Near-infrared spectra of laboratory H 2O–CH 4 ice mixtures

Abstract

We present 1.25–19 μm infrared spectra of pure solid CH 4 and H 2O/CH 4=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH 4 in solid H 2O with those of pure CH 4. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH 4 in solid H 2O are estimated. Using the two largest ( ν 3 + ν 4 ) and ( ν 1 + ν 4 ) near-IR absorptions of CH 4 at 2.324 and 2.377 μm (4303 and 4207 cm −1), respectively, as examples, we show that peaks of CH 4 in solid H 2O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH 4. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H 2O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH 4 absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies.