UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier II. The case of hydrogenated carbon grains

We present the results of a laboratory program aimed at studying the effect s induced by energetic UV photons on hydrogenated carbon particles. Experim ents have been performed under simulated diffuse and dense interstellar med ium conditions. To monitor the effects of UV irradiation on grains IR spect roscopy has been used. In both circumstances UV photons lead to a reduction of the aliphatic 3.4 mum band. An estimation of the destruction cross sect ion by UV photons for the hydrogenated carbon particles has been derived fr om the reduction of the 3.4 mum intensity band as a function of the UV flue nce. The results of the present work, together with previous laboratory dat a, can shed light on the enigmatic difference observed for the 3.4 mum band between dense and diffuse interstellar medium clouds. This difference is c ompatible with the transformation of hydrogenated carbon particles produced by UV photons and hydrogen atoms and with the changes of the grain propert ies in the two environments.