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 HO: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 HO, 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 HO and CO ranging from 1:4 to 4:1.Results.A quantitative analysis of the band profiles and band strengths of HO 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 HO 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 HO:CO ice mixtures. It is shown that for mixing ratios of 1:0.5 HO:X and higher, the HO bending mode offers a good tracer to distinguish between CO or CO in HO 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 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 HO:CO laboratory ice system. From the change in the HO 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 are mixed in. © 2007 ESO.