Upper panels: representative transmission functions calculated from theoretical photoabsorption cross sections of important molecular species, with column densities: N(H) = 10²², N(H₂) = 10²⁰, and ; and simulating an excitation temperature of 50 K for all molecular absorption. Lower panels: probability per Å of H₂ emission following a cosmic-ray induced ionisation event. Spectra are shown assuming two different values for the H₂ ortho:para ratio.