UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier I. The case of aliphatic and aromatic molecular species

Experiments simulating the processing of various hydrocarbon species under diffuse and dense cloud conditions by UV irradiation were performed. The re sults indicate that such molecules will be efficiently dehydrogenated in in terstellar space. It is argued that the presence of hydrogen in the aliphat ic grain material in diffuse clouds results: from an equilibrium between de hydrogenation by UV processing and re-hydrogenation by the impinging atomic gas. In dense clouds, the presence of the ice layer will prevent the re-hy drogenation process, causing the carbonaceous grain material to be graduall y de-hydrogenated if UV photons are able to penetrate into the dense medium . The implications of this study for the evolution of the carbonaceous comp onent of dust in the interstellar medium are discussed.