We study the chemical evolution of H 2 O:CO:NH 3 ice mixtures irradiated with soft X-rays, in the range 250 − 1250 eV. We identify many nitrogen-bearing molecules such as e.g., OCN − , NH + 4 , HNCO, CH 3 CN, HCONH 2 , and NH 2 COCONH 2 . Several infrared features are compatible with glycine or its isomers.During the irradiation, we detected through mass spectroscopy many species desorbing the ice. Such findings support either the infrared identifications and reveal less abundant species with not clear infrared features. Among them, m/z = 57 has been ascribed to methyl isocyanate (CH 3 NCO), a molecule of prebiotic relevance, recently detected in protostellar environments.During the warm up after the irradiation, several infrared features including 2168 cm −1 band of OCN − , 1690 cm −1 band of formamide, and the 1590 cm −1 band associated to three different species, HCOO − , CH 3 NH 2 and NH + 3 CH 2 COO survive up to room temperature. Interestingly, many high masses have been also detected. Possible candidates are methyl-formate, (m/z = 60, HCOOCH 3 ), ethanediamide (m/z = 88, NH 2 COCONH 2 ), and N-acetyl-L-aspartic acid (m/z = 175). This latter species is compatible with the presence of the m/z = 43, 70 and 80 fragments.Photo-desorption of organics is relevant for the detection of such species in the gas-phase of cold environments, where organic synthesis in ice mantles should dominate. We estimate the gas-phase enrichment of some selected species in the light of a protoplanetary disc model around young solartype stars.

We study the photo-desorption occurring in H 2 O:CO:NH 3 ice mixtures irradiated with monochromatic (550 and 900 eV) and broad band (250-1250 eV) soft X-rays generated at the National Synchrotron Radiation Research Center (Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen (m/z = 1) to complex species with m/z = 69 (e.g., C 3 H 3 NO, C 4 H 5 O, C 4 H 7 N), supporting the enrichment of the gas phase.At low number of absorbed photons, substrate-mediated exciton-promoted desorption dominates the photo-desorption yield inducing the release of weakly bound (to the surface of the ice) species; as the number of weakly bound species declines, the photo-desorption yield decrease about one order of magnitude, until porosity effects, reducing the surface/volume ratio, produce a further drop of the yield.We derive an upper limit to the CO photo-desorption yield, that in our experiments varies from 1.4 to 0.007 molecule photon −1 in the range ∼ 10 15 − 10 20 absorbed photons cm −2 . We apply these findings to a protoplanetary disk model irradiated by a central T Tauri star.

Pure CO ice has been irradiated with electrons of energy in the range 150 − 1000 eV with the Interstellar Energetic-Process System (IEPS). The main products of irradiation are carbon chains C n (n = 3, 5, 6, 8, 9, 10, 11, 12), suboxides, C n O (n = 2, 3, 4, 5, 6, 7), and C n O 2 (n = 1, 3, 4, 5, 7) species. CO 2 is by far the most abundant reaction product in all the experiments. The destruction cross-section of CO peaks at about 250 eV, decreases with the energy of the electrons and is more than one order of magnitude higher than for gas-phase CO ionization. The production cross-section of carbon dioxide has been also derived and is characterized by the competition between chemistry and desorption.Desorption of CO and of new species during the radiolysis follows the electron distribution in the ice. Low energy electrons having short penetration depths induce significant desorption. Finally, as the ice thickness approaches the electron penetration depth the abundance of the products starts to saturate. Implications on the atmospheric photochemistry of cold planets hosting surface CO ices are also discussed.

CO 2 ice has a phase transition at 35 K when its structure changes from amorphous to crystalline. Using Reflection absorption Infrared Spectroscopy (RAIRS), Öberg et al. (2009) observed that the photodesorption yield of CO 2 ice deposited at 60 K and irradiated at 18 K is 40% lower than that of CO 2 ice deposited and irradiated at 18 K. In this work, CO 2 ices were deposited at 16-60 K and UV-irradiated at 16 K to rule out the temperature effect and figure out the relationship between photodesorption yield and ice structure. IR spectroscopy is a common method used for measurement of the photodesorption yield in ices. We found that undetectable C atoms produced in irradiated CO 2 ice can account for 33% of the amount of depleted CO 2 molecules in the ice. A quantitative calibration of QMS was therefore performed to convert the measured ion current into photodesorption yield. During various irradiation periods, the dominant photodesorbing species were CO, O 2 , and CO 2 , and their photodesorption yields in CO 2 ices deposited at different temperature configurations were almost the same, indicating that ice morphology has no effect on the photodesorption yield of CO 2 ice. In addition, we found that the lower desorption yield reported by Martín-Doménech et al. (2015) is due to a linear relationship between the photodesorption yield and the combination of energy distribution of Microwave-Discharge Hydrogen-flow Lamp (MDHL) and UV absorption cross section of ices.

Non-thermal desorption of inter-and circum-stellar ice mantles on dust grains, in particular ultraviolet photon-induced desorption, has gained importance in recent years. These processes may account for the observed gas phase abundances of molecules like CO toward cold interstellar clouds. Ice mantle growth results from gas molecules impinging on the dust from all directions and incidence angles. Nevertheless, the effect of the incident angle for deposition on ice photo-desorption rate has not been studied. This work explores the impact on the accretion and photodesorption rates of the incidence angle of CO gas molecules with the cold surface during deposition of a CO ice layer. Infrared spectroscopy monitored CO ice upon deposition at different angles, ultraviolet-irradiation, and subsequent warm-up. Vacuum-ultraviolet spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp. Molecules ejected from the ice to the gas during irradiation or warm-up were characterized by a quadrupole mass spectrometer. The photodesorption rate of CO ice deposited at 11 K and different incident angles was rather stable between 0 and 45 • . A maximum in the CO photodesorption rate appeared around 70 • -incidence deposition angle. The same deposition angle leads to the maximum surface area of water ice. Although this study of the surface area could not be performed for CO ice, the similar angle dependence in the photodesorption and the ice surface area suggests that they are closely related. Further evidence for a dependence of CO ice morphology on deposition angle is provided by thermal desorption of CO ice experiments.