Temperature-dependent photoabsorption cross sections in the VUV-UV region. I. Methane and ethane

Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of methane (CH4) and ethane (C2H6) from their respective absorption thresholds to 120 nm, with a spectral bandwidth (FWHM) of 0.06 nm and at three different temperatures, i.e., 370, 295, and 150 K. Only moderate temperature effects are observed in the changes of cross-section values of these two molecules and are attributed to their high vibrational frequencies of the ground electronic states and their repulsive potential surfaces of the excited electronic states. When the gas temperature decreases from 360 to 150 K, the percentage changes of cross sections amount to a maximum of +/-30% in CH4 at 142.5 mm and +/-20% in C2H6 at 142.3 and 155 nm. The well-known vibrational progressions of C2H6 exhibit pronounced temperature effects in their band profiles which become narrower and sharper as the gas temperature decreases. The data presented are an extension of our effort to provide the required data to the planetary atmospheres community and will have an important impact on our understanding of the atmospheres of the giant planets. (C) 2003 Elsevier Ltd. All rights reserved.