Left: commonly adopted ionizing spectra, ﹩{J}_{\nu }﹩, for IGM/CGM ionization studies. The spectra are computed for z = 0.2 using Cloudy (version c13.05; Ferland et al. 2013) based on (1) the Haardt & Madau (HM05) model (solid curve), (2) a quasar-only model from HM05 (dotted curve), (3) the revised Haardt & Madau (2012, hereafter HM12) model (dashed–dotted curve), and (4) a hybrid model that includes the default HM05 background and a local radiation field due to leakage from an L_* galaxy at 50 kpc (red dashed curve; see the main text for details). Right: frequency-dependent photo-ionization cross sections of neutral hydrogen, H⁰, neutral silicon Si⁰, and silicon ions Si⁺ and Si²⁺ from Verner et al. (1996). While the photo-ionization cross-section of hydrogen is sharply peaked near 1 Rydberg, the relatively flat frequency dependence of the ionization cross sections of silicon ions beyond 1 Rydberg indicate that accurate estimates of the ionization fractions of these ions depend more strongly on the accuracy of the spectral slope of J_ν at high energies. The same conclusion also applies to other heavy ions, such as C⁺, ﹩{{\rm{C}}}^{2+}﹩, ﹩{{\rm{C}}}^{3+}﹩, and Mg⁺ commonly seen in the CGM.