1. G. Herzberg: Spectra of Diatomic Molecules, 2nd ed. (Van Nostrand, Princeton, NJ 1950)
2. Yu.E. Perlin: Usp. Fiz. Nauk 80, 553–595 (1963) [English transi.: Sov. Phys.—Usp.]
3. K.K. Rebane: Elementary Theory of Vibrational Structure of Impurity Spectra in Crystals (Nauka, Moscow 1968) [Russian ed.]
4. A.M. Stoneham: Theory of Defects in Solids. Electronic Structure of Defects in Insulators and Semiconductors (Clarendon, Oxford 1975) Chap. 17
5. Yu.E. Perlin, B.S. Tsukerblat: The Effects of Electron-Vibrational Interactions in the Optical Spectra of Paramagnetic Impurity Ions (Shtiintsa, Kishinev 1974) [in Russian]