13CO and 13CO2 ice mixtures with N2 in photon energy transfer studies

Abstract

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