THE ABSORPTION-SPECTRUM OF C2H2 AROUND NU(1)- ENERGY STANDARDS IN THE1.5 MU-M REGION AND VIBRATIONAL CLUSTERING(NU(3) )

We have recorded the Fourier transform absorption spectrum of acetylen e, C2H2, at high resolution, around 6500 cm(-1). The positions of the strongest rovibrational lines are measured with respect to the rovibra tional lines in 3-0 of CO. They provide secondary calibration standard s 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 Sigm a(u)(+) symmetry, in addition to the bright combination level (1010(0) 0(0)). This is demonstrated to strictly fit the predicted anharmonic r esonance pattern in that region, which permits the vibrational assignm ent of those extra transitions. Study of the relative intensities of t he reported vibrational transitions suggests the need to include new q uartic anharmonic couplings. This is supported by the rovibrational an alysis of the cold bands around 8500 cm(-1), involving the (1110(0)0(0 )) bright level, which is also presented.