Abstract
The purpose of this paper is to lay a consistent theoretical framework for the discussion of a series of forthcoming experimental papers on the absorption and emission of light by benzene crystals. Emphasis will be placed not only on the usual Davydov splittings but also on k≠O states and band-to-band transitions in neat (pure) crystals, and on orientational effects, site splittings, shifts, and resonance pair spectra in isotopic mixed crystals. The role of translationally equivalent interactions will be discussed. The interchange-group concept is used in order to simplify the theoretical analysis. From the four possible interchange groups D2, C2υa, C2υb, C2υc, the D2 group is found to be the most convenient for the classification of benzene exciton functions. A differentiation between static and dynamic interactions is made in the limit of Frenkel excitons, and the concepts of site distortion energy and the ideal mixed crystal are introduced to aid in this distinction. Data pertaining to site shifts and splittings and resonance and quasiresonance interaction terms for the B2u1 electronic exciton band and the vibrational ν12(b1u), ν15(b2u), and ν18(e1u) exciton bands are discussed in order to illustrate briefly the theoretical concepts.
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