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We present the results of a laboratory program aimed at studying the effects induced by energetic UV photons on hydrogenated carbon particles. Experiments have been performed under simulated diffuse and dense interstellar medium conditions. To monitor the effects of UV irradiation on grains IR spectroscopy has been used. In both circumstances UV photons lead to a reduction of the aliphatic 3.4 μm band. An estimation of the destruction cross section by UV photons for the hydrogenated carbon particles has been derived from the reduction of the 3.4 μm intensity band as a function of the UV fluence. The results of the present work, together with previous laboratory data, can shed light on the enigmatic difference observed for the 3.4 μm band between dense and diffuse interstellar medium clouds. This difference is compatible with the transformation of hydrogenated carbon particles produced by UV photons and hydrogen atoms and with the changes of the grain properties in the two environments.

UV photodestruction of CH bonds and the evolution of the 3.4 $\mathsf{\mu}$m feature carrier