Wavelength dependence of the assumed UV illumination and the photo-dissociation cross sections. The thick black line shows the unshielded empty-space Draine FUV field, the dashed line shows an extension of the Draine field longward of 2000 Å suggested by van Dishoeck & Black (1982). The blue, green and red lines show the FUV radiation field inside the reference cloud at 95% R_tot for the WD01-7, WD01-21, and WD01-25 dust models, respectively. Left panel: the blue filled curve shows the continuum absorption cross section σ_cont of CH² that leads to dissociation, the black dots with drop lines show the corresponding line absorption cross section σ_line. Right panel: same as left panel but for the SiH⁺ photo-dissociation cross sections.