Difference-frequency laser spectroscopy of the 3ν2+ν3 band of C2H

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Abstract
A spectrum of C2H 2Π–2Σ+ transition centered at 2928 cm−1 has been measured in the gas phase using a high resolution difference-frequency infrared laser spectrometer. The C2H molecules are produced in a hollow cathode discharge through a mixture of C2H2 (∼20 mTorr) and H2 (∼500 mTorr). The discharge amplitude modulation technique is used to enhance sensitivity and selectivity. About 125 lines are rotationally analyzed and accurate molecular constants are determined through a least-squares fit. The band is assigned to the 3ν2+ν3 combination band. The assignment is consistent with the observed 2Π–2Σ+ symmetry and is in good agreement with the recent theoretical calculation of the vibrational frequency and the spin–orbit coupling constant. The relatively large spin–orbit coupling constant of the (0 3 1) state is indicative of strong vibronic interaction between the à and X̃ states. A small perturbation appears near the end of observed Q-branch series, which is probably caused by a Σ vibronic state. New assignments of the progressions (0 v2 0) and (0 v2 1) toward higher v2 are proposed based on the existing gas phase and rare gas matrix data with the help of recent ab initio theoretical predictions.