The stationary states of condensed systems such as crystals
are characterized by energy bands. These energy bands are described
by a dispersion relation and a density function. Within the Frenkel
tight-binding method, the physical quantities that determine the band
structure are the intermolecular resonance interactions.
The density functions for the first excited singlet states of
crystalline benzene and naphthalene are determined experimentally
from spectral data involving band ↔ band transitions. The experimental
results are not in complete agreement with a transition
octopole model for the intermolecular interactions.
Mixed molecular crystals provide theoretically and experimentally
tractable systems for studying the properties of molecular
aggregates. This knowledge is basic to understanding the liquid and
biological states and may in the future be of significant technological
importance. Spectroscopic observations on isotopic mixed crystals
of naphthalene are made to determine the energy of the crystal states
that correlate with the 1B2u state of the free naphthalene molecule.
The spectral data for the dilute crystals are interpreted in terms of
a one-particle Green's function and are consistent with the band
structure as observed in band ↔ band transitions. The transition
energies of guest levels disagree with a model involving configuration
interaction with charge transfer states. New theoretical models are
suggested, and the data available for evaluating these models are
outlined.
Very high resolution spectra at 4.2 °K reveal fine structure in
the 1B2u ← 1Ag and 3BIu → 1Ag electromc trans1t10ns of the naphthalene
mixed crystals. Some of the structure corresponds to the resonance
splitting of pairs of guest molecules in the host lattice. In the Frenkel
tight-binding approximation, this structure gives directly the intermolecular
excitation transfer matrix elements responsible for the
exciton mobilities and the energy band structures.
Optical spectra of 13CC5H6-C6H6 mixed crystals show that the
shallow impurity 13CC5H6 shifts the 1B2u factor group components by
2 cm-1 and increases the linewidth by 5 cm-1 in going from 6% to 50%
13CC5H6. The effect is explained qualitatively by an extension of the
Frenkel exciton theory to the mixed crystal system.
Exciton structure in the two lowest ungerade triplet states of
crystalline naphthalene is reported. For the lowest state the calculated
splitting of 40 cm-1 is in good agreement with the expe...