with the lower state the first excited bending level (0,1,0) of the ground electronic state and upper state the 2Σ− component of the (0,1,0) excited bending level of the excited electronic state. From these observations a value of 1.30 Å for the CC bond length in the excited electronic state and a value of ∼375 cm−1 for the bending frequency of the X̃ ground electronic state are obtained.