  We report on the preparation of hydrogenated amorphous carbon nano-particles
whose spectral characteristics include an absorption band at 217.5 nm with the
profile and characteristics of the interstellar 217.5 nm feature. Vibrational
spectra of these particles also contain the features commonly observed in
absorption and emission from dust in the diffuse interstellar medium. These
materials are produced under slow deposition conditions by minimizing the flux
of incident carbon atoms and by reducing surface mobility. The initial
chemistry leads to the formation of carbon chains, together with a limited
range of small aromatic ring molecules, and eventually results in carbon
nano-particles having an sp2/sp3 ratio = 0.4. Spectroscopic analysis of
particle composition indicates that naphthalene and naphthalene derivatives are
important constituents of this material. We suggest that carbon nano-particles
with similar composition are responsible for the appearance of the interstellar
217.5 nm band and outline how these particles can form in situ under diffuse
cloud conditions by deposition of carbon on the surface of silicate grains.
Spectral data from carbon nano-particles formed under these conditions
accurately reproduces IR emission spectra from a number of Galactic sources. We
provide the first detailed fits to observational spectra of Type A and B
emission sources based entirely on measured spectra of a carbonaceous material
that can be produced in the laboratory.
