EXTREME-ULTRAVIOLET EXTINCTION MEASUREMENTS ON HYDROGENATED AND DEHYDROGENATED AMORPHOUS-CARBON GRAINS

We present, for the first time, extinction measurements performed in the extreme-ultraviolet spectral region down to about 50 nm, on both dehydrogenated and hydrogenated amorphous carbon grains produced in the laboratory. The results evidence a typical extinction feature at 85-95 nm which can be attributed to an active sigma + pi plasmon. This feature is present in all samples we have analyzed, in agreement with the fact that hydrogenation cannot inhibit this kind of resonance. On the other hand, dehydrogenated amorphous carbon particles show an extinction ''hump'' at around 240 260 nm, which is lacking in the hydrogen-rich sample. In agreement with other authors, we interpret this band in terms of a collective excitation of the pi electrons which corresponds to the pi --> pi* transition. Present results allow to better characterize the spectral properties of a class of solid particles which have been invoked in various theoretical models of cosmic dust and provide useful hints to interpret the trend of both interstellar and circumstellar extinction curves.