Absolute intensities and pressure effects in the infrared fundamental bands of methane and its deuterated species

Abstract

The infrared fundamental bands of methane and its deuterated species have been examined in dilute mixtures with helium, at densities from ~100 to ~1000 amagat. There are a total of 24 active fundamentals for CH4, CH3D, CH2D2, CD3H, and CD4, although in some cases only groups of overlapping bands can be observed. For the thirteen separate bands or groups of hands, the density variation of the spectral profiles and the integrated intensities has been studied. The contours have been corrected for slit-width effects by an iterative procedure. There is progressive disappearance of the rotational structure with increasing density, until at the highest pressures only broad, almost featureless bands remain. For the stretching modes, the integrated intensity Ӷ decreases linearly with helium density; the relative decrease is the same for bands at ~3000 cm-l and ~2200 cm-l, and equal to 1.53 x 10~4 / amagat. For the deformation modes, the initial decrease in Ӷat low densities is followed by a region of constant or slightly increasing intensity…