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Dielectric functions of partly amorphized GaAs layers produced by deep ion implantation of different doses of 270‐keV As+ ions in crystalline (c‐) GaAs have been measured from 1.5 to 6.0 eV by spectroscopic ellipsometry. We show that these dielectric functions cannot be described as physical mixtures of amorphous (a‐) and c‐GaAs, as such models exhibit strong deviations with respect to the data near 3 eV. We determine representations of these dielectric functions as finite sums of harmonic oscillators, which allows us to describe these spectra as analytic functions of a single parameter related to the amount of damage. In this harmonic oscillator approximation (HOA), we show that damage profiles of ion‐implanted material can be nondestructively determined from ellipsometric spectra in terms of multilayer models. In a representative case, good agreement is found between damage profiles determined nondestructively in the HOA and destructively by chemical etching.