Evolution of the PAH 7.7 μm over 11.3 μm intensity ratio as a function of distance into the H ii region, tracing the ionization state of the PAH molecules (e.g. Bauschlicher 2002; Szczepanski & Vala 1993). The intensities of the bands are measured with PAHFIT (Smith et al. 2007). The ratio shows a systematic rise with distance into the H ii region, indicating that the molecules reside within the ionized gas, and a dramatic drop in the region closest to the ionizing star. The numbers correspond to the regions drawn in Fig. 4; the dotted lines show the 1σ uncertainty constrained by the quality of the spectra. The upper axis plots the ionization parameter γ (Tielens 2005).