Continuing Investigations Of Ortho-para-dependent Pressure Broadening In The ν₁ + ν₃ Band Of Acetylene

\begin{wrapfigure}{l}{0pt} \includegraphics[scale=0.32]{gamma.eps} \end{wrapfigure} In 2016, Iwakuni et al.\footnotemark[1] measured the $\nu_1 + \nu_3$ vibrational combination band of acetylene using a dual-frequency comb based spectrometer. They reported an alternation in the self-pressure broadening coefficients of even and odd rotational levels, which correspond to the para- and ortho- nuclear spin states. This can occur if relaxation involving resonant energy transfer between molecules with common nuclear spin symmetry is important, because ortho-ortho collisions are statistically more probable than para-para ones. Subsequently several authors\footnotemark[2] \footnotemark[3] have disputed these findings. 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.\footnotemark[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. Further low temperature measurements have been planned to try to limit influence of the hot-band lines. \footnotetext{1. K. Iwakuni, S. Okubo, K. M. T. Yamada, H. Inaba, A. Onae, F.-L. Hong, and H. Sasada, \textit{Phys. Rev. Lett.} \textbf{117}, (2016).} \footnotetext{2. K. K. Lehmann, \textit{J. Chem. Phys.} \textbf{146}, 094309 (2017).} \footnotetext{3. J.-M. Hartmann and H. Tran, \textit{Phys. Rev. Lett.} \textbf{119}, (2017).} \footnotetext{4. E.C. Gross, K. A. Tsang, and T.J. Sears, \textit{J. Chem. Phys.} \textbf{154}, 054305 (2021).