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Context. In dense clouds, hydrogenation reactions on icy dust grains are keyin the formation of molecules, like formaldehyde, methanol, and complex organicmolecules (COMs). These species form through the sequential hydrogenation of COice. Although molecular hydrogen (H2) abundances can be four orders ofmagnitude higher than those of free H-atoms in dense clouds, H2 surfacechemistry has been largely ignored; several laboratory studies show that H2does not actively participate in non-energetic ice chemistry because of thehigh activation energies required. Aims. For the example of CO icehydrogenation, we experimentally investigated the potential role of H2molecules on the surface chemistry when energetic processing (i.e., UVphotolysis) is involved. We test whether additional hydrogenation pathwaysbecome available upon UV irradiation of a CO:H2 ice mixture and whether thisreaction mechanism also applies to other chemical systems. Methods. Ultra-highvacuum (UHV) experiments were performed at 8~20 K. A pre-deposited solidmixture of CO:H2 was irradiated with UV-photons. Reflection absorption infraredspectroscopy (RAIRS) was used as an in situ diagnostic tool. Single reactionsteps and possible isotopic effects were studied by comparing results fromCO:H2 and CO:D2 ice mixtures. Results. After UV-irradiation of a CO:H2 icemixture, two photon-induced products, HCO and H2CO, are unambiguously detected.The proposed reaction mechanism involves electronically excited CO in thefollowing reaction steps: CO + h{\nu}->CO*, CO* + H2->HCO + H where newlyformed H-atoms are then available for further hydrogenation reactions. The HCOformation yields have a strong temperature dependence for the investigatedregime, which is most likely linked to the H2 sticking coefficient. Finally,the astronomical relevance of this photo-induced reaction channel is discussed.

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