Re-evaluation of ortho-para-dependence of self pressure-broadening in the ν + ν band of acetylene.

Optical frequency comb-referenced measurements of self pressure-broadened line profiles of the R(8) to R(13) lines in the ν + ν combination band of acetylene near 1.52 µm are reported. The analysis of the data found no evidence for a previously reported [Iwakuni et al., Phys. Rev. Lett. 117(14), 143902 (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 the use of an accurate line profile model and careful accounting for weak background absorptions due to hot band and lower abundance isotopomer lines. The data were adequately fit using the quadratic speed-dependent Voigt profile model, neglecting the small speed-dependent shift. Parameters describing the most probable and speed-dependent pressure-broadening, most probable shift, and the line strength were determined for each line. Detailed modeling of the results of Iwakuni et al. showed that their neglect of collisional narrowing due to the speed-dependent broadening term combined with the strongly absorbing data recorded and analyzed in transmission mode were the reasons for their results.