Vacuum-UV absorption spectroscopy of interstellar ice analogues. III.
  Isotopic effects
  This paper reports the first measurements of solid-phase vacuum-ultraviolet
(VUV) absorption cross-sections of heavy isotopologues present in icy dust
grain mantles of dense interstellar clouds and cold circumstellar environments.
Pure ices composed of D2O, CD3OD, 13CO2, and 15N15N were deposited at 8 K, a
value similar to the coldest dust temperatures in space. 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 as the VUV
source. Prior to this work, we have recently submitted a similar study of the
light isotopologues (Cruz-Diaz, Mu\~noz Caro and Chen). The VUV spectra are
compared to those of the light isotopologues in the solid phase, and to the gas
phase spectra of the same molecules. Our study is expected to improve very
significantly the models that estimate the VUV absorption of ice mantles in
space, which have often used the available gas phase data as an approximation
of the absorption cross sections of the molecular ice components. We will show
that this work has also important implications for the estimation of the
photodesorption rates per absorbed photon in the ice.
