Hydrogenation of carbon grains by exposure to hydrogen atoms: Implications for the 3.4 μm interstellar absorption band

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The 3.4 μm absorption band provides direct evidence for the presence of organic compounds in the diffuse interstellar medium. There is, however, a distinct absence of the band in the spectra of molecular cloud dust. The difference between diffuse and dense environments represents a strong constraint for any description of the formation and evolution of the band carrier material. Laboratory research under simulated interstellar medium conditions is of crucial interest for improving our understanding of the nature of this interstellar dust component. In fact, the composition of carbonaceous materials in the interstellar medium can vary in response to several evolutionary factors. Irradiation from UV photons and cosmic rays, as well as chemical interactions with the interstellar gas, can drive grain transformation. Here we discuss the results of experiments aimed at studying the interaction of nano-sized carbon grains with atomic hydrogen. H atoms induce the formation of CH bonds in carbon grains. After hydrogen exposure, the infrared spectrum of carbon grains is similar to that of the aliphatic component observed toward the Galactic center. The efficiency of the hydrogenation process and the amount of carbon which is necessary to reproduce the intensity of the interstellar feature are reported. Finally, implications for the formation and evolution of the organic materials responsible for the 3.4 μm band are discussed.