H-atom bombardment of CO₂, HCOOH, and CH₃CHO containing ices

Abstract

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 3 CHO) 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 3 CHO 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 × 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 3 CHO does react with H-atoms. At most 20% is hydrogenated to ethanol (C 2 H 5 OH) 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 2 CO and CH 3 OH (15-50%). The methane production yield is expected to be equal to the summed yield of H 2 CO and CH 3 OH and therefore CH 4 most likely evaporates partly after formation due to the high exothermicity of the reaction. The reaction rates for CH 3 CHO destruction depend on ice temperature and not on ice thickness. The results are discussed in an astrophysical context.

Formation rates of complex organics in UV irradiated CH3OH-rich ices

“…Experimental studies show that dissociative reactions may be the favored outcome of hydrogenating larger molecules and fragments (e.g. Bisschop et al 2007a), hampering the build-up of large quantities of complex molecules. Quantitative experiments are, however, still lacking for most reactions.…”

Hydrogenation reactions in interstellar CO ice analogues

“…Astronomical observations, detailed laboratory studies, and progress in UHV surface techniques allow experimental verification of the initial surface reaction schemes, as introduced by Tielens, Hagen, and Charnley (Tielens & Hagen 1982;Tielens & Charnley 1997). Recently the formation of water was demonstrated in hydrogenation schemes starting from solid molecular oxygen (Miyauchi et al 2008;Ioppolo et al 2008) and the formation of ethanol from acetaldehyde (Bisschop et al 2007a). The first solid-state astrochemical laboratory studies focused on the formation of formaldehyde and methanol by H-atom bombardment of CO ice.…”

THz and mid-IR spectroscopy of interstellar ice analogs: methyl and carboxylic acid groups

“…Although non-energetic surface reaction pathways leading to the formation of the more complex species studied in this work have not yet been investigated in the laboratory, several models indicate that those species should be formed in the solid phase and could participate in the formation of even more complex species. 11 For instance, Bisschop et al 52 studied the surface hydrogenation of acetaldehyde and found that it converts to ethanol (C 2 H 5 OH) or to CH 4 , H 2 CO, and CH 3 OH.…”