Optical absorption of pure and mixed silver halides a low temperatures
Joesten, Burdette Lee
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https://hdl.handle.net/2142/25049
Description
Title
Optical absorption of pure and mixed silver halides a low temperatures
Author(s)
Joesten, Burdette Lee
Issue Date
1966
Doctoral Committee Chair(s)
Brown, Frederick C.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
optical absorption
silver halides
AgCl AgBr mixed crystals
Language
en
Abstract
The low level indirect optical absorption of pure AgCl and
AgBr has been measured between 4.20 K and0 77 K . The corresponding
indirect optical absorption of AgCl~AgBr mixed crystals has also
been measured in the same temperature range. The temperature
dependence of the absorption spectra of pure AgCl and AgBr is
explained by the theory of phonon~assisted indirect transitions
consistent with an electronic band structure with a conduction
band of standard form at k = 0 and a valence band with at least one - maximum away from k == O. For pure Agel,'! the temperature dependence
indicates that phonons of two energies are important in the indirect
transitions. The equivalent phonon temperatures are 95°K ~ 20K
o 0 for the primary phonon and 154 K ~ 9 K for the second phonon. The
4.7°K absorption spectrum of AgCl has structure which suggests that
the spin=orbit splitting at the maximum of the valence band in the
(111) direction is 0.025 eVa
The temperature dependence of the AgBr absorption spectrum
is explained primarily by indirect transitions with the emission or
40 0 absorption of 9 K ± 2 K phonons. There is some indication that
phonons near 139° K ~ 4° K also ' participate in the indirect
transitions.
At 4.20K, addition of AgBr to AgCl causes the absorption
edge to shift continuously from the pure AgCl threshold at 3.252 eV
to the pure AgBr threshold at 2.691 eV. ~he effect of the alloying
on the pure crystal absorption spectra is somewhat different for
the two extremes of mixing. In AgBr-rich mixed crystals ( > 90 mol ~
AgBr in Agel) a temperature independent component is observed with a
sharp threshold at an energy precisely half-way between the indirect
phonon emission and absorption thresholds (which still appear with
phonon energy 0.0082 eV as in pure AgBr). This suggests that in
the alloy non-vertical transitions occur without the aid of phonons
to conserve crystal momentum. For dilute concentrations of AgBr
in AgCl « 1 mol~) a weak shoulder and tail extends approximately
0.06 eV to longer wavelengths from the phonon-assisted indirect
threshold. Above 2 mol % AgBr in AgCl the absorption coefficient
increases steeply to values considerably greater than that of
pure AgBr at the same wavelength.
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