Amorphous materials lack long-range order in one of several possible ways. What happens in an amorphous solid following electronic excitation may differ from what occurs when a crystalline solid is similarly excited. In this chapter, we compare the behaviours following excitation of crystalline and amorphous materials. In amorphous materials, electronic excitation induces photo-luminescence and generates defects, as in crystals. Excitation of amorphous materials can lead both to distinctive gradual structural changes and to changes of macroscopic properties. In this chapter, we compare the processes induced by the excitation of specific materials in their crystalline and amorphous forms. Photo-induced processes in amorphous materials have been reviewed by Shimakawa et al. (1995) and by Morigaki (1999).

There are two major complications in referring to amorphous materials (see Stoneham (1987b), pp 4–6). The first issue concerns whether a solid is amorphous or not. Normally, an experimenter will call a solid ‘amorphous’ when there are diffraction rings, rather than spots. A theorist will say it is amorphous when there is no long-range order. These two descriptions are not equivalent. For example, quasi-crystalline and para-crystalline materials may show diffraction spots without having long-range order. Likewise, diffraction rings may indicate a powder or micro-crystalline material. The second complication is that whereas one can be sure what is meant by a crystalline system in thermal equilibrium, an amorphous material is not so well defined.