$^{13}$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.