The alkali halide surfaces were the first for which full surface phonon dispersion curves were measured. Most of the early work was mainly concerned with the low-energy modes for the lighter alkali halides, such as LiF and NaF, and a full survey of a more representative sampling of these crystals was never made. A recent theoretical calculation has also suggested that relaxation effects might provide interesting features on the surface phonon modes especially for the optical modes of the heavier compounds. The work of this dissertation is an effort to measure the surface dynamics of other, usually heavier, alkali halide crystals. This work describes measurements on the surfaces of RbCl(001), RbBr(001), KBr(001), and NaI(001) with the inelastic He atom scattering technique. The energy gain or loss and the momentum change is measured by the time of flight method. For the four crystals, the surface phonon dispersion curves are obtained along both high symmetry directions. The general agreement between the theory and the experimental results is very good. Some new features of the surface vibration have been found. First, the optical mode, crossing resonance and acoustic modes were clearly seen from the KBr surface over the entire zone. The strong interaction coupling between the He and the optical mode is rather unique. Secondly, crossing resonances (S$\sb8$ mode) were observed for the KBr, RbBr and NaI surfaces. This observation suggests that this mode is a general feature of all the alkali halide surfaces. The origin of this mode might be due to the perpendicular polarized bulk acoustic motion. Thirdly, NaI results show that bound-state resonance effects could strongly enhance the bulk mode especially when there is a high density of phonon states. Finally, there is no experimental evidence supporting any surface relaxation effects. In fact, the experimental / results seem to agree better with the unrelaxed calculations. In addition to the inelastic scattering measurements, we also did angular distribution, low pressure He beam scattering and a temperature-dependent dynamical study. Information such as the surface corrugation, He-surface potential, bound state energies, the surface Debye-Waller temperature and a measure of multi-phonon contributions can be obtained from these experimental results. / Source: Dissertation Abstracts International, Volume: 51-01, Section: B, page: 0266. / Major Professor: J. G. Skofronick. / Thesis (Ph.D.)--The Florida State University, 1989.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_78155 |
| Contributors | Chern, Gung., Florida State University |
| Source Sets | Florida State University |
| Language | English |
| Detected Language | English |
| Type | Text |
| Format | 273 p. |
| Rights | On campus use only. |
| Relation | Dissertation Abstracts International |
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