Vibrational exciton cluster states,percolation and pairwise interactions: Benzene A2u band

Infrared spectra of isotopic mixed benzene crystals (C₆H₆/C₆D₆) were obtained at about boiling nitrogen temperature, with, emphasis on the umbrella mode (at about 700 cm^?1). These are interpreted in terms of guest cluster spectra (monomers, pairs, etc.) up to about 55% mole. Static percolation sets in at about 50%, resulting in an extended exciton band. The line positions, line intensities and the percolation concentration all agree quantitatively with an effective 2-dimensional square lattice exciton interaction topology. This means that very short range, nearest neighbor interchange equivalent, excitation exchange interactions dominate over the long range transition-dipole type terms even in this most intense, dipole allowed IR band. These results fully support our recent picture of the umbrella mode exciton band, in both solid and liquid benzene. Similarities between these solid and liquid exciton bands are pointed out. The existence of pseudolocalized states, in the middle of the energy band of the extended states, is illustrated.