The role of the multi-mode vibrational spectrum of crystals with struc
tural defects in optical phenomena such as intracenter absorption and
luminescence, internal photoeffect, and resonant Raman scattering is e
xamined. A comparison is performed between the quasimolecular model an
d those with more comprehensive account of the electron coupling with
the quasicontinual vibrational spectrum. It is shown, that the multi-m
ode character of the impurity crystal vibrational subsystem must be ta
ken into consideration to interpret adequately the optical experiments
. A review of theoretical and experimental results concerning this pro
blem is presented.