Pure gas-phase PDR models of the Orion Bar. Continuous curves show fractional abundances as a function of cloud depth, in logarithm scale to better display the irradiated edge of the PDR, using the new reaction rates listed in Table 1. The gray dotted curve shows f₂, the fraction of H₂ that is in vibrationally excited levels v ≥ 2 (right axis scale). Dashed curves are for a model using standard reaction rates.