Electronic excitation is a means to change materials properties. It offers both novel types of change and better-controlled changes. The new developments in laser technologies, in electron beam methods, and also in the use of particle beams have led to a wide range of new opportunites, both scientific and technological.

The basic ideas are not new. Photography is over 150 years old. Photochromics have been on sale for decades. Photolithography, electron lithography, and ablation are standard tools. Photosynthesis is nearly as old as life, and incorporates some of the ideas we shall describe. What has changed in the last few years has been the gradual growth in insight in areas which are not usually connected, and a vast increase in the technology available to manipulate materials by excitation in a controlled way. Our aim is to analyse the important features of the changes induced by electronic excitation, to identify what is critical, and to provide a basis from which materials modification can be developed successfully.

We shall emphasise the mechanisms underlying materials modifications in inorganic non-metals following electronic excitation. Metals and organics are mentioned mainly when they show interesting parallels with the inorganic insulators which we emphasise. It would take at least another book to cover the photochemistry of organic and biological systems. We shall not (and could not) cover every aspect of electronic excited states, which would need to include vast areas of photonics.