$^{13}$CO and $^{13}$CO$_2$ ice mixtures with N$_2$ in photon energy
  transfer studies
  In dense clouds of the interstellar medium, dust grains are covered by ice
mantles, dominated by H$_2$O. CO and CO$_2$ are common ice components observed
in infrared spectra, while infrared inactive N$_2$ is expected to be present in
the ice. Molecules in the ice can be dissociated, react or desorb by exposure
to secondary ultraviolet photons. Thus, different physical scenarios lead to
different ice mantle compositions. This work aims to understand the behaviour
of $^{13}$CO : N$_2$ and $^{13}$CO$_2$ : N$_2$ ice mixtures submitted to
ultraviolet radiation in the laboratory. Photochemical processes and
photodesorption were studied for various ratios of the ice components.
Experiments were carried out under ultra-high vacuum conditions at 12K. Ices
were irradiated with a continuous emission ultraviolet lamp simulating the
secondary ultraviolet in dense interstellar clouds. During the irradiation
periods, fourier-transform infrared spectroscopy was used for monitoring
changes in the ice, and quadrupole mass spectrometry for gas-phase molecules.
In irradiated $^{13}$CO$_2$ : N$_2$ ice mixtures, $^{13}$CO, $^{13}$CO$_2$,
$^{13}$CO$_3$, O$_2$, and O$_3$ photoproducts were detected in the infrared
spectra. N$_2$ molecules also take part in the photochemistry, and N-bearing
molecules were also detected: NO, NO$_2$, N$_2$O, and N$_2$O$_4$.
Photodesorption rates and their dependence on the presence of N$_2$ were also
studied. As it was previously reported, $^{13}$CO and $^{13}$CO$_2$ molecules
can transfer photon energy to N$_2$ molecules. As a result, $^{13}$CO and
$^{13}$CO$_2$ photodesorption rates decrease as the fraction of N$_2$
increases, while N$_2$ photodesorption is enhanced with respect to the low
UV-absorbing pure N$_2$ ice.
