Electronic and Vibrational Excitons in Molecular Crystals

Abstract

This review is the third in the Annual Reviews of Physical Chemistry (1) during the past eight years that treats the optical properties of molecular crystals. Two other reviews (2) on this same type of subject matter, but with slightly different emphasis, have appeared recently in the literature. Besides these review articles, other recent treatises are strongly recommended to the reader. Knox's book (3) , for example, presents a much broader presentation of exciton theory and practice than this review attempts to do. For those readers who are interested in a unified approach, not just to the Frenkel exciton problem in organic crystals, but also to other kinds of excitons in other kinds of crystals, Knox's book is strongly recommended as a starting point. A condensed but rather clear discussion of many of the important aspects of space group theory and time reversal symmetry as they apply to the general energy band problem is to be found in the book by Callaway (4) . Other good sources for this type of information are the classic paper by Koster (5) and the books on applied group theory by Tinkham (6) and Mariot (7) . Craig & Walmsley (8) outline the fundamental theoretical material neces­ sary for an understanding of the subject matter presented in this review. A fairly recent paper by Davydov (9) corrects, clarifies, and extends Davydov's own classical work on the molecular exciton problem. Because of the exist­ ence of this publication, the original book in Russian ( 10) and its later English translation ( 1 1 ) , which describes Davydov's early work, are now to be valued more for their historical content and bibliographies than as a source for study of modern molecular exciton theory. Since Hochstrasser's review, a large number of new experimental and theoretical techniques have been introduced leading to many significant advances in molecular exciton theory. The field is a much more polished one than it was in 1966, and its scope is considerably broader. The field too has taken on new significance in recent years because of the partial shift of