Models of the PDR component in d203-506. Left panels: H₂ relative populations and abundances. The upper panel shows the normalized distribution of H₂(v) populations across different vibrational levels (solid curves), along with the rotationally excited H₂(v = 0, J = 7, 9, 11) levels (dashed curves). The lower panel displays abundance profiles of ${\text{H}}_2^{+}$, ${\text{H}}_3^{+}$, and HOC⁺. Continuous curves show results for models using state-dependent rate constants for reactions (2), (9), (10), and for H₂(v≥4) photoionization (the complete reference model). Dotted curves are for models neglecting H₂ photoionization. Dashed curves are for models neglecting H₂ photoionization and using the thermal rate constants for reactions (2), (9), and (10). Right panels: chemical formation rates. ${\text{H}}_2^{+}$ formation rates as a function of depth into the PDR (upper box). Contribution (in %) to the ${\text{H}}_3^{+}$ formation rate as a function of depth into the PDR (lower box).