Re-evaluation of ortho-para-dependence of self-pressure broadening in the $v_1+v_3$ band of acetylene

Abstract

Measurements of self-pressure broadened line profiles of the R(8)−R(13) lines in the v 1 +v 3 band of acetylene near 1.52µm are reported. The analysis of the data found no evidence for a previously reported [Iwakuni et al. Phys. Rev. Letts. 117, 143902(5) 2016] systematic alternation in self-pressure broadened line widths with the nuclear spin state of the molecule. The present work brought out the need for a careful accounting of weak background absorptions due to hot-band and lower abundance isotopomer lines as well as the requirement for the use of an accurate line profile model. The data were adequately fit using the quadratic speed-dependent Voigt profile model, and parameters describing the average and speed dependent broadening and shift, and line strengths were determined for each line. Smaller contributions to the full HTP including the rate of velocity-changing collisions and the correlation between velocity-and state-changing collisions were only marginally determined and did not significantly improve the overall quality of the line profile fitting. The results of Iwakuni et al. derived from the use of an approximate (Voigt) line profile model combined with data taken in a regime where peak transmission was very small and pressure effects accounted for only a fraction of the observed line widths.