Activation of an ultraviolet resonance in hydrogenated amorphous carbon grains by exposure to ultraviolet radiation

The results of an experiment aimed at simulating the UV processing of hydrogenated amorphous carbon grains that occurs in the interstellar medium are presented. UV exposure of these grains induces significant changes in the UV-visible spectrum and, in particular, activates a resonance at 215 +/- 2 nm, very close to the position of the interstellar extinction bump. This is the first reported laboratory observation of the activation of a UV resonance in hydrogenated carbon grains irradiated by UV photons. The spectral variations depend on the UV dose deposited in the samples; as the dose increases, the band becomes more intense while its peak position remains stable. We attribute the band to pi-pi* electronic transitions in sp(2) ringed clusters that form the grains and interpret the spectral variations in terms of structural changes of the grains. This interpretation is confirmed by the behavior of the optical gap, which indicates an increase of the sp(2) clustering degree as a function of the grain processing. The results of the present experiment suggest that it is unlikely that hydrogenated amorphous carbon grains can be transformed into pure graphite grains by UV processing in a typical diffuse-cloud timescale. The possibility that the interstellar bump is due to pi-pi* transitions in graphitic clusters that form small carbon grains is analyzed. This hypothesis requires a definite internal structure for the bump carrier. A process able to determine the grain structure is briefly discussed.