An exciton is an electronic excitation wave consisting of an
electron-hole pair which propagates in a nonmetallic solid. Since the
pioneering research of Fren- kel, Wannier and the Pohl group in the
1930s, a large number of experimental and theoretical studies have been
made. Due to these investigations the exciton is now a well-established
concept and the electronic structure has been clarified in great detail.
The next subjects for investigation are, naturally, dynamical processes
of excitons such as excitation, relaxation, annihilation and molecule
formation and, in fact, many interesting phenomena have been disclosed
by recent works. These excitonic processes have been recognized to be
quite important in solid-state physics because they involve a number of
basic interactions between excitons and other elementary excitations. It
is the aim of this quasi monograph to describe these excitonic processes
from both theoretical and experimental points of view. we take a few To
discuss and illustrate the excitonic processes in solids, important and
well-investigated insulating crystals as playgrounds for excitons on
which they play in a manner characteristic of each material. The
selection of the materials is made in such a way that they possess some
unique properties of excitonic processes and are adequate to cover
important interactions in which excitons are involved. In each material,
excitonic processes are described in detail from the experimental side
in order to show the whole story of excitons in a particular material.