Re-evaluation of ortho-para-dependence of self pressure-broadening in the ν 1 + ν 3 band of acetylene

Abstract

Optical frequency comb-referenced measurements of self pressure-broadened line profiles of the R(8) to R(13) lines in the ν1 + ν3 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.

CONTINUING INVESTIGATIONS OF ORTHO-PARA-DEPENDENT PRESSURE BROADENING IN THE ν₁ + ν₃ BAND OF ACETYLENE

“…Our recently published results, using a frequency-comb stabilized laser spectrometer found no experimental evidence of the reported effect in the R(8)−R(13) lines of the band. 4 Analysis of the data needed careful accounting of weak underlying absorptions due to hot-bands and lower abundance isotopomer lines. Detailed modeling of Iwakuni et al results suggest the alternation is an artifact of the fits neglecting collisional narrowing in a strongly absorbing regime.…”