VIBRATIONAL AND ELECTRONIC EXCITON STRUCTURE IN BENZENE.

Author Institution: Department of Chemistry, California Institute of TechnologyExperimental data on the exciton splitting in the benzene $^{1}B_{2u}$ electronic state and the exciton splitting in a number of ground state vibrational levels are presented. Both ultraviolet and infrared spectra of a number of isotopic pure and isotopic mixed crystals are studied. In the $^{1}B_{2u}$ state the exciton interactions among nearest neighbors are all positive $(M_{I II} = 6.9 cm^{-1}, M_{I III} = 12.5 cm^{-1}, M_{I IV} = 11.7 cm^{-1})$. It should be noted that these interaction energies are far larger than those based on an earlier interpretation by Fox and Schnepp. The static field shift D is found to be $-226 cm^{-1}$. By observing the (0-0) transition in absorption and emission, the bottom of the exciton band is shown to be of B symmetry and with $k = 0$. The ground state gerade vibrations, observed in the electronic emission spectrum, are found to have an exciton splitting of the order of $10 cm^{-1}$. The exciton splittings for ungerade vibrations, observed in infrared absorption studies, are of the same size but of opposite structure, one of the B components being the lowest. The static field shifts are $1-10 cm^{-1}$, while the site group splittings are about $5 cm^{-1}$. Exciton interactions at near resonance have been observed. Nearest neighbor interactions are discussed in terms of magnitude and relative sign