INFRARED SPECTRA AND THERMODYNAMIC PROPERTIES OF CO₂/METHANOL ICES

Abstract

Ices of mixtures of carbon dioxide and methanol have been studied in a range of temperatures relevant for starforming regions, comets, polar caps of planets and satellites, and other solar system bodies. We have performed temperature-programmed desorption measurements and recorded IR spectra of various types of samples. The presence of two slightly different structures of CO 2 is manifest. A distorted CO 2 structure is characterized by bandshifts between 5 cm −1 (ν 3 ) and 10 cm −1 (ν 2 ) with respect to normal CO 2 . If the samples are heated above 130 K, the distorted CO 2 sublimates and only the normal structure remains. The latter can stay trapped until the sublimation of crystalline methanol (150 K). The desorption energy (E d ∼ 20 kJ mol −1 ) of CO 2 from methanol ice, and the specific adsorption surface area (6 m 2 g −1 ) of amorphous CH 3 OH ice, have been determined. CO 2 does not penetrate into crystalline ice. Whereas the desorption energy is similar to that of CO 2 /H 2 O samples, the specific surface of methanol is much smaller than that of amorphous solid water (ASW). The interaction of CO 2 molecules with water and methanol is similar but ices of CH 3 OH are much less porous than ASW. The inclusion of CO 2 into previously formed ices containing these two species would take place preferentially into ASW. However, in processes of simultaneous deposition, methanol ice can admit a larger amount of CO 2 than water ice. CO 2 /CH 3 OH ices formed by simultaneous deposition admit two orders of magnitude more CO 2 than sequentially deposited ices. These findings can have direct relevance to the interpretation of observations from protostellar environments (e.g., RAFGL7009S) and comet nuclei.

Surface science investigations of the role of CO 2 in astrophysical ices

“…In this case, even following a dose of only 20 L of CO 2 , the spectrum already contains a component that looks like that expected for pure CO 2 ice. In agreement with this observation, previous work [34] has shown that CH 3 OH traps less CO 2 than ASW. The RAIR spectrum for CO 2 adsorbed on top of CH 3 OH : H 2 O (bottom spectrum, figure 6) also contains a feature that can be assigned to CO 2 interacting with H 2 O, as seen for layered and mixed binary ices.…”

“…Finally, the pre-exponential values for the desorption of CO 2 were determined to be 9.9 × 10 14±0.9 s −1 and 1.1 × 10 30±1.5 mol m −2 s −1 for monolayer and multilayer CO 2 , respectively. Comparing the kinetic parameters for CO 2 desorption determined here and those cited in the literature [20,32,34,46,51], some striking differences are evident. Ulbricht et al [51] assumed a desorption order of zero from qualitative inspection of their data for multilayer ices, but calculated a first-order pre-exponential factor of 6 × 10 14±1 s −1 .…”

“…Ulbricht et al [51] assumed a desorption order of zero from qualitative inspection of their data for multilayer ices, but calculated a first-order pre-exponential factor of 6 × 10 14±1 s −1 . Sandford & Allamandola [20], Gálvez et al [32] and Maté et al [34] also assumed a desorption order of zero, but used a firstorder pre-exponential factor of 3 × 10 12 s −1 , which is clearly not accurate. The determination of accurate kinetic parameters is of great importance in modelling desorption in molecular clouds and star-forming regions in the ISM [54].…”

Laboratory spectra of CO2 vibrational modes in planetary ice analogs

“…The splitting and increase of intensity of the 2m 2 + m 3 CO 2 mode is likely due to the interaction of H 2 O À CH 3 OH complexes trapping CO 2 molecules in separate pockets of ice (Section 3.2.2, Blake et al (1991);Tielens (2005)). The presence of CH 3 OH appears to strongly influence many absorption features of CO 2 , as has been demonstrated in previous studies (e.g., Sandford and Allamandola, 1993;Marchand et al, 2006;Maté et al, 2009;White et al, 2009;White, 2010). The influence of CH 3 OH on the absorption profiles is strong due to the possibility of stronger bonds between CH 3 OH and H 2 O, thus forming more complex structures trapping CO 2 molecules (Collings et al, 2004).…”

Methanol ice on the surface of minor bodies in the solar system

“…4. The spectra of methanol obtained in the crystalline form are very similar to that assigned to the metastable phase by Lucas et al (2005) or Mate et al (2009). These authors formed this phase by condensation in the 130−145 K temperature range or obtained by vapor deposition at 85 K followed by heating of the sample to 130 K, keeping the pressure at ∼10 7 mbar.…”