THE ABSORPTION-SPECTRUM OF C2H2 AROUND NU(1)+NU(3) - ENERGY STANDARDS IN THE 1.5 MU-M REGION AND VIBRATIONAL CLUSTERING

We have recorded the Fourier transform absorption spectrum of acetylene, C2H2, at high resolution, around 6500 cm(-1). The positions of the strongest rovibrational lines are measured with respect to the rovibrational lines in 3-0 of CO. They provide secondary calibration standards in that range with an accuracy of 3 x 10(-4) cm(-1). The rotational analysis of the data gives evidence of five vibrational levels of Sigma(u)(+) symmetry, in addition to the bright combination level (1010(0)0(0)). This is demonstrated to strictly fit the predicted anharmonic resonance pattern in that region, which permits the vibrational assignment of those extra transitions. Study of the relative intensities of the reported vibrational transitions suggests the need to include new quartic anharmonic couplings. This is supported by the rovibrational analysis of the cold bands around 8500 cm(-1), involving the (1110(0)0(0)) bright level, which is also presented.