| Icy grain mantles in the interstellar medium play an important role in the chemistry of molecular clouds. The vibrational absorption features of solid-state materials are detected by infrared (IR) spectroscopy, and the study of laboratory ice analogs provides a means by which they may be interpreted. The general methodology behind laboratory astrophysics is discussed. The absorption strengths of a molecule's IR features are important parameters used to calculate its interstellar abundance, and results presented show that these strengths are only weakly dependent on the ice environment. Results of ultraviolet (UV) irradiation experiments on nine pure ices are presented and show a remarkable complexity in certain cases. Pure methane (CH4), formaldehyde (H2CO) and methanol (CH3OH) ices are shown to be efficient producers of complex organics. These experiments provide a basis by which more complex UV photochemistries may be evaluated.;Spectra of interstellar carbon dioxide (CO2) taken by the Short-Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO) at a resolving power gamma/Deltagamma ≈ 1500 are presented for 14 lines of sight. The selected sources include a background field star (Elias 16), massive protostars (Elias 29, GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS9, NGC 7538 IRS1, S 140, W3 IRS5, and W33 A), and sources associated with the Galactic Center (SgrA*, GCS3I, and GCS4). These sight-lines probe a diverse range of environments, but measured column densities of CO2 fall in the range 10--23% relative to water (H2O): this ratio displays remarkably little variation for such a physically diverse sample. Comparisons with laboratory data indicate that interstellar CO2 generally exists in two phases, one polar (H2O-dominant) and nonpolar (CO2-dominant). The observed profiles may also be reproduced when the nonpolar components are replaced with thermally annealed ices. Formation and evolutionary scenarios for grain mantle chemistry are discussed. Results show that thermal annealing, rather than energetic processing due to UV or cosmic rays, dominates the evolution of the CO2-bearing ices. |
