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

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.