Laboratory spectra through the mid-infrared (4000 to 500 cm-1 [2.5-20 micrometers]) have been used to calculate the optical constants (n and k) and integrated absorption coefficients (A) for a variety of pure and mixed molecular ices of relevance to astrophysics. The ices studied were H2O, CH3OH, CO2, OCS, CH4, CO2 + CH4, CO2 + OCS, CO + CH4, CO + OCS, O2 + CH4, O2 + OCS, N2 + CH4, N2 + OCS, H2O + CH4, H2O + OCS, and H2O + CH3OH + CO + NH3. In addition, the measurements have been extended through the far-infrared (500 to 50 cm-1 [20-200 micrometers]) for the H2O, CH3OH, and H2O + CH3OH + CO + NH3 ices.

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To better constrain and quantify the composition of material in the diffuse interstellar medium (ISM), absorption spectra between 3600 and 2700 cm-1 (2.8 and 3.7 microns) have been taken of objects which have widely varying amounts of visual extinction along different lines of sight. The spectra of these objects contain a broad feature centered at approximately 3300 cm-1 (approximately 3.0 microns), attributed to O-H stretching vibrations, and/or a feature near 2950 cm-1 (3.4 microns) attributed to C-H stretching vibrations. The lack of correlation between the strengths of these two bands indicates that they do not arise from the same molecular carrier. The features in the 3100-2700 cm-1 (3.2-3.7 microns) region fall into one of two classes. We attribute the first class of features to material in the diffuse ISM on the basis of the similarity between the band profiles along the very different lines of sight to Galactic center source IRS 7 and VI Cygni #12. Similar features are also reported for Galactic center source IRS 3, Ve 2-45, and AFGL 2179. Higher resolution spectra of the objects OH 01-477 and T629-5, which are known to be M stars, are dominated by a series of narrow bands in this region. These bands are largely due to OH in the stars' photospheres. While the spectra of OH 01-477 and T629-5 are likely to contain C-H absorption from diffuse ISM dust, the strength of the overlapping photospheric OH features presently prevents us from quantifying the depths of the interstellar C-H feature towards these objects. The interstellar feature for Galactic center source IRS 7 has subpeaks near 2955, 2925, and 2870 cm-1 (+/- 5 cm-1), which we attribute to C-H stretching vibrations in the -CH2- and -CH3 groups of aliphatic hydrocarbons. These band positions fall within 5 cm-1 of the values normal for saturated aliphatics. The absence of a distinct band near 2855 cm-1 suggests that the material contains small amounts of electronegative groups like -O-H or -C triple bond N. The relative strengths and profiles of the 2955 and 2925 cm-1 features towards five objects suggests an average diffuse ISM line-of-sight -CH2-/-CH3 ratio of about 2.5, indicating the presence of relatively complex organic materials. The strengths of the subpeaks at 2925 and 2955 cm-1, due to -CH2- and -CH3 groups, respectively, correlate with visual extinction, strongly suggesting that the C-H stretching band is a general feature of the material along different lines of sight in the diffuse ISM. We find average ratios of A nu/tau(2925 cm-1) = 240 +/- 40 and A nu/tau(2955 cm-1) = 310 +/- 90 for the objects we have observed. We deduce that 2.6%-35% of the cosmic carbon in the ISM is tied up in the carrier of this band with the most likely value falling near 10%. The interstellar C-H band is remarkably similar to the feature in lab residues produced by irradiating analogs of dense molecular cloud ices. This is consistent with a model in which the hydrocarbon component in the diffuse interstellar medium consists of complex hydrocarbons ...

High spectral resolution (nu/delta nu = 900) studies in the 3100-2600 cm-1 (3.2-3.9 microns) range are presented of the protostars NGC 7538 IRS 9, W33A, W3 IRS 5, and S140 IRS 1. This is the spectral region in which the fundamental C-H stretching vibrations of aliphatic hydrocarbons fall. Well-resolved absorption bands at about 2825 cm-1 (3.54 microns) and 2880 cm-1 (3.47 microns) were found superposed on the low-frequency wing of the strong O-H stretch feature. The 2880 cm-1 (3.47 microns) band, a new interstellar feature, is moderately strong in the spectra of all four objects studied. The 2825 cm-1 (3.54 microns) band, previously detected toward W33A, is also in the spectrum of NGC 7538 IRS 9. The relative strength of these two bands varies, showing that they are associated with two different carriers. On the basis of comparisons with laboratory spectra, the 2825 cm-1 (3.54 microns) band is assigned to methanol (CH3OH), in agreement with the earlier work of Grim et al. (1991). This assignment is further supported by a pair of weak absorptions centered at 2600 and 2540 cm-1 (3.85 and 3.94 microns) in the spectrum of W33A recently reported by Geballe (1991). These features compare very well with laboratory spectra of CH3OH/H2O ice mixtures. The CH3OH/H2O ratio derived from the 2825 cm-1 methanol band and the 3250 cm-1 (3.08 microns) H2O feature are 0.13 and 0.40 for NGC 7538 IRS 9 and W33A, respectively. These values are smaller than the ratios of 0.61 and 0.54 derived using the 1460 cm-1 (6.85 microns) band assigned to CH3OH and the 1665 cm-1 (6.00 microns) H2O band. These apparent discrepancies may be due to a combination of scattering effects within the molecular cloud, uncertainties associated with the baselines for the 2825 cm-1 feature, and the presence of other interstellar grain materials that absorb at 1460 cm-1 (6.85 microns). Nonetheless, after H2O, CH3OH is the most abundant known interstellar ice constituent. The new band at about 2880 cm-1 (3.47 microns) falls near the position for C-H stretching vibrations in tertiary carbon atoms. The strength of this feature, in combination with the lack of strong features associated with primary (-CH3) and secondary (-CH2-) carbon atoms, suggests that the carrier of the new feature has a diamond-like structure. We therefore tentatively attribute this new feature to interstellar "diamonds." The detection of this band in the spectra of all four dense molecular clouds suggests that the carrier is ubiquitous in dense clouds. Band-strength analysis indicates that a minimum of a few percent of the available cosmic carbon is tied up in this material.