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

Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: bisschop@strw.leidenuniv.nl

Received: 3 July 2007
Accepted: 11 September 2007

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 (CO₂), formic acid (HCOOH) and acetaldehyde (CH₃CHO) react with H-atoms in the solid state at low temperatures and to derive reaction rates and production yields.

Methods.Pure CO₂, HCOOH and CH₃CHO 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 CO₂ and HCOOH ice does not result in detectable reaction products. The upper limits on the reaction rates are ≤710^-17 cm² 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, CH₃CHO does react with H-atoms. At most 20% is hydrogenated to ethanol (C₂H₅OH) and a second reaction route leads to the break-up of the C–C bond to form solid state CH₄ (~20%) as well as H₂CO and CH₃OH (15–50%). The methane production yield is expected to be equal to the summed yield of H₂CO and CH₃OH and therefore CH₄ most likely evaporates partly after formation due to the high exothermicity of the reaction. The reaction rates for CH₃CHO destruction depend on ice temperature and not on ice thickness. The results are discussed in an astrophysical context.