A method of calculating optical transitions from a non-degenerated electronic state to a twofold degenerated or quasi-degenerated electronic state in an impurity center of a crystal is proposed. The vibronic interaction with non-totally symmetrical local modes and phonons is taken into account. The latter interaction results in the decay of vibronic levels and in the replacement of vibronic lines in the spectrum by phonon-assisted bands. The results are illustrated by the calculations of optical spectra in the Debye-Van Hove model of phonons. (C) 2012 Elsevier B. V. All rights reserved.