Introduction

The question of identifying the IR emission bands (3-12 $\mu$m) is an active subject for more than 10 years, especially because the amount of energy contained in these features may be as high as 30% of the total energy in some objects, and because their observation requires an alternative way of exciting the particles than thermal heating (Sellgren 1984). The exact identification of the interstellar mixture has not yet been done.
The review on the PAH model present status is given in this book by F. Salama. We will draw here some interpretation of ISO observations related to the PAH features, and give some comparison with experimental results on these compounds.