On the action of UV photons on hydrogenated fulleranes C60H36 and C60D36

Fulleranes, the hydrogenated fullerenes C60H36 and C60D36 have been synthesized in n-hexane where they show an absorption maximum at 217 nm in their electronic absorption spectra with molar extinction coefficients epsilon(217) = 17 140 and 16 480 L cm-1 mol-1, respectively, which correspond to absorption cross-sections of order 6500 Mbarn, about 10 times higher than for C-60. We find that the absorption curves measured for these two fulleranes fit well the wavelength, width and shape of the ultraviolet (UV) bump (217 nm) in the extinction curve of the diffuse interstellar medium. If present in the interstellar medium these fulleranes would contribute significantly to the bump extinction, although interstellar carbon budget considerations indicate that C60H36 cannot be the only contributor. The photolysis of fulleranes with monochromatic light at 245 nm causes a shift of the absorption maximum from 217 nm to longer wavelengths. This band shift of the absorption maximum has already been observed in the photolysis or in the thermal processing of hydrogenated carbon dust. The band shift is due to the release of molecular hydrogen, a process which causes the aromatization of the carbon dust. C60H36 and C60D36 are photolyzed at a rate k(C60H36) = 2.45 x 10-3 s-1 while k(C60D36) = 2.27 x 10-3 s-1. This implies an isotope effect so that C60H36 is photolyzed 1.08 times faster than C60D36. In the interstellar medium, this implies a deuterium enrichment of the hydrogenated interstellar carbon dust. The presence of a measurable isotope effect suggests that the photolysis of C60H36 and C60D36 involves the rupture of the C-H and C-D bond with release of H-2. C60H36 releases completely hydrogen when heated to 630 degrees C under inert atmosphere producing back C-60 as shown by FT-IR spectroscopy. Thus in the interstellar medium an equilibrium should occur between the hydrogenation of C-60 with atomic hydrogen and the emission of molecular hydrogen because of the action of UV photons or thermal processing. Interstellar fullerenes and interstellar carbon dust containing fullerenes and fullerene-like structures should play a key role in the formation of molecular hydrogen starting from atomic hydrogen and as carriers of the UV bump of the interstellar extinction curve.