The role of torsional hot bands in modeling atmospheric ethane

The strong and sharp (P)Q(3) subbranch of the v(7) band of ethane near 2976 .8 cm(-1) is relatively free of interfering lines of methane, water and ozo ne and has been utilized as the signature of ethane for atmospheric monitor ing. This subbranch is unresolvable at the Doppler limit, even at low tempe rature. However, the rotational structure and torsional splittings have bee n obtained by subDoppler molecular-beam spectroscopy, along with the air-br oadening coefficients and their temperature dependence, so that the fundame ntal band subbranch can be well characterized, apart from minor perturbatio ns evident at high J. This still leaves a significant fraction of the obser ved structure and intensity in this region unassigned. The strong temperatu re dependence of this extraneous structure enables us to attribute it to to rsional hot bands, which improves the quantitative estimates of atmospheric ethane. We also discuss various approximations to the ethane partition fun ction, accounting for the highly anharmonic torsional mode, needed for the accurate scaling of the intensities over the wide range of atmospheric temp eratures. (C) 1999 Elsevier Science Ltd. All rights reserved.