Photoprocessing of astrophysical ice analogs using the Interstellar Astrochemistry Chamber
Abstract
UV-photodesorption is a plausible non-thermal desorption process in dark clouds, which is required to explain the presence of molecules in the gas phase. Models of ice photoprocessing depend on the vacuum ultraviolet (VUV) absorption cross section of the ice. In the past, gas phase cross section values were used as an approximation due to the lack of reported VUV-absorption cross sections of most molecules present in interstellar ice mantles (with the exception of H2O, CO2, and NH3). ISAC is an ultra-high-vacuum (UHV) set-up where pure ices composed of H2O, CO, CO2, CH3OH, NH3, CH4, H2S, N2, and O2 were deposited at 8 K. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV-absorption spectra of the ice samples were collected in the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged hydrogen flow lamp. We provide VUV-absorption cross sections of the reported molecular ices. H2S presents the highest absorption in the 120-160 nm range, while solid N2 has the lowest VUV-absorption cross section, which is about three orders of magnitude lower than that of other species. Isotopic effects were studied for D2O, 13CO2, CD3OD, and 15N2. Our method allows fast and readily available VUV spectroscopy of ices without the need of using a synchrotron beamline. Photodesorption rates of pure ices, expressed in molecules per absorbed photon, can be derived from our data.