Relative intensity of the final yield for N(HCO) (black column) and N(DCO) (red column), i.e., the saturation value (α), obtained after CO:H₂ and CO:D₂ eice UV irradiation for 60 min with a flux of 6 × 10¹² photons cm⁻² s⁻¹ at temperatures ranging from 8 to 20 K, respectively. The white columns are the initial N(HCO) before subtraction of the extra HCO contribution from the contamination of residual H₂ or H₂O in the UHV chamber. The column densities have been calibrated by the initial CO ice thickness, and further normalized to the maximum ΔN(HCO) in the CO:H₂ experiment at 8 K, i.e., 2.5 × 10¹⁴ molecules cm⁻². The orange points are the reported relative intensity of H₂ sticking coefficients on olivine substrate at 8–14 K in Acharyya (2014).