Band profiles and band strengths in mixed H2O:CO ices

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 H2O: 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 H2O, 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 H2O and CO ranging from 1: 4 to 4: 1. Results. A quantitative analysis of the band profiles and band strengths of H2O 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 H2O 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 H2O: ice mixtures. It is shown that for mixing ratios of 1: 0.5 H2O: X and higher, the H2O bending mode offers a good tracer to distinguish between CO2 or CO in H2O 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 H2O: CO laboratory ice system. From the change in the H2O 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 CO2 are mixed in.