Presents a unified theoretical treatment, which is complete and rigorous but nonetheless readable. The theoretical treatment requires a variety of mathematical and physical tools. To keep the main text uncluttered, these tools are developed in comprehensive Appendices to which cross-references are made in the main text. These Appendices also ensure that the main text is useful to readers with a wide variety of scientific backgrounds and experience. These include not only spectroscopists, but also chemists, physicists, biochemists and analytical chemists. The presentation is such that postgraduate and postdoctoral students as well as more established research workers will find it valuable.