Electron-induced chemistry in simple molecular ices: Fundamental reaction principles revisited

Abstract

Electron-induced reactions play a key-role in astrochemical processes as vast numbers of secondary electrons are released when any ionizing radiation such as UV photons, X-rays, or cosmic rays penetrate the icy mantle around interstellar dust particles. Subsequently, these secondary electrons can interact with molecules in their environment producing reactive fragments which, in turn, may trigger a reaction sequence. Eventually, new organic molecules are formed some of which might be important for the emergence of life on earth. However, while there are many data on the identification of irradiation products under various conditions, the mechanisms that drive their formation just start to emerge. In particular, there is a lack of knowledge on the electron-molecule interactions that underlie product formation. In this thesis, electron-induced reactions have been investigated in pure CH3OH, and in binary ices, namely CO/H2O, C2H4/CH3OH, and CO/CH3OH. The results of these studies provide important insights into electron-induced reactions in interstellar ices as H2O, CO, and CH3OH are all major constituents of these ices. Detailed knowledge on the reaction mechanisms has been inferred from the dependences of product yields on electron energy which reveals the underlying electron-molecule interactions. Additional information on a mechanism were obtained by comparing the energy dependence of a particular product with those of its side products. This enables one to identify the relevant intermediates and to discriminate among several possible reaction scenarios. On the basis of this approach, several reaction classes were identified to play a role under electron-irradiation of the investigated mixtures. Namely, these are radical recombination, hydrogenation of CO and C2H4, oxidation of CO to CO2, and addition reactions between a radical and CO or C2H4.