13CO and 13CO2 Ice Mixtures with N2 in Photon Energy Transfer Studies

In dense clouds of the interstellar medium, dust grains are covered by ice mantles, dominated by H_2O. 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 ^13CO : N_2 and ^13CO_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 ^13CO_2 : N_2 ice mixtures, ^13CO, ^13CO_2, ^13CO_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_2O, and N_2O_4. Photodesorption rates and their dependence on the presence of N_2 were also studied. As it was previously reported, ^13CO and ^13CO_2 molecules can transfer photon energy to N_2 molecules. As a result, ^13CO and ^13CO_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.