ON THE ELECTRONIC-STRUCTURE OF SMALL CARBON GRAINS OF ASTROPHYSICAL INTEREST

In a previous paper Mennella et al. (1995a) studied the evolution of t he UV spectrum of small carbon grains due to thermal annealing in the range 250-800 degrees C. The spectral variations were interpreted in t erms of internal structural rearrangement of the grains caused by hydr ogen loss. The electronic transitions (sigma-sigma and pi-pi*) of the sp(2) clusters forming the grains were indicated as the major factors responsible for determining their extinction properties. In this pape r we present the results of new measurements aimed at probing the heat -induced structural changes. The thermal evolution of the optical gap and of the Raman spectrum, both sensitive to the sp(2) clustering degr ee, confirms that the observed spectral changes do depend on structura l variations. In fact, the pi electron delocalization of the sp(2) clu sters determines a link between structural and electronic properties i n carbons. We find a basic correlation between the UV peak position an d the optical gap. It is interpreted in terms of a dependence of the d ipole matrix momentum of pi transitions on the sp(2) cluster size. The attribution of the spectral changes to the grain internal structure i s corroborated by morphological analyses. Scanning and transmission el ectron microscope images show that the fluffy structure of the samples as well as the dimension and the shape of the single grains do not ch ange after the annealing process. In the astrophysical context, the pr esent results can be relevant for the attribution of the 217.5 nm feat ure, as they show that the internal structure of carbon grains, having sizes similar to those expected for the ''bump'' carriers, controls t he interaction with UV photons.