Vacuum-UV spectroscopy of interstellar ice analogs. I. Absorption
  cross-sections of polar-ice molecules
  The VUV absorption cross sections of most molecular solids present in
interstellar ice mantles with the exception of H2O, NH3, and CO2 have not been
reported yet. Models of ice photoprocessing depend on the VUV absorption cross
section of the ice to estimate the penetration depth and radiation dose, and in
the past, gas phase cross section values were used as an approximation. We aim
to estimate the VUV absorption cross section of molecular ice components. Pure
ices composed of CO, H2O, CH3OH, NH3, or H2S were deposited at 8 K. The column
density of the ice samples was measured in situ by infrared spectroscopy in
transmittance. VUV 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.
Our results agree with those previously reported for H2O and NH3 ices.
Vacuum-UV absorption cross section of CH3OH, CO, and H2S in solid phase are
reported for the first time. H2S presents the highest absorption in the 120-160
nm range. Our method allows fast and readily available VUV spectroscopy of ices
without the need to use a synchrotron beamline. We found that the ice
absorption cross sections can be very different from the gas-phase values, and
therefore, our data will significantly improve models that simulate the VUV
photoprocessing and photodesorption of ice mantles. Photodesorption rates of
pure ices, expressed in molecules per absorbed photon, can be derived from our
data.
