The Mössbauer effect in surface studies Fe57 on Ag
Godwin, Robert Paul
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https://hdl.handle.net/2142/25046
Description
Title
The Mössbauer effect in surface studies Fe57 on Ag
Author(s)
Godwin, Robert Paul
Issue Date
1966
Doctoral Committee Chair(s)
Frauenfelder, Hans
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Mossbauer effect
crystal surface studies
Einstein model
Mossbauer recoilless fraction
Mossbauer second-order Doppler shift
Fe57 on Ag
many-body
Language
en
Abstract
The Mossbauer effect for atoms located at crystal
surfaces is discussed. We show that the Einstein model is
valid for describing qualitatively the mean square displacement
of lattice atoms. The mean square displacement
(Mossbauer recoilless fraction) and mean square velocity
(MOssbauer second-order Doppler shift) of atoms in one and
three-dimensional crystals with free surfaces are treated
in a many-body approach The simple models indicate that at
high temperatures surface atom mean square displacements are
on the order of twice bulk atom mean square displacements
(or equivalently that surface atom Debye temperatures are
about 1//2 times the corresponding bulk values) and that
surface effects penetrate only a few atomic spacings into a
crystal. A possible temperature dependence of the Mossbauer
isomer shift due to anharmonic binding forces at surfaces
is pointed out. The experimental difficulties faced in a
surface Mossbauer study, performed under ultrahigh vacuum
conditions and the apparatus used in an attempt to overcome
them are described. M~ssbauer spectra of Fe57 on LiF, in
Ag, and £n Ag have been obtained in the temperature range
100-400oK. The LiF experiment is assumed to be a:measurement
of Fe on W which had contaminated the LiF. This
experiment yielded a broad line which we interpret as a
quadrupole split pair of lines due to a field gradient
normal to the surface of about -4 x 1016 volts/cm2
e The
Fe in Ag experiment gave a single narrow line shifted at
room temperature (.056 + .002) cm/sec with respect to iron
in iron, a temperature gradient of line position
-(7.5 ± 0.5) x 10-5 cm/(sec.deg), and an effective Debye
temperature obtained from the temperature dependence of the
recoilless fraction of (253 + 12)oK. The Fe on Ag measurements
yielded an asymmetric line interpreted as a quadrupole
split line due to a field gradient of -3 x 1016 volts/cm2•
A very small isomer shift with respect to iron in iron was
observed along with a temperature dependence of the line
position of -(5.1 + 0.6) x 10-5cm/(sec.deg). An effective
Debye temperature of (380 + 30)oK was found for Fe on Ag.
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