H2 chemistry in interstellar ices: the case of CO ice hydrogenation in UV irradiated CO:H2 ice mixtures

TL;DR

This study explores how UV irradiation of CO and H2 ice mixtures can facilitate hydrogenation reactions on icy dust grains, revealing a new photo-induced pathway for molecule formation relevant to dense interstellar clouds.

Contribution

It demonstrates that UV irradiation enables H2 molecules to participate in surface chemistry, leading to the formation of HCO and H2CO from CO ice, a process previously considered unlikely.

Findings

UV irradiation produces HCO and H2CO from CO:H2 ice mixtures.

HCO formation depends strongly on temperature, likely due to H2 sticking.

Photo-induced reactions could significantly impact interstellar molecule synthesis.

Abstract

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.