Low frequency light scattering in fused silica and rubidium silver iodide
Field, Robert Allen
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https://hdl.handle.net/2142/25090
Description
Title
Low frequency light scattering in fused silica and rubidium silver iodide
Author(s)
Field, Robert Allen
Issue Date
1978
Doctoral Committee Chair(s)
Klein, Miles V.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
low frequency light scattering
fused silica
rubidium silver iodide
Raman spectroscopy
Rayleigh-Brillouin spectra
Language
en
Abstract
We have measured the low frequency Raman spectra and the Rayleigh-Brillouin spectra of an amorphous solid,silicon dioxide, and
a crystalline electrolyte, rubidium silver iodide. In fused silica the Raman spectra from 2K to 600K are independent of frequency shift from 4 to 10 cm-I• The scattering is attributed
to the relaxation by thermal activation at high temperatures of
the excess low frequency two-level states that are responsible for the anomalous magnitude and temperature dependence of the specific heat observed in fused silica and many other amorphous solids. The integrated intensity of the excess scattering is 10-2 times the Brillouin intensity at room temperature, consistent with the density of excess states deduced from specific heat studies. No decrease in scattering was observed in the highest temperature spectra. A double grating monochromator was used for analyzing these spectra.
In RbAg I at 90C the longitudinal acoustic phonon was observed
4 5
by Brillouin scattering at a frequency shift consistent with known and measured physical properties of the crystal. The absolute intensity was determined by comparison with fused silica. The Rayleigh Ratio is 8xlO-7 cm-I and the Pockels coefficient is estimated to be 0.1. The Brillouin linewidth was instrumental and therefore less than 0.3 GHz. A triple-pass Fabry-Perot interferometer was used to analyze these spectra.
The Rayleigh-Brillouin spectra of our RbAg I crystals were
. 4 5
dominated by an intense Rayleigh peak of width less than 10 MHz and Rayleigh Ratio of .06 cm -1 . Removal of this feature with an iodine filter enabled us to observe a dynamic central peak wider than the
350 MHz half-width of the iodine filter. An analysis and comparison of data from an interferometer and from a double monochromator lead to
the conclusion that the central peak was best fit by two Lorentzians. The broader peak has a halfwidth of 40±6 GHz and a Rayleigh Ratio of
4x10-7 cm-(one-half of the value for the Brillouin peak). The narrower peak has a halfwidth of roughly 9±3 GHz and a Rayleigh Ratio of
1
1.6xI0-6 cm-(twice the value for the Brillouin peak). The intensities and widths of these two features are consistent with the motions of mobile silver ions that dwell at a site 80% of the time and fly between sites 20% of the time.
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