Knowledge Bank

The strong and sharp $Q0$ and subbranches of the $\nu 2$ perpendicular band of $C2H6$ near $3000cm-1$ are located in atmospheric windows and have been observed in numerous remote monitoring measurements of ethane using infrared solar absorption $spectroscopy1$. We report here differnce-frequency laser measurements of the air broadening and shifting of these Q branches at T = 296 and 160 K in order to improve the quantitative analysis of the atmospheric spectra. The air-broadened spectra can be fit to high accuracy by convolving the Doppler-limited spectra with single shifted Lorentzians of $HWHM0.671(13)cm-1/MPaat296K$ and $\gamma \propto Tn$ exponents n = 1.02(2). The peculiar Doppler contours of these Q branches are unresolved, even at low $temperatures2$, so we have recorded their sub-Doppler molecular beam optothermal spectra to be able to model the blended contours. We have resolved torsional tunneling splittings and an anomalous $A1-A2-type$ K doubling in with the individual J components exhibiting distinctive nuclear-spin $weights3$. These splittings are induced by high-order torsional background perturbing states with localized irregularities making it difficult to model the spectra with simple J(J + 1) power series.