Torsional Tunneling andA1–A2Splittings and Air Broadening of therQ0andpQ3Subbranches of the ν7Band of Ethane

• Published in: Journal of molecular spectroscopy 1996-01, Vol.175 (1), p.21-30
• Main Authors: Pine, A.S., Stone, S.C.
• Format: Article
• Language: English
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1006/jmsp.1996.0004

The strong and sharprQ0andpQ3subbranches of the ν7perpendicular band of C2H6near 3000 cm−1are located in atmospheric windows and have been observed in several remote monitoring measurements of ethane using infrared solar absorption spectroscopy. We report here difference-frequency laser measurements of the air broadening and shifting of theseQbranches atT= 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 with broadening coefficients of ∼0.67 cm−1/MPa at 296 K and temperature exponents (γ ∝T−n) ofn∼ 1. The peculiar Doppler contours of theseQbranches are unresolved, even at the lower temperatures. Therefore, in order to model their blended contours, we have recorded their sub-Doppler fine structure using a color-center laser optothermal molecular beam spectrometer. We have resolved torsional tunneling splittings in bothQsubbranches and anomalousA1–A2splittings inpQ3with the individualJcomponents exhibiting distinctive nuclear-spin weights. These splittings are induced by high-order torsional background perturbing states with localized irregularities, making it difficult to model the spectra with simpleJ(J+ 1) power series.