DIFFERENCE-FREQUENCY LASER SPECTROSCOPY OF THE 3-NU-2-3 BAND OF C2H(NU)

A spectrum of C2H 2PI-2SIGMA+ transition centered at 2928 cm-1 has bee n measured in the gas phase using a high resolution difference-frequen cy infrared laser spectrometer. The C2H molecules are produced in a ho llow cathode discharge through a mixture of C2H2 (approximately 20 mTo rr) and H-2 (approximately 500 mTorr). The discharge amplitude modulat ion technique is used to enhance sensitivity and selectivity. About 12 5 lines are rotationally analyzed and accurate molecular constants are determined through a least-squares fit. The band is assigned to the 3 nu2 + nu3 combination band. The assignment is consistent with the obse rved 2PI-2SIGMA+ symmetry and is in good agreement with the recent the oretical calculation of the vibrational frequency and the spin-orbit c oupling constant. The relatively large spin-orbit coupling constant of the (0 3 1) state is indicative of strong vibronic interaction betwee n the A and X states. A small perturbation appears near the end of obs erved Q-branch series, which is probably caused by a SIGMA vibronic st ate. New assignments of the progressions (0 upsilon2 0) and (0 upsilon 2 1 ) toward higher upsilon2 are proposed based on the existing gas ph ase and rare gas matrix data with the help of recent ab initio theoret ical predictions.