Gas-grain PDR models leading to the formation of s-H₂S (shown as black curves). Continuous colored curves show gas-phase fractional abundances as a function of depth into the cloud. ϵ refers to the efficiency of the chemical desorption reaction s-H + s-H₂S → SH + H₂ (see text). Left panel: gas-grain high E_b model (high adsorption binding energies for S and SH, see Table 4). Right panel: low E_b model.