Absorption of the ionizing radiation throughout the gas slab. Top: shape of the radiation field spectrum for each zone calculated in Cloudy for the radiation-bounded sector of model M_NM, from the illuminated edge of the cloud (blue) to the last zone (green). The vertical lines show the 1, 1.8, 4, 8, 20, and 2000 Ryd energy values. The spectrum is heavily and rapidly absorbed right above 4 Ryd due to He²+ absorption. Bottom: evolution of the integrated flux across the cloud depth (normalized to the value at the illuminated edge) for different energy ranges in Rydberg. The vertical lines show the ionization front, PDR, and C/CO transitions from left to right. The radiation field >20 Ryd is able to penetrate in the neutral gas (top panel) but this contributes little to the total escape fraction (bottom panel). The kink for the 1.8−4 Ryd range is due to secondary ionization following He recombination.