Diode laser spectroscopic measurement and impact theoretical analysis of collision broadening of acetylene band transitions

A near infrared diode laser spectrometer has been used for measurement of line shapes of the rotational transitions of the () overtone-combination band of acetylene perturbed by oxygen and air. Pressure-broadening coefficients and line strength parameters are extracted from least squares fitting of the spectra with Voigt profiles. A detailed calculation of pressure-broadening coefficients for the self, nitrogen and oxygen broadening cases has been performed by using semi-classical impact theory based on quadrupolar and dispersion interactions. The relative importance of the intermolecular interactions has been investigated and it is found that for these weak interaction cases dispersive terms have a dominant role in line broadening. This is more pronounced for the O₂-broadening case. We have included all the non-resonant terms till convergence is achieved. The cutoff parameter b₀ of the Anderson procedure has been determined separately for each non-resonant process. In order to include the orientation effect of the molecules at the moment of collision we have considered an average kinetic collision diameter which sets the lower limit of b₀. This improves the result considerably and the calculated values are close to the observed results. The J-dependence of the half width is also reported and shows decrease for large J-values. Received 24 November 1999