Laser spectroscopy of the nu(1)+3 nu(3) absorption band in (C2H2)-C-12. II. Self-collisional lineshift measurements

We have recorded the spectrum of the nu(1)+3 nu(3) band in (C2H2)-C-12, aro und 12676 cm(-1), using an autoscan Ti:Sa spectrometer. Pressure-dependent line positions were determined from an absorption spectrum recorded using a long White-type multiple-pass cell. An optoacoustic cell with a constant, low acetylene pressure allowed reference line position wavenumbers to be me asured simultaneously. In different experiments, the multiple-pass cell was filled with increasing acetylene pressures. Self-collisional lineshifts we re determined for more than 30 P and R lines in the band. Line wavenumbers for the stronger lines were separately measured from an optoacoustic spectr um calibrated using the results of a previous investigation in the literatu re [X. Zhan and L. Halonen, J. Mol. Spectrosc. 160, 464 (1993)] and correct ed for their pressure dependence using the presently determined self-collis ional lineshifts. Lineshift parameters that are significantly different fro m the overall behavior in the band are unexpectedly observed for the R(17) and P(19) lines. This result is explained in terms of a different intermole cular behavior of the molecule in the upper J=18 rotation level, attributed to an intramolecular Coriolis-type coupling with a nearby state tentativel y assigned as containing important excitation in nu(5), the cis-bend mode. (C) 1999 American Institute of Physics. [S0021-9606(99)00241-X].