Context : A chemical scenario was proposed for photon-dominated regions ( PDRs ) according to which UV photons from nearby stars lead to the evaporation of very small grains ( VSGs ) and the production of gas-phase polycyclic aromatic hydrocarbons ( PAHs ) .

Aims : Our goal is to achieve better insight into the composition and evolution of evaporating very small grains ( eVSGs ) and PAHs through analyzing the infrared ( IR ) aliphatic and aromatic emission bands .

Methods : We combined spectro-imagery in the near- and mid-IR to study the spatial evolution of the emission bands in the prototypical PDR NGC 7023 .
We used near-IR spectra obtained with the IRC instrument onboard AKARI to trace the evolution of the 3.3 \mathrm { \mu m } and 3.4 \mathrm { \mu m } bands , which are associated with aromatic and aliphatic C - H bonds on PAHs .
The spectral fitting involved an additional broad feature centered at 3.45 \mathrm { \mu m } that is often referred to as the plateau .
Mid-IR observations obtained with the IRS instrument onboard the Spitzer Space Telescope were used to distinguish the signatures of eVSGs and neutral and cationic PAHs .
We correlated the spatial evolution of all these bands with the intensity of the UV field given in units of the Habing field G _ { 0 } to explore how their carriers are processed .

Results : The intensity of the 3.45 \mathrm { \mu m } plateau shows an excellent correlation with that of the 3.3 \mathrm { \mu m } aromatic band ( correlation coefficient R = 0.95 ) and a relatively poor correlation with the aliphatic 3.4 \mathrm { \mu m } band ( R=0.77 ) .
This indicates that the 3.45 \mathrm { \mu m } feature is dominated by the emission from aromatic bonds .
We show that the ratio of the 3.4 \mathrm { \mu m } and 3.3 \mathrm { \mu m } band intensity ( I _ { 3.4 } / I _ { 3.3 } ) decreases by a factor of 4 at the PDR interface from the more UV-shielded layers ( G _ { 0 } \sim 150 ,I _ { 3.4 } / I _ { 3.3 } = 0.13 ) to the more exposed layers ( G _ { 0 } > 1 \times 10 ^ { 4 } ,I _ { 3.4 } / I _ { 3.3 } = 0.03 ) .
The intensity of the 3.3 \mathrm { \mu m } band relative to the total neutral PAH intensity shows an overall increase with G _ { 0 } , associated with an increase of both the hardness of the UV field and the H abundance .
In contrast , the intensity of the 3.4 \mathrm { \mu m } band relative to the total neutral PAH intensity decreases with G _ { 0 } , showing that their carriers are actively destroyed by UV irradiation and are not efficiently regenerated .
The transition region between the aliphatic and aromatic material is found to correspond spatially with the transition zone between neutral PAHs and eVSGs .

Conclusions : We conclude that the photo-processing of eVSGs leads to the production of PAHs with attached aliphatic sidegroups that are revealed by the 3.4 \mathrm { \mu m } emission band .
Our analysis provides evidence for the presence of very small grains of mixed aromatic and aliphatic composition in PDRs .