Vibronic interaction effects constitute a new field of investigation in
the physics and chemistry of molecules and crystals that combines all
the phenomena and laws originating from the mixing of different
electronic states by nuclear displacements. This field is based on a new
concept which goes beyond the separate descriptions of electronic and
nuclear motions in the adiabatic approximation. Publications on this
topic often appear under the title of the lahn-Thller effect, although
the area of application of the new approach is much wider: the term
vibronic interaction seems to be more appropriate to the field as a
whole. The present understanding of the subject was reached only
recently, during the last quarter of a century. As a result of intensive
development of the theory and experiment, it was shown that the
nonadiabatic mixing of close-in-energy elec- tronic states under nuclear
displacements and the back influence of the modified electronic
structure on the nuclear dynamics result in a series of new effects in
the properties of molecules and crystals. The applications of the theory
of vibronic in- of spectroscopy [including visible, ultraviolet, in-
teractions cover the full range frared, Raman, EPR, NMR, nuclear
quadrupole resonance (NQR), nuclear gam- ma resonance (NOR),
photoelectron and x-ray spectroscopy], polarizability and magnetic
susceptibility, scattering phenomena, ideal and impurity crystal physics
and chemistry (including structural as well as ferroelectric phase
transitions), stereochemistry and instability of molecular (including
biological) systems, mechanisms of chemical reactions and catalysis.