First experimental confirmation of the CH3O + H2CO -> CH3OH + HCO reaction: expanding the CH3OH formation mechanism in interstellar ices

Abstract

The successive addition of H atoms to CO in the solid phase has been hitherto regarded as the primary route to form methanol in dark molecular clouds. However, recent Monte Carlo simulations of interstellar ices alternatively suggested the radical-molecule H-atom abstraction reaction CH3O + H2CO -> CH3OH + HCO, in addition to CH3O + H -> CH3OH, as a very promising and possibly dominating (70 - 90 %) final step to form CH3OH in those environments. Here, we compare the contributions of these two steps leading to methanol by experimentally investigating hydrogenation reactions on H2CO and D2CO ices, which ensures comparable starting points between the two scenarios. The experiments are performed under ultrahigh vacuum conditions and astronomically relevant temperatures, with H:H2CO (or D2CO) flux ratios of 10:1 and 30:1. The radical-molecule route in the partially deuterated scenario, CHD2O + D2CO -> CHD2OD + DCO, is significantly hampered by the isotope effect in the D-abstraction process, and can thus be used as an artifice to probe the efficiency of this step. We observe a significantly smaller yield of D2CO + H products in comparison to H2CO + H, implying that the CH3O-induced abstraction route must play an important role in the formation of methanol in interstellar ices. Reflection-Absorption InfraRed Spectroscopy (RAIRS) and Temperature Programmed Desorption-Quadrupole Mass Spectrometry (TPD-QMS) analyses are used to quantify the species in the ice. Both analytical techniques indicate constant contributions of ~80 % for the abstraction route in the 10 - 16 K interval, which agrees well with the Monte Carlo conclusions. Additional H2CO + D experiments confirm these conclusions.

10 pages, 1 table, 6 figures. Accepted in the Astrophysical Journal Letters

References in corpus (8)

• Experimental evidence for Glycolaldehyde and Ethylene Glycol formation by surface hydrogenation of CO molecules under dense molecular cloud conditions
• Formation of COMs through CO hydrogenation on interstellar grains
• Methanol formation via oxygen insertion chemistry in ice
• Carbon Atom Reactivity with Amorphous Solid Water: H${}_2$O Catalyzed Formation of H${}_2$CO
• Tunneling Rate Constants for H2CO+H on Amorphous Solid Water Surfaces
• A New "Non-energetic" Route to Complex Organic Molecules in Astrophysical Environments: The C + H${}_2$O $\to$ H${}_2$CO Solid-state Reaction
• A cryogenic ice setup to simulate carbon atom reactions in interstellar ices
• Methoxymethanol Formation Starting from CO-Hydrogenation

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