Laser spectroscopy of the ν1+3ν3 absorption band in 12C2H2.: II.: Self-collisional lineshift measurements

We have recorded the spectrum of the nu(1)+3 nu(3) band in (C2H2)-C-12, around 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 measured simultaneously. In different experiments, the multiple-pass cell was filled with increasing acetylene pressures. Self-collisional lineshifts were determined for more than 30 P and R lines in the band. Line wavenumbers for the stronger lines were separately measured from an optoacoustic spectrum calibrated using the results of a previous investigation in the literature [X. Zhan and L. Halonen, J. Mol. Spectrosc. 160, 464 (1993)] and corrected for their pressure dependence using the presently determined self-collisional lineshifts. Lineshift parameters that are significantly different from 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 intermolecular behavior of the molecule in the upper J=18 rotation level, attributed to an intramolecular Coriolis-type coupling with a nearby state tentatively assigned as containing important excitation in nu(5), the cis-bend mode. (C) 1999 American Institute of Physics. [S0021-9606(99)00241-X].