Photochemistry is the underlying mechanism for all of photobiology. When a molecule absorbs a photon of light, its electronic structure changes, and it reacts differently with other molecules. The energy that is absorbed from light can result in photochemical changes in the absorbing molecule, or in an adjacent molecule. The energy can also be given off as heat, or as lower energy light, i.e., fluorescence or phosphorescence, in order to return the molecule to its ground state. Each type of molecule has a different preference for which of these different mechanisms it uses to get rid of absorbed photon energy, e.g., some prefer fluorescence over chemistry. The importance of the subject has gained added dimensions with the intensive researches in the field of solar energy utilization and advances in laser technology and their applications. It is expected that such an approach will promote the basic understanding of the methodology and an appreciation for the use of sophisticated instruments needed in photochemical studies. The pace of development in the field has been very fast and more could not be included because of the constraint of keeping the size of the book within limits. Like most scientific disciplines, photochemistry utilizes the SI or metric measurement system. This book “Photochemistry” is provides all necessary information.
