Density of states and σ–π mixing in hydrogenated amorphous carbon films

Abstract In this work we analyse the real and imaginary parts of the complex dielectric functions of two a-C:H samples, which are available over a wide energy range (2–25 eV). Values of the sp3/sp2 ratios matching those determined by XANES can be obtained by an optical analysis of these data, provided that an asymmetric Gaussian density of states model (for π-like states) and suitable matrix element values (for σ-like states) are used. These results also suggest that bonding and anti-bonding states sitting ∼2 eV above and below Fermi level are not pure π-states, but that some π–σ mixing is occurring. The origin and the effects of such a mixing are discussed.