$^{13}$CO and $^{13}$CO$_2$ ice mixtures with N$_2$ in photon energy transfer studies

Abstract

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 gasphase 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.

Mechanism of Indirect Photon-induced Desorption at the Water Ice Surface

“…In such systems, indirect desorption processeswhere an excited matrix species transfers energy to a different species which desorbs -play an important role. Indirect photodesorption has been studied for the astrophysically relevant CO-ice environment [15][16][17][18][19] but the exact mechanism remains elusive. Water ice is a more complex but very important system for which indirect desorption has been little explored.…”