Vibrational Dynamics in Disordered Molecular Crystals by Picosecond Coherent Raman and Photon Echo Spectroscopies (Exciton, Low Temperature)
Chronister, Eric Lee
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https://hdl.handle.net/2142/77312
Description
Title
Vibrational Dynamics in Disordered Molecular Crystals by Picosecond Coherent Raman and Photon Echo Spectroscopies (Exciton, Low Temperature)
Author(s)
Chronister, Eric Lee
Issue Date
1985
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Language
eng
Abstract
An understanding of vibrational dynamics in complex molecules is a fundamental problem in chemical physics and is important for discerning how vibrational relaxation effects actual chemical reaction rates. The natural states of a pure crystal, however, are delocalized excitons and are described by the language of solid state physics. In this thesis the important processes involved in vibrational exciton (vibron) dynamics are differentiated by investigating vibron relaxation in isotopically and chemically disordered crystals at low temperature by picosecond time-delayed Coherent Anti-Stokes Raman Scattering (psCARS) and photon echo spectroscopy (PE).
Picosecond laser spectroscopy is used to observe vibrational dynamics directly in the time domain. PsCARS is very useful for the investigation of pure and heavily disordered crystals since the Raman effect has a small cross section and consequently low net absorption. The photon echo measurements are used to examine vibron relaxation above the electronic excited state and can be used on very dilute chemically mixed crystals since the laser induced transitions are dipole allowed. The experiments performed were all done at low temperature (1.5(DEGREES)K and 10(DEGREES)K) where the vibrational relaxation rates are slowed allowing the effect of crystal disorder to be investigated.
The results of this study have shown that vibrational relaxation in low temperature molecular crystals depends on a variety of effects. In a pure crystal the effect of molecular symmetry has been shown to be important in determining whether fundamental or combination vibrations are involved in vibrational relaxation. The studies of isotopically mixed crystals have shown that one and two site processes can be of importance and have shown that relaxation is greatly affected by a lower energy state of the same normal coordinate on the heavier isotope. In addition, the photon echo experiments look at isolated guest molecules in a chemically different host and have shown that relaxation of the guest can be slower than the host modes of nearly equal energy, thus indicating single site decay with the emission of bulk phonons.
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