Near-infrared laboratory spectra of solid H 2O/CO 2 and CH 3OH/CO 2 ice mixtures

Abstract

We present near-IR spectra of solid CO 2 in H 2O and CH 3OH, and find they are significantly different from that of pure solid CO 2. Peaks not present in either pure H 2O or pure CO 2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO 2 ( 2 ν 3 ) overtone near 2.134 μm (4685 cm −1), that is absent from our spectrum of pure solid CO 2, is prominent in the spectra of H 2O/CO 2=5 and 25 mixtures. Second, a 2.74-μm (3650 cm −1) dangling OH feature of H 2O (and a potentially related peak at 1.89 μm) appear in the spectra of CO 2–H 2O ice mixtures, but are probably not diagnostic of the presence of CO 2. Other CO 2 peaks display shifts in position and increased width because of intermolecular interactions with H 2O. Warming causes some peak positions and profiles in the spectrum of a H 2O/CO 2=5 mixture to take on the appearance of pure CO 2. Absolute strengths for absorptions of CO 2 in solid H 2O are estimated. Similar results are observed for CO 2 in solid CH 3OH. Since the CO 2 ( 2 ν 3 ) overtone near 2.134 μm (4685 cm −1) is not present in pure CO 2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO 2 is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies.