Analysis of Gas Phase Clusters Made from Laser-Vaporized Icosahedral Al-Pd-Mn
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Analysis of Gas Phase Clusters Made from Laser-Vaporized Icosahedral Al-Pd-Mn J.A. Barrow*, E.F. Rexer**, D.J. Sordelet*, M.F. Besser*, C.J. Jenks*, S.J. Riley** and P.A. Thiel* * Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011 ** Chemistry Division, Argonne National Laboratory, Argonne, IL, 60439 ABSTRACT Laser vaporization of an icosahedral Al-Pd-Mn sample with detection by time-of-flight mass spectrometry is used to probe metal clusters made from the alloy. After sample vaporization, clusters form by gas aggregation and may contain several to hundreds of atoms. Multi-photon ionization/fragmentation of these clusters yields mass spectra showing many cluster sizes with enhanced intensity. Clusters are identified at masses near those of pseudoMackay and Bergman clusters; however, these clusters do not appear special relative to neighboring clusters. Results of this study and its relationship to the proposed cluster structures in quasicrystalline materials are discussed. INTRODUCTION Both pseudo-Mackay icosahedral (PMI) and Bergman type cluster models have been proposed as a basic motif for the quasicrystalline structure [1,2]. Many experiments have been done which provide support for cluster models. Quasicrystals are known to have low thermal and electrical conductivity. Measurements of these properties suggest a hierarchical localization. This would be consistent with electron localization inherent to a cluster-based structure [3]. Studies of cleavage properties and plasticity have been interpreted within the context of stable clusters as structural sub-units [4,5]. In addition, neutron scattering studies were performed on the liquid melt of quasicrystal-forming alloys during the solidification process [6]. In these studies short-range icosahedral order was identified. The cluster models proposed do begin to describe the bulk quasicrystalline structure; however, such models require clusters to be overlapping and interlaced. It is difficult to explain how these clusters could assemble in an energetically favorable way. A cluster jellium model has been proposed to describe the structure growth [3]. In this model the inflation is driven by attempts to obtain a stabilizing closed-shell electronic configuration for the clusters. It may be useful to investigate quasicrystal structure with experiments designed to probe isolated metal clusters. An understanding of the fundamental interactions between atomic species in the clusters may provide insight regarding electronic and geometric stability. Additionally, studying the chemical properties of the clusters may aid our understanding of the metal surface since the clusters have a high percentage of surface atoms. The method of laser vaporization is used in our study. After vaporizing the sample, the atoms condense into clusters, which are subsequently detected by time-of-flight mass spectrometry (TOF-MS). This technique has proven to be very successful for the study of both electronic and geometric structure of gas phase metal clusters. Th
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