First Experimental Confirmation of the CH<sub>3</sub>O + H<sub>2</sub>CO → CH<sub>3</sub>OH + HCO Reaction: Expanding the CH<sub>3</sub>OH Formation Mechanism in Interstellar Ices
Julia C. Santos, K.-J. Chuang, Thanja Lamberts, G. Fedoseev, S. Ioppolo, H. Linnartz
Abstract
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 CH 3 O + H 2 CO → CH 3 OH + HCO, in addition to CH 3 O + H → CH 3 OH, as a very promising and possibly dominating (70%–90%) final step to form CH 3 OH in those environments. Here, we compare the contributions of these two steps leading to methanol by experimentally investigating hydrogenation reactions on H 2 CO and D 2 CO ices, which ensures comparable starting points between the two scenarios. The experiments are performed under ultrahigh vacuum conditions and astronomically relevant temperatures, with H:H 2 CO (or D 2 CO) flux ratios of 10:1 and 30:1. The radical-molecule route in the partially deuterated scenario, CHD 2 O + D 2 CO → CHD 2 OD + 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 D 2 CO + H products in comparison to H 2 CO + H, implying that the CH 3 O-induced abstraction route must play an important role in the formation of methanol in interstellar ices. Reflection-absorption infrared spectroscopy and temperature-programmed desorption-quadrupole mass spectrometry 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 calculations. Additional H 2 CO + D experiments confirm these conclusions.