Fractional abundances, N (X)/N (H₂), of SiO and N₂H⁺ relative to H₂, calculated across shocks propagating at 5 (left panels), 10 (middle panels), and 20 (right panels) km s⁻¹ in the initial media density n_H = 10² (red squares), 10³ (green circles), 10⁴ (cyan diamonds), and 10⁵ (blue triangles) cm⁻³, as function of irradiating radiation field G₀ in Mathis unit. In the upper panels for SiO, the filled gray areas indicate the observed SiO abundances (5 × 10⁻¹–1 × 10⁻¹⁰) toward DR18.