Date of Award

Summer 1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Committee Director

Gilbert R. Hoy

Committee Member

John Adam

Committee Member

Mark Havey

Committee Member

Gary E. Copeland

Committee Member

Desmond Cook

Abstract

Since 1985 nuclear-resonant scattering, and in particular nuclear forward-scattering, of synchrotron radiation has been observed in a select few Mossbauer isotopes including: 57Fe, 119Sn, 169Tm. For the first time, nuclear forward-scattering from the 9.4 keV Mossbauer level in 83Kr has been observed. A large resonance effect, comparable to that in 57Fe, has been observed in a variety of systems containing Kr including bulk crystals and physisorbed Kr-on-exfoliated graphite. Previous nuclear-resonant scattering experiments using synchrotron radiation have consisted of demonstration experiments. Nuclear forward-scattering from Kr has been applied to study the lattice dynamics of near-monolayer Kr-on-graphite by measurement of the probability for recoil-free scattering, i.e., Lamb-Mossbauer factor, (fLM) as a function of both relative coverage on the substrate and sample temperature. This represents the first practical application of the nuclear forward-scattering technique to a previously unsolved physical problem.

It was found that fLM of the physisorbed Kr displays a coverage-dependent behavior, indicating a change of the in-plane fLM due to neighbor-neighbor interactions in the layer. At low coverages, the slope of fLM with temperature is reminiscent of anharmonic behavior in the bulk, while at near-monolayer coverages, the slope of fLM with temperature is like a harmonic 3-D solid. A measurement of fLm at low temperatures through the commensurate-incommensurate transition of Kr-on-graphite shows a dramatic decrease and slow recovery in the transition region. This behavior is shown to be consistent with a simple calculation of the interaction energy of the incommensurate layer.

The nuclear forward-scattering technique is shown to be a powerful tool, complementary to conventional x-ray diffraction measurements, as the time-resolved resonant scattering is free from any non-resonant graphite background and independent of the static structure of the system.

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DOI

10.25777/tp0z-wg54

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