Influence of applied uniaxial stress upon the ultraviolet absorption bands in KBr and KI
LeCorgne, Denis York
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https://hdl.handle.net/2142/25855
Description
Title
Influence of applied uniaxial stress upon the ultraviolet absorption bands in KBr and KI
Author(s)
LeCorgne, Denis York
Issue Date
1970
Doctoral Committee Chair(s)
Compton, W.D.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
applied uniaxial stresses
ultraviolet absorption bands
Potassium bromide (KBr)
Potassium iodide (KI)
interstitial atom
Language
en
Abstract
The influence of uniaxial stress upon three types of imperfections
occurring in the alkali halide crystal lattice has been investigated.
The imperfections are the interstitial atom, the interstitial ion, and
the negative ion vacancy.
The interstitial atom, or H center, is a paraelastic defect which
assumes a preferential crystal orientation in the field of an external
mechanical stress. From the results of the reorientation kinetics -
studies, it was possible to show that H centers are not stable in the
KBr crystal lattice above 2SoK. At temperatures higher than 2SoK, the
H centers are transformed into two new paraelastic defects, H(ii) and
H(iii), possessing the same optical absorption band as the H center but
differing both from the H' center and from each other in their reorientation
kinetics. A study of the wavelength dependence of the H, H(ii),
and VI (Na+) centers s~owed the 'existence of three similar-polarized
transitions for each of these defects. One of these transitions,
located at 230 run for all of the defects studied, was determined to
be too high in energy to be explained by the simple X2
- level scheme.
In addition, a comparison of various properties of the four defects
indicates that the last three can be described as perturbed H centers.
Dichroism measurements, performed as a function of temperature and
wavelengt, h on the 230-nm I band in KBr, showed this band to be a composite
of a band at 234 nm due to the I center and a band at 230 nm attributed
to the H center. The I center dichroism was isolated and was observed
under various experimental conditions. The results of these observations
are consistent with a body-centered model for the I center in which the
I-center absorption band is attributed to the excitation of a p-like
electron on the interstitial Br- ion.
Similar measurements were also perfonned on the a band in KI. The
a-band dichroism measurements were found to be consistent with an electronic
transition from an s-like ground state to a p-like excited state, indicating
that the a center is best described as a quasi-molecule.
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