Clusters containing both metallic and non-metallic elements have been synthesised by pulsed laser vaporisation of a solid target, followed by entrainment and nucleation of the vaporised material in a pulsed supersonic expansion with an inert carrier gas. The resulting cluster distributions were characterised by laser photoionisation time-of-flight mass spectrometry. Such spectra display intensity anomalies, or so-called 'magic numbers', associated with enhanced geometric and/or electronic stabilities, for particular cluster sizes. These are known as shell effects. An overview of the shell effects used to account for cluster magic numbers is presented. Details of the types of electronic and geometric structures which can be used to rationalise the existence of particularly stable clusters are discussed. The abundance patterns and intensity anomalies found in the cluster mass spectra obtained in this work are interpreted in these terms. Studies of clusters of the alkaline earth elements of group-IIA reveal a competition between pre-crystalline icosahedral structures and fragments of the bulk crystal form. These are the first reported laser vaporisation experiments on such systems. Barium clusters containing up to 32 atoms showed a preference for an icosahedral geometry, even on the inclusion of a small number of oxygen atoms. In comparison, strontium clusters of a similar size were found to attract more oxygen atoms, and to adopt a very different structure based on the face-centred cubic (fcc) lattice of bulk strontium oxide. These observations are related to the known structures of other group-IIA metal clusters and cluster oxides. In order to examine shell effects in more complex systems, photoionisation mass spectra have also been recorded for iron clusters containing up to several hundred atoms. In this case, intensity anomalies in the cluster distributions could not be attributed to one particular shell effect, but involved a competition between both electronic and geometric shell structure.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657800 |
| Date | January 1994 |
| Creators | Miller, Jon Ferguson |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12660 |
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