The effects arising from quadratic electron-phonon interaction and anharmonicity on the optical spectra of imperfections in nonmetals are examined. The absorption or emission line-shape function is expressed in terms of Kubo's formula and is reduced to a trace over phonon states only via the application of the Barrie-Sharpe method. This trace is evaluated with the aid of quantum field-theoretic techniques. It is shown that both the quadratic electron-phonon interaction and anharmonicity modify

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... he phonon spectrum, and the influence of this modification on the optical spectra is studied in detail with regard to position, intensity, width, Stokes' shift and mirror reflection symmetry.