Lifetimes in Excited States

Abstract

This review attempts to cover developments within the past year or two up to the end of January 1971. Our understanding is now approaching the level where we can begin to comprehend experimental results in connection with theoretical developments. Nevertheless, though our understanding is just beginning, we are already faced with a large body of data, as well as a large body of theoretical models and discussions. Hence, some arbitrary selection had to be made here to emphasize those contributions which are particularly helpful, admittedly in the eyes of the writers, in demonstrating fundamental principles. Our apologies in this matter are perhaps somewhat mitigated by the presence of some outstanding books and reviews which cover in more complete detail some of the points neglected in this review. In particular, there is the recent treatise by Birks on photophysics (1) as well as the excellent discussion of the triplet state by McGlynn, Azumi & Kinoshita (2) and the book by Becker on fluorescence and phosphorescence (3). Interest in the field has also been furthered by several conferences: in particular, the Conference at Loyola University in Chicago in 1968 (4), the Conference on Radiationless Transitions in Paris in 1969 (5), the Delaware Luminescence Conference in 1969 (6), the EUCHEM Conference in Elmau in 1970, and the Columbus Conference in 1970. At this writing another EUCHEM Conference on Radiationless Processes is announced for April 1971 in Wales. Several reviews have appeared, most notable the one by Henry & Kasha covering theoretical approaches (7), as well as the ones by Birks & Munro (8) and by Jortner, Rice & Hochstrasser (9). This review will limit itself to a discussion of the processes pursuant to the absorption of light and the formation of an excited electronic state. In particu­ lar, singlet states may be formed which then can fluoresce or undergo various nonradiative processes. Also, triplet states can be formed which then phos­ phoresce or undergo other radiationless processes. Radiationless processes would collectively describe the typical case of photochemical reaction, the change in electronic state with the conservation of spin (internal conversion), or the change in electronic state with a change in spin (intersystems crossing), such as singlet to triplet crossing. The processes have characteristic rate constants, or lifetimes (T = k-1), which characterize the rate process and may