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Context. The properties of hydrogenated amorphous carbon (a-C:H) dust areknown to evolve in response to the local conditions. Aims. We present anadaptable model for the determination of the optical properties oflow-temperature, interstellar a-C:H grains that is based on the fundamentalphysics of their composition. Methods. The imaginary part of the refractiveindex, k, for a-C:H materials, from 50 eV to cm wavelengths, is derived and thereal part, n, of the refractive index is then calculated using theKramers-Kronig relations. Results. The formulated optEC(s) model allows adetermination of the complex dielectric function, epsilon, and refractiveindex, m(n, k), for a-C:H materials as a continuous function the band gap, Eg ,which is shown to lie in the range = -0.1 to 2.7 eV. We provide expressionsthat enable a determination of their optical constants and tabulate m(n, k, Eg) for 14 different values of Eg . We explore the evolution of the likelyextinction and emission behaviours of a-C:H grains and estimate the relevanttransformation time-scales. Conclusions. With the optEC(s) model we are able topredict how the optical properties of an a-C:H dust component in theinterstellar medium will evolve in response to, principally, the localinterstellar radiation field. The evolution of a-C:H materials appears to beconsistent with many dust extinction, absorption, scattering and emissionproperties, and also with H2 molecule, daughter PAH and hydrocarbon moleculeformation resulting from its photo-driven decomposition.

Variations on a theme - the evolution of hydrocarbon solids: II. Optical property modelling - the optEC(s) model