This highly topical volume will stimulate interest in this expanding field, and point the way towards future directions in research. The major objectives of this established series are to integrate theory and practice and to bring together different branches of both academic and industrial research through the presentation of critical review articles in fundamental and applied spectroscopy. The policy of the editors is to commission authoritative reviews by acknowledged leaders in the various fields of spectroscopy. Thus each volume presents a carefully composed picture of the 'state of the art for a particular area. For each volume the subject matter is presented in a manner which is Comprehensible to the non- expert, for whom the series will continue to provide a valuable introduction, and a timely overview of topics in spectroscopy which are of current interest and importance. At the same time the involved expert will find much to engage his or her attention. The series is ofinterest to research scientists and technologists, to teachers and both graduate and undergraduate students.