Excitons in Crystalline Benzene

Citation

Colson, Steven Douglas (1968) Excitons in Crystalline Benzene. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/B02E-Q937. https://resolver.caltech.edu/CaltechETD:etd-09122002-100424

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part I. Experimental procedure The spectrographs and excitation sources used to study the absorption and emission spectra of crystalline benzene are described and their relative merits discussed. A new purification method is also described. This method, in which the benzene is vacuum refluxed over an ~100[degree]C cesium mirror, can be used in conjunction with zone refining techniques to reduce the spectroscopic impurity level to ~10[superscript -6] mole percent. Also see Part III in this regard. A modified Bridgman type crystal growing technique that is used to produce high quality, low temperature crystals is described in detail. The development of these experimental techniques was essential to the success of the experiments described in Parts II and III. Part II. Singlet Exciton Band Structure of Crystalline Benzene The Frenkel theory for localized excitations in molecular crystals is redeveloped and extended for the case of benzene to lay a consistent theoretical framework for the interpretation of pure and mixed crystal spectra. Three types of dipole selection rules are proved for [...] transitions in a restricted Frenkel limit. They are: 1) [delta][...] =0; 2) general selection rules based on interchange symmetry; and 3) the [...] selection rule for centrosymmetric crystals. These are particularly important in the interpretation of exciton band [...] exciton band transitions. Furthermore, the interchange group and ideal mixed crystal concepts are introduced to provide a clear, precise definition of the static and dynamic contributions to the energy of the crystal states. Emphasis is placed upon the relative signs and magnitudes of the dynamic coupling constants rather than on crystal splittings since these constants give directly the anisotropy of the resonance interactions in the crystal. Experimental data from the absorption and emission spectra of the [superscript 1]B[subscript 2u] state of pure and isotopic mixed benzene crystals are also presented. From these data it can be concluded that the line shapes and transition energies are very sensitive to the effects of crystal strain. Exciton band [...] exciton band transition line shapes are interpreted in terms of the density of states function of the [superscript 1]B[subscript 2u] exciton band. This experimental function is found to be inconsistent with that predicted from the data obtained from the variation of energy denominators method of analysis of this exciton band by Nieman and Robinson. The variation of energy denominators method is investigated with more extensive and improved experiments. It is found to be inappropriate for the study of the [superscript 1]B[subscript 2u] state of crystalline benzene, and these new experiments lead to a different interpretation of this band. The resonance coupling constants obtained from this new interpretation are used to calculate the density of states function for a crystal of 32,000 molecules. This calculated function is in excellent agreement with that obtained from the exciton band [...] exciton band transition line shapes. Exciton band [...] exciton band transitions involving the lowest singlet of crystalline naphthalene are also reported and discussed. Part III. Triplet excitons in Crystalline Benzene: Location of the First and Second Triplets; Phosphoresence and Trap - Trap Energy Transfer in Isotopic mixed Crystals. The first and second triplets of solid benzene are observed using O[subscript 2] and NO perturbation techniques. The absorption spectra of long benzene crystals of ultrahigh spectroscopic purity are discussed in terms of the unperturbed transition to the second triplet state. The phosphorescence spectrum of isotopic mixed benzene crystals is analyzed with special emphasis being placed on the effects of the static crystal field. The fluorescence and absorption spectra are used to supplement this analysis. The phosporescence and fluorescence spectra of three component isotopic mixed benzene crystals are studied as a function of guest concentration, excitation lamp intensity, and, temperature. Some very interesting effects are observed at high lamp intensities and interpreted in terms of triplet - triplet annihilation of trapped excitation. Triplet excitation is found to be transferred between two different traps which are not more than ~20 [angstroms] apart in this system.

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