Knowledge Bank

We have measured absolute line intensities in two bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C2H2$ near $7.5\mu m$, namely the $\nu 4+\nu 5(\Sigma u+)-0(\Sigma g+)$ and $\nu 4+\nu 5(\Delta u)-0(\Sigma g+)$ bands, using Fourier transform spectroscopy with an accuracy estimated to be better than 2%. Using theoretical predictions from $Watsona$, the observation of the forbidden $\nu 4+\nu 5(\Delta u)-0(\Sigma g+)$ band and the Herman-Wallis behavior exhibited by its rotational lines were studied quantitatively in terms of two types of interactions affecting the levels involved by the band: $\ell$-type resonance and Coriolis interaction. In the case of the $\nu 4+\nu 5(\Sigma u+)-0(\Sigma g+)$ band, the influences of $\ell$-type resonance is also confirmed. We also attributed the intensity asymmetry observed between the R and P branches of that latter band to a Coriolis interaction with $\ell =1$ levels. We did not observe the $\nu 4+\nu 5(\Sigma u-)-0(\Sigma g+)$ band, consisting only of a Q branch, in agreement with Watson's prediction.