Dynamics of Phonons in Crystals as Studied by Picosecond Coherent Anti-Stokes Raman Spectroscopy
Schosser, Claire Lynn
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https://hdl.handle.net/2142/70256
Description
Title
Dynamics of Phonons in Crystals as Studied by Picosecond Coherent Anti-Stokes Raman Spectroscopy
Author(s)
Schosser, Claire Lynn
Issue Date
1984
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
The dynamics of the motions of ions and molecules in crystals are of fundamental interest, but due to the short correlation times of the motions (generally in the picosecond or subpicosecond range) the study of the dynamics is a difficult experimental problem. At the same time, theoretical progress in understanding the dynamics has been difficult due to the complexity of the crystal potential function. In this thesis the picosecond coherent anti-Stokes Raman (CARS) technique is applied to the study of intermolecular and intramolecular phonons in organic molecular crystals and ionic crystals. In conjunction with recent theoretical calculations on the phonon dynamics in naphthalene, the experimental results lead to an understanding of the mechanism and pathways for decay of the phonons over a wide temperature range.
In the picosecond CARS technique, a Nd:YAG laser synchronously pumps two picosecond tunable lasers. These lasers are tuned so that their difference frequency matches the frequency of the phonon transition of interest. The interaction of the beams within the sample coherently populates the phonon of interest; the state of coherence is probed by part of the higher frequency laser which travels a variable delay line. The intensity of the picosecond CARS signal is proportional to the absolute square of the correlation function of the phonon normal coordinate.
It is found from the temperature dependence of the decay rate that at low temperatures the phonons decay by spontaneous emission into two lower frequency phonons. The decay rate due to cubic anharmonicity is proportional to the sum of the two phonon density of states and the square of the cubic anharmonic matrix element for each emission and absorption process. For naphthalene external phonons these cubic anharmonic processes dominate the decay rate; the matrix elements for each process are approximately equal. For internal phonons in molecular crystals, a contribution from a pure phase decay process in addition to the lifetime process is noted. For LaF(,3), the high Debye frequency and low anharmonicity would have been expected to yield a long lifetime for the 78 cm('-1) optical phonon. However, a small symmetry distortion creates additional state density, lowering the lifetime of the phonon and resulting in a type of symmetry selection rule for phonon decay processes for LaF(,3). Other crystals with similar distortions are predicted to exhibit this selection rule as well.
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