Electron-Induced Processing of Methanol Ice

Abstract

The 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.

Electron irradiation and thermal chemistry studies of interstellar and planetary ice analogues at the ICA astrochemistry facility

“…Spectra have been offset vertically for clarity and methane (CH 4 ). The formation of these products is in line with the results reported by prior studies of electron irradiation of frozen CH 3 OH [8,15,[19][20][21][22].…”

“…Increases in the 160 K spectrum at wavenumbers <1000 cm −1 are due to changes in the high-temperature background profile that are not taken into account by the background spectrum measured at 20 K concentration of this molecule within the ice structure (Figs. 5 and ν1, ν3 H2O 14.00 [37] ment with the findings of several other studies considering the electron irradiation of CH 3 OH, such as those by Bennett et al [21] and Schmidt et al [22]. Our thermal processing of a mixed H 2 O:SO 2 ice of compositional ratio 3:5 demonstrated that as the ice mixture was warmed from 20 K, molecular reactions resulted in the formation of new products, namely HSO − 3 and S 2 O 2− 5 .…”

“…Studies by Bennett et al [21] and Schmidt et al [22] have provided detailed information as to the likely formation routes of these molecules. Initial fragmentation of CH 3 OH may occur via the loss of hydrogen (yielding the neutral radicals CH 3 O and CH 2 OH), the loss of oxygen (yielding CH 4 ), or the simultaneous (one-step) loss of two hydrogen atoms (yielding H 2 CO).…”

Molecular synthesis in ices triggered by dissociative electron attachment to carbon monoxide

“…While this ionization-driven reaction mechanism is temptingly simple, a deeper insight into the reactions occurring in the ice mixtures is of utmost importance to arrive at a comprehensive understanding of the electron-induced chemistry in ices. The potential complexity of the reaction networks is obvious considering the wealth of products that can be formed even in a pure ice as exemplified by the case of pure methanol (CH 3 OH) ice [15,20]. As another example, ethanol (C 2 H 5 OH) formation was also observed as consequence of electron attachment (EA) to C 2 H 4 in the presence of H 2 O [18] indicating that different reaction mechanisms can contribute to the formation of a particular product.…”

“…In an effort to unravel electron-induced chemistry in ices, we have recently conducted comprehensive studies by post-irradiation thermal desorption spectrometry (TDS) on thin layers of different pure and mixed ices [20][21][22][23]. In each case, the dependence of product formation on electron energy was monitored for several products.…”