Context : This work is conducted as part of the “ SPECPDR ” program dedicated to the study of very small particles and chemistry in photo-dissociation regions with the Spitzer Space Telescope ( SST ) .

Aims : We study the evolution of the Aromatic Infrared Bands ( AIBs ) emitters across the illuminated edge of the Horsehead nebula and especially their survival and properties in the HII region .

Methods : We present spectral mapping observations taken with the Infrared Spectrograph ( IRS ) at wavelengths 5.2-38 \mu m. The spectra have a resolving power of \lambda / \Delta \lambda = 64 - 128 and show the main aromatic bands , H _ { 2 } rotational lines , ionised gas lines and continuum .
The maps have an angular resolution of 3.6-10.6″and allow us to study the nebula , from the HII diffuse region in front of the nebula to the inner dense region .

Results : A strong AIB at 11.3 \mu m is detected in the HII region , relative to the other AIBs at 6.2 , 7.7 and 8.6 \mu m , and up to an angular separation of \sim 20 ″ ( or 0.04 pc ) from the ionisation front .
The intensity of this band appears to be correlated with the intensity of the [ NeII ] at 12.8 \mu m and of H \alpha , which shows that the emitters of the 11.3 \mu m band are located in the ionised gas .
The survival of AIB emitters in the HII region could be due to the moderate intensity of the radiation field ( G _ { 0 } \sim 100 ) and the lack of photons with energy above \sim 25 eV .
The enhancement of the intensity of the 11.3 \mu m band in the HII region , relative to the other AIBs can be explained by the presence of neutral PAHs .

Conclusions : Our observations highlight a transition region between ionised and neutral PAHs observed with ideal conditions in our Galaxy .
A scenario where PAHs can survive in HII regions and be significantly neutral could explain the detection of a prominent 11.3 \mu m band in other Spitzer observations .