Energy States and Intermolecular Integrations in Molecular Aggregates--Crystalline Naphthalene

Citation

Hanson, David Marvin (1969) Energy States and Intermolecular Integrations in Molecular Aggregates--Crystalline Naphthalene. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4GN0-9K47. https://resolver.caltech.edu/CaltechTHESIS:06022017-101853292

Abstract

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 ¹B_2u 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 ¹B_2u ← ¹A_g and ³B_Iu → ¹A_g 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 ¹B_2u 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 experimental result.

The Raman scattering tensor for each vibrational mode is determined by polarized Raman scattering from oriented single crystals. The experimental data when compared with the phosphorescence spectrum of the C₁₀H₈ - C₁₀D₈ mixed crystal allow unambiguous vibrational assignments to be made and provide a measure of the intermolecular interactions in the crystal. It is found that the crystal effects on the gerade vibrations are small. Frequency shifts are 2-4 cm^-1; exciton splittings are less than 1 cm^-1; and intensities are described qualitatively by the oriented gas model.

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