While photoconductivity has been observed with many ionic
solids, in the general case the first absorption maximum in the
long wavelength end of the absorption spectrum does not correspond
to the energy required to elevate an electron into the conductance
band, but rather to a lower energy which is associated with the
formation as an excited species called an exciton (2). This is
regarded as an electron associated with its positive hole and is
thus a neutral species: so despite the fact that it is mobil:: it
does not contribute to the electronic conductivity of the solid.
It has been found to be a kinetically significant factor in many
The excited species normally returns to the ground state by
a radiationless transition, i.e. by dissipating its energy thermally
to the lattice. However, if the potential energy curves of the
ground and excited states do not overlap then reversion to the
ground state can only take place by re- emission of radiation,
i.e. luminescence. Another form of fluorescence, phosphorescence,
takes place if the excited species is trapped in a metastable
state at an impurity centre (copper, silver or gold, in zinc
sulphide) and re- emission of radiation takes place after a certain
time interval, dependent on the stability of the metastable state.
These phenomena represent effects of radiation which are
reversible since the crystal can be restored to its ground state.
In certain salts, the silver halides, metallic azides and
oxalates, the effect of radiation may be permanent and result in
a definite chemical change.