Mechanistical Studies on the Irradiation of Methanol in Extraterrestrial Ices

Abstract

Pure ices of amorphous methanol, CH 3 OH(X 1 A 0 ), were irradiated at 11 K by 5 keV electrons at 100 nA for 1 hr. These energetic electrons simulate electronic energy transfer processes that occur as interstellar ices, comets, and icy solar system bodies are subjected to irradiation from MeV ions and secondary electrons produced in this process. The results were analyzed quantitatively via absorption-reflection-absorption Fourier transform infrared ( FTIR) spectroscopy, with the identification of new species aided by high-level electronic structure calculations. The unimolecular decomposition of methanol was found to proceed via the formation of (1) the hydroxymethyl radical, CH 2 OH(X 2 A 00 ), and atomic hydrogen, H( 2 S 1=2 ), (2) the methoxy radical, CH 3 O(X 2 A 0 ), plus atomic hydrogen, (3) formaldehyde, H 2 CO(X 1 A 1 ) plus molecular hydrogen, H 2 (X 1 AE þ g ), and (4) the formation of methane, CH 4 (X 1 A 1 ), together with atomic oxygen, O( 1 D). The accessibility of the last channel indicates that the reverse process, oxygen addition into methane to form methanol, should also be feasible. A kinetic model is presented for the decomposition of methanol into these species, as well as the formyl radical, HCO(X 2 A 0 ), and carbon monoxide, CO(X 1 AE þ ). During the subsequent warming up of the sample, radicals previously generated within the matrix were mobilized and found to recombine to form methyl formate, CH 3 OCHO(X 1 A 0 ), glycolaldehyde, CH 2 OHCHO(X 1 A 0 ), and ethylene glycol, HOCH 2 CH 2 OH(X 1 A). Upper limits for the production of these species by the recombination of neighboring radicals produced during irradiation as well as during the warm-up procedure are presented. The generation of these molecules by irradiation of ices in the solid state and their subsequent sublimation into the gas phase can help explain their high abundances as observed toward hot molecular cores and underlines their importance in astrobiology.

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

“…HCO can be identified from its ν 2 band between 1840 and 1860 cm −1 (Gerakines et al 1996;Bennett et al 2007). No stable species considered in this study absorbs in this region.…”

“…A few studies exist on the formation of complex molecules from ion bombardment of pure CH 3 OH ice, though similarly to the UV photolysis experiments, the formation of more complex molecules than CH 3 OH is in general not quantified (Baratta et al 2002;Bennett et al 2007). Bennett et al (2007) identified HCOOCH 3 , HOCH 2 CHO and (CH 2 OH) 2 , mainly based on comparison with calculated spectra and desorption patterns following irradiation, but provided only upper limits of their formation rates.…”

“…Bennett et al (2007) identified HCOOCH 3 , HOCH 2 CHO and (CH 2 OH) 2 , mainly based on comparison with calculated spectra and desorption patterns following irradiation, but provided only upper limits of their formation rates. The quantitative data from both studies include formation rates of small molecules and CH 3 OH and H 2 CO ionbombardment dissociation rates.…”

“…Radicals: OH, CH 3 , HCO, CH 3 O, CH 2 OH OH absorbs at 3423 and 3458 cm −1 from a study on pure H 2 O ice photolysis by Gerakines et al (1996), using transmission spectroscopy. Bennett et al (2007) calculated the band position to be at 3594 cm −1 using the hybrid density functional theory with the 6-311G(d,p) basis set and refining the output with the coupled cluster CCSD(T) method -these calculations have typical gas-phase band-position uncertainties of 0.5-1% or <50 cm −1 (Galabov et al 2002). There are some shallow bands in the 3400-3600 cm −1 wavelength region in our CH 3 OH experiments, that disappear between 30 and 50 K, and that are also present in the CD 3 OH ices but not in the CH 3 OD ones.…”

“…No stable species considered in this study absorbs in this region. Bennett et al (2007) calculated the band positions of a large number of unstable species that can theoretically form in an irradiated CH 3 OH ice and none of these have absorption features within 50 cm −1 of 1840 or 1860 cm −1 . Furthermore the band enhancement in the CO ice mixture and the low temperature at which the bands disappear all point to HCO.…”