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Published online by Cambridge University Press: 25 September 2002
We present ISOCAM observations (5-18 μm) of illuminated surfaces of molecular clouds. The emission properties of the transiently heated small particles, whichdominate the emission observed byISOCAM, are analysed in relation with the spatial structure of the material. Striking spatial variations of the infrared colour(5-8.5 μm/12-18 μm) are detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features(especially at 7.7 μm) relatively to the continuum emission increasing towards long wavelengths. Compared to the intensity of the continuum emission,the intensity of the aromatic features are significantly fainter at the illuminated surfaces of dense structures than in low density regions surrounding these densestructures. This effect could be the signature of photo-chemical evolution, size segregationdue to grain dynamics in uni-directional radiation fields, or abundance variations of very small particles.