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

By Keith Cowing
May 25, 2022
Filed under
First Experimental Confirmation of the CH3O + H2CO -> CH3OH + HCO reaction: expanding the CH3OH formation mechanism in interstellar ices
Spectra of the TPD-QMS experiments obtained after codeposition of H2CO + H (upper) and D2CO + H (lower) at 12 K for 360 min. The m/z signals of 32 (red), 34 (blue) and 35 (green) correspond to, respectively, CH3OH, CHD2OH and CHD2OD. No signal is detected for m/z = 35 at the desorption temperature of methanol (∼140 K).

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.

Julia C. Santos, Ko-Ju Chuang, Thanja Lamberts, Gleb Fedoseev, Sergio Ioppolo, Harold Linnartz

Comments: 10 pages, 1 table, 6 figures. Accepted in the Astrophysical Journal Letters
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2205.12284 [astro-ph.GA] (or arXiv:2205.12284v1 [astro-ph.GA] for this version)
Submission history
From: Julia C. Santos
[v1] Tue, 24 May 2022 18:00:03 UTC (1,339 KB)
Astrobiology, Astrochemistry

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