The Interpretation of Factor-Group Splittings in Weak Crystal Transitions

Abstract

Abstract The Davydov splitting in weak ultraviolet absorption systems such as those of benzene and naphthalene has been interpreted according to two different theoretical schemes. In the first the benzene crystal spectrum1 and the naphthalene spectrum2 were treated in the framework of neutral exciton theory.3 Then, since the transition dipole moment is too small to give appreciable intermolecular coupling, it is necessary to invoke higher moments of the transition, and to assign values to them to give agreement between measured and calculated Davydov splitting. The required values for naphthalene are O3c 3 = 12 A3 and O1c 3=-9 A3 which, though within physically possible limits, are somewhat higher than given by molecular orbital wave functions. The alternative proposal4 ascribes the splitting primarily to coupling between the neutral exciton state and a charge transfer state lying closely above it. In this theory the splitting depends on the separation between the two states, as well as on the properties ...