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| - H-atom bombardment of CO 2, HCOOH, and CH 3CHO containing ices
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| - Context.Hydrogenation reactions are expected to be among the most important surface reactions on interstellar ices. However, solid state astrochemical laboratory data on reactions of H-atoms with common interstellar ice constituents are largely lacking. Aims.The goal of our laboratory work is to determine whether and how carbon dioxide (CO 2), formic acid (HCOOH) and acetaldehyde (CH 3CHO) react with H-atoms in the solid state at low temperatures and to derive reaction rates and production yields. Methods.Pure CO 2, HCOOH and CH 3CHO interstellar ice analogues are bombarded by H-atoms in an ultra-high vacuum experiment. The experimental conditions are varied systematically. The ices are monitored by reflection absorption infrared spectroscopy and the reaction products are detected in the gas phase through temperature programmed desorption. These techniques are used to determine the resulting destruction and formation yields as well as the corresponding reaction rates. Results.Within the sensitivity of our set-up we conclude that H-atom bombardment of pure CO 2 and HCOOH ice does not result in detectable reaction products. The upper limits on the reaction rates are ≤7 $\times$10 -17 cm 2 s -1 which make it unlikely that these species play a major role in the formation of more complex organics in interstellar ices due to reactions with H-atoms. In contrast, CH 3CHO does react with H-atoms. At most 20% is hydrogenated to ethanol (C 2H 5OH) and a second reaction route leads to the break-up of the C-C bond to form solid state CH 4 (~20%) as well as H 2CO and CH 3OH (15-50%). The methane production yield is expected to be equal to the summed yield of H 2CO and CH 3OH and therefore CH 4 most likely evaporates partly after formation due to the high exothermicity of the reaction. The reaction rates for CH 3CHO destruction depend on ice temperature and not on ice thickness. The results are discussed in an astrophysical context.
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