Band profiles and band strengths in mixed H₂O:CO ices

Abstract

Context. Laboratory spectroscopic research plays a key role in the identification and analysis of interstellar ices and their structure. To date, a number of molecules have been positively identified in interstellar ices, either as pure, mixed or layered ice structures. Aims. Previous laboratory studies on H 2 O:CO ices have employed a "mix and match" principle and describe qualitatively how absorption bands behave for different physical conditions. The aim of this study is to quantitatively characterize the absorption bands of solid CO and H 2 O, both pure and in their binary mixtures, as a function of partner concentration and temperature. Methods. Laboratory measurements based on Fourier transform infrared transmission spectroscopy are performed on binary mixtures of H 2 O and CO ranging from 1:4 to 4:1. Results. A quantitative analysis of the band profiles and band strengths of H 2 O in CO ice, and vice versa, is presented and interpreted in terms of two models. The results show that a mutual interaction takes place between the two species in the solid, which alters the band positions and band strengths. It is found that the band strengths of the H 2 O bulk stretch, bending and libration vibrational bands decrease linearly by a factor of up to 2 when the CO concentration is increased from 0 to 80%. By contrast, the band strength of the free OH stretch increases linearly. The results are compared to a recently performed quantitative study on H 2 O:CO 2 ice mixtures. It is shown that for mixing ratios of 1:0.5 H 2 O:X and higher, the H 2 O bending mode offers a good tracer to distinguish between CO 2 or CO in H 2 O ice. Additionally, it is found that the band strength of the CO fundamental remains constant when the water concentration is increased in the ice. The integrated absorbance of the 2152 cm −1 CO feature, with respect to the total integrated CO absorption feature, is found to be a good indicator of the degree of mixing of CO in the H 2 O:CO laboratory ice system. From the change in the H 2 O absorption band strength in laboratory ices upon mixing we conclude that astronomical water ice column densities on various lines of sight can be underestimated by up to 25% if significant amounts of CO and CO 2 are mixed in.

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

“…CO is usually present on grains in different mixtures: pure CO, polar CO and CO mixed with CO 2 . Laboratory spectra of CO mixed with either H 2 O or CH 3 OH show similar changes in the 4.7 μm CO feature, broadening and redshifting (Bisschop 2007;Bottinelli et al in prep;Bouwman et al 2007). The polar CO component at 2139 cm −1 , normally ascribed to CO in a water-rich ice, can therefore also be due to a methanol-rich mixture.…”

Quantification of segregation dynamics in ice mixtures

“…The two mixed-ice Gaussians have been observed previously in several studies and are attributed to two different CO-H 2 O interactions (Collings et al 2003). The Gaussian parameters are free variables during the fit except for the FWHM of the pure CO Gaussian, which is constrained to be within 0.5 cm −1 of the pure CO ice value in Bouwman et al (2007). The resulting Gaussian parameters for all H 2 O:CO experiments are reported in Table 5.…”

“…The same band strengths are used to calculate absolute ice thicknesses, but in the thin ice experiments the band strengths are first scaled to account for the longer absorption pathway in RAIRS (Öberg et al 2009), which results in a ∼50% uncertainty in the absolute ice thickness in these experiments. Based on the results of previous spectroscopic studies on mixed ices, the H 2 O abundance is determined from the bending mode rather than the stretching feature (Öberg et al 2007;Bouwman et al 2007).…”

Hydrogenation reactions in interstellar CO ice analogues

“…Bouwman et al (2007) strongly affected by the ice composition, the total ice thickness determined using a constant band strength should vary in time, whereas the real thickness is constant. Since this does not occur, we estimate that the change in band strength caused by the change in ice composition is negligible and well within our margins of error.…”