H2 chemistry in interstellar ices: the case of CO ice hydrogenation in
  UV irradiated CO:H2 ice mixtures
  Context. In dense clouds, hydrogenation reactions on icy dust grains are key
in the formation of molecules, like formaldehyde, methanol, and complex organic
molecules (COMs). These species form through the sequential hydrogenation of CO
ice. Although molecular hydrogen (H2) abundances can be four orders of
magnitude higher than those of free H-atoms in dense clouds, H2 surface
chemistry has been largely ignored; several laboratory studies show that H2
does not actively participate in non-energetic ice chemistry because of the
high activation energies required. Aims. For the example of CO ice
hydrogenation, we experimentally investigated the potential role of H2
molecules on the surface chemistry when energetic processing (i.e., UV
photolysis) is involved. We test whether additional hydrogenation pathways
become available upon UV irradiation of a CO:H2 ice mixture and whether this
reaction mechanism also applies to other chemical systems. Methods. Ultra-high
vacuum (UHV) experiments were performed at 8~20 K. A pre-deposited solid
mixture of CO:H2 was irradiated with UV-photons. Reflection absorption infrared
spectroscopy (RAIRS) was used as an in situ diagnostic tool. Single reaction
steps and possible isotopic effects were studied by comparing results from
CO:H2 and CO:D2 ice mixtures. Results. After UV-irradiation of a CO:H2 ice
mixture, two photon-induced products, HCO and H2CO, are unambiguously detected.
The proposed reaction mechanism involves electronically excited CO in the
following reaction steps: CO + h{\nu}->CO*, CO* + H2->HCO + H where newly
formed H-atoms are then available for further hydrogenation reactions. The HCO
formation yields have a strong temperature dependence for the investigated
regime, which is most likely linked to the H2 sticking coefficient. Finally,
the astronomical relevance of this photo-induced reaction channel is discussed.
