Photoelectron Spectroscopy Investigations on the Stability, Energetics, and Dynamics of Gas-phase p-block Clusters

open_access
Open Access
Author:
Melko, Joshua Jennings
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 10, 2011
Committee Members:
Albert Welford Castleman Jr., Dissertation Advisor/Co-Advisor
Albert Welford Castleman Jr., Committee Chair/Co-Chair
Nicholas Winograd, Committee Member
Ayusman Sen, Committee Member
Robert John Santoro, Committee Member
Keywords:
Assembled Nanomaterials
Clusters
Photoelectron Spectroscopy
Abstract:
Through the use of mass spectrometry and photoelectron spectroscopy, as well as theoretical calculations, the studies within this dissertation investigate the geometric and electronic structures of small clusters of p-block elements. There is a focus on identifying cluster stoichiometries that exhibit enhanced stability, as part of a synergistic collaboration to build assembled nanomaterials out of cluster building blocks. Further, the work in this dissertation reveals the guiding principles governing cluster behavior in the small size regime and various theoretical models are used to rationalize the enhanced stability of certain clusters. Investigations of AlnX (n = 1-6, X = As, Sb) clusters establish that Al3As and Al3Sb are planar, all-metal, aromatic molecules, while Al5As and Al5Sb are spherical clusters that possess large energy gaps due to closed electronic shells. The stability of these molecules is subtly affected by the identity of the non-aluminum atom, and this leads to a tunability of cluster properties. These ideas are developed through additional studies on lead-indium clusters, in which it is shown that both aromatic and closed electronic shell species are present, but that lead often drives the geometries towards close-packed arrangements. Thus, by controlling the amount of lead within the cluster, one can control the geometry. Additionally, within the lead-indium system a thorough investigation of Pb2In2 and PbIn3- reveals new rules for all-metal aromaticity, showing that despite large electronegativity differences of the constituent atoms and different geometric arrangements, aromatic character can be preserved in heavy p-block clusters, in contrast to the lighter aluminum analogs. To further explore the behavior of the heavy p-block clusters, experiment and theory are used to reveal remarkable similarities in the electronic structures of PbxSby- and BixSny- clusters. It is found that the energy and ordering of electrons within these clusters is not affected by the identity of the nuclear core. Thus, the properties of one cluster (or assembled material) may be mimicked through a substitution of different elements. Finally, this dissertation describes the invention of a new method for determining the angular momentum character of molecular orbitals. Photoelectron imaging experiments coupled with calculations reveal the degree of s-p hybridization in the valence electrons of small aluminum clusters. The Al3- cluster possesses a large degree of s-p hybridization, while Al4- is completely unhybridized. Further, the degree of hybridization continues to vary markedly for larger clusters. This work represents the first demonstration of an experimental technique that can directly probe the level of hybridization within molecular orbitals.