Schmidt, Fabian
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Schmidt, Fabian
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Schmidt, Fabian
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Item-typ:Veröffentlichung, Electron-induced chemistry in simple molecular ices: Fundamental reaction principles revisited(2021-09-21); ; ; 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.Dissertation343 384 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Electron-Induced Processing of Methanol IceThe formation of methane (CH4), formaldehyde (H2CO), ethylene glycol (HOCH2CH2OH), methoxymethanol (CH3OCH2OH), dimethyl ether (CH3OCH3), and ethanol (CH3CH2OH) upon electron irradiation of condensed multilayer adsorbates of CH3OH as a model of cosmic CH3OH ice has been monitored by the combined use of electron-stimulated and thermal desorption experiments. The energy-dependent relative yields of all products were measured between 2.5 and 20 eV, and the reaction mechanisms of product formation were deduced. The energy dependences of the yields of HOCH2CH2OH, CH3OCH2OH, CH3OCH3, and CH3CH2OH agree closely with their threshold at the lowest electronic excitation energy of CH3OH. The formation of these products is consequently ascribed to the reactions of radicals formed by the dissociation of neutral electronically excited states and, at higher energy, also by ionization and subsequent proton transfer to an adjacent CH3OH. These electron–molecule interactions also can contribute to the nonresonant formation of H2CO and CH4; these latter products are also produced through resonances around 4 eV reported previously from anion electron-stimulated desorption (ESD) experiments and around 13 eV seen earlier in the energy-dependent yield of carbon monoxide (CO). The present results constitute the most complete data set on the energy dependence of product formation during low-energy electron exposure of condensed CH3OH so far. They provide a comprehensive picture of the reactions triggered by electron impact with energies in the range between 2.5 and 20 eV as representative of low-energy secondary electrons that are released from condensed material, for instance, under the effect of cosmic radiation.Wissenschaftlicher ArtikelBand:5Heft:244 44