C-H bond formation in carbon grains by exposure to atomic hydrogen:: The evolution of the carrier of the interstellar 3.4 micron band

We present the results of a systematic study on the interaction of nano-sized carbon grains with atomic hydrogen. The effects of H processing have been analyzed by infrared spectroscopy. The samples were irradiated with fluences ranging between 9.2x10(16) and 1.3x10(20) H atoms cm(-2). Hydrogen atoms lead to the activation of the aliphatic C-H stretching and bending modes, whose intensity increases with exposure until saturation of hydrogenation. The plateau value of the absorption coefficient per unit mass of material at 3.4 mum is 1.6x10(3) cm(2) g(-1). The estimated cross section of C-H bond formation by H atoms for carbon particles is sigma(f) = (1.9+/-0.5) x 10(-18) cm(2) per H atom, as derived from the behavior of the 3.4 mum band intensity as a function of the H atom fluence. We have found that the C-H bond formation depends on the structure of the carbon material that is exposed to atomic hydrogen. In view of the basic role of the hydrogenation of carbon particles by H atoms in interpreting the presence of the 3.4 mum band in the diffuse interstellar medium, the behavior of carbon materials under H processing becomes a fundamental criterion for constraining their applicability as analogs of the interstellar aliphatic component. The 3.4 mum band and the doublet at 6.86 and 7.26 mum of carbon particles processed by H atoms reproduce the peak positions and the relative intensities of those observed in the spectrum of interstellar dust toward the Galactic center. The estimation of the formation cross section of C H bonds by H atoms, together with the previously determined destruction cross section by UV photons, allows a complete description of the evolution of the interstellar aliphatic carbon component due to UV and H processing. The conclusion of our analysis is that the interstellar component (i.e., the C-H bonds in the CH2 and CH3 groups) responsible for the 3.4 mum stretching band and the associated bending features at 6.85 and 7.25 mum is formed in the diffuse medium, since the carrier readily loses memory of its birthsite, wherever it is, because of interstellar processing, which determines a new equilibrium value for its degree of hydrogenation.