Near-IR Spectra of CO₂ in H₂O

Solid H₂O is ubiquitous in the outer Solar System (Roush, 2001), and it is inevitable that CO₂ will come into intimate contact with H₂O at various temperatures and in varied proportions. For example, reflectance spectra of icy Galilean and Saturnian satellites show strong NIR absorptions of CO₂ and H₂O. Although good NIR spectra of pure CO₂ and CO₂ in N₂ have been published (Hansen 1997, Quirico & Schmitt 1997, Gerakines et al., 2005), to our knowledge no NIR spectra of CO₂ in H₂O are available. The interaction between CO₂ and H₂O (and CH₃OH) on a molecular level has been shown to cause significant changes in the position and profile of CO₂ peaks in the mid-IR (Sandford & Allamandola 1990; Dartois et al., 1999; Ehrenfreund et al., 1999; Palumbo and Baratta 2000), so it seems reasonable that the presence of CO₂ could change NIR CO₂ peaks as well. Indeed, in this paper we show that one such peak exists, the classically 'forbidden' 2ν₃ overtone near 2.134 micron. This could potentially complicate the interpretation of IR of spectra of ices on outer Solar System bodies.

On this web page we display eight figures with NIR spectra of CO₂-H₂O and CH₃OH-CO₂ ice mixtures at temperatures from 15 to 150 K. We highlight the enhancement of the putative CO₂ (2ν₃) overtone near 2.134 micron (4685 cm^-1) and its potential as an observational (spectral) indicator of whether solid CO₂ is a pure material or intimately mixed with other molecules. As far as we know this is by far the most sensitive indicator CO₂ intermolecular interactions. Even in cases where spectra of a surface show H₂O and CO₂ together, they could still be in small patches or in layers. Only a molecular approach would allow us to distinguish between these types of mixtures.

Figure 1. The 1.75-22 micron (5700-450 cm^-1) IR spectrum of an H₂O/CO₂ = 5 ice mixture at 15 K. In addition to the broad absorptions of amorphous solid H₂O and the sharper CO₂ fundamentals one sees the 'forbidden' 2ν₃ overtone of CO₂ at 2.135 micron (4684 cm^-1). This feature is prominent in spectra of mixtures but is not seen in spectra of pure CO₂ (see Fig. 3).

Figure 2. The 2.667-2.817 micron (3750-3550 cm^-1) IR spectrum of pure CO₂ (above) compared with that of an H₂O/CO₂ = 5 ice mixture at 15 K (below). The ~2.70 and 2.78 micron (3702 & 3592 cm^-1) absorptions of CO₂ in H₂O are significantly broader and shifted to longer wavelength than those of pure CO₂. The central broad feature near 2.74 micron (3650 cm^-1) is an absorption of H₂O, not CO₂, so it does not appear in the upper spectrum. The lower spectrum drops off to the right because of the large 3 micron H₂O band (see Fig. 1).

Figure 3. The 2.08-2.27 micron (4800-4400 cm^-1) IR spectra of pure CO₂ and H₂O compared with that of an H₂O/CO₂=20 ice. The ν2v3 overtone of the asymmetric stretch of CO₂ near 2.135 micron (4684 cm^-1) is absent from the spectra of pure CO₂ and pure H₂O, but present in the spectra of the CO₂ mixed with H₂O. The position and profile of the this overtone is sensitive to ice composition and concentration, not unusual behavior for a classically 'forbidden' feature. The baselines of the spectra rich H₂O in drop off to the left because of the 2 micron H₂O band (see Fig. 1).

Figure 4. The 2.08-2.27 micron (4800-4400 cm^-1) IR spectra of pure CO₂ and H₂O compared with various H₂O, CH3OH, & CO₂ containing ice mixtures at 15 K. The 2ν3 overtone of the asymmetric stretch of CO₂ near 2.135 micron (4684 cm^-1) is also present in the spectra of these mixed ices.

Figure 5. The 1.908-2.105 micron (5240-4750 cm^-1) IR spectrum of pure CO₂ (above) compared with that of an H₂O/CO₂ = 5 ice mixture (below) at 15 K. The large broad dip crossing the entire lower trace is caused by H₂O. The smaller peaks at ~ 1.97, 2.01, and 2.07 micron (5083, 4963, and 4827 cm^-1) are from CO₂ in H₂O. Compared to pure CO₂ these absorptions from the mixture are significantly broader and shifted to longer wavelength.

Figure 6. Temperature dependence of the 2.67-2.82 micron (3750-3550 cm^-1) IR spectrum of an H₂O/CO₂ = 5 ice mixture. The broad features at ~2.70 and 2.79 micron (3700 and 3590 cm^-1) of CO₂ in H₂O are visible over the entire temperature range. The sharper peaks superimposed upon them that appear, broaden, and then diminish with warming are similar to those produced by pure CO₂.

Figure 7. The 2.0-22 micron (5000-450 cm^-1) IR spectrum of an CH3OH/CO₂ = 5 ice mixture. As in figures 1, 3 and 4, the 2ν3 overtone of CO₂ is clearly visible at ~2.14 micron (4682 cm^-1), and the pair of CO₂ peaks at ~2.70 and 2.79 micron are also present. The other absorptions between 2 and 3 micron are caused by methanol.