H₂ formation efficiencies on 10 K and 125 K surfaces as a function of gas temperature, with the sticking in physisorbed sites S_phys taken as in Cuppen et al. (2010). Top: total H₂ formation, governed by mechanisms M1 (2 physisorbed atoms) and M5 (physisorbed atom arriving in H dimer) for 10 K dust, and governed by mechanisms M3 (direct chemisorption with H dimer) and M2 (direct chemisorption with H monomer). Bottom: same as top panel but for H₂ formation involving only chemisorbed atoms. At low gas temperatures, H₂ is formed through mechanism M3 (direct chemisorption with H dimer). For T_gas > 1000 K, H₂ is formed though mechamism M2 (direct chemisorption with H monomer).