An investigation of the photoexcitation dynamics and stability of clusters

by Davis, Kevin Matthew.

iii The research discussed in this thesis has two different themes. First, photoexcitation studies delve further into the fundamental phenomena of clusters. One of the processes considered is the emission of a delayed atomic ion upon photoexcitation of a molecular beam of metal­carbon clusters. The origin of this delayed atomic ion is uncovered by simultaneously studying the delayed ionization of zirconium and titanium carbides using species produced with a mixed metal alloy rod in a standard laser vaporization source coupled to a time­of­flight mass spectrometer. These experiments have provided the ability to observe, for the first time, both the zirconium and titanium delayed atomic ions concurrently. Through an extensive investigation of clusters formed under these conditions, the origin of the delayed atomic ions is elucidated and a mechanism is proposed. The mechanism states that the delayed atomic ion is a product of a delayed ion­pair separation that is formed due to the excitation of the MC2 molecule to a compilation of Rydberg states near its ionization threshold. The detection of C2­ following excitation of MxCy clusters provides further evidence of this mechanism being the source of the delayed M+ emission. Furthermore, the design and calibration of an anion photoelectron spectrometer is discussed in this thesis. Copper anions are isolated using a time­of­flight mass spectrometer. The copper anion is then excited to its neutral state by removing an electron. The photodetached electron is analyzed by a “velocity map imaging” photoelectron spectrometer, which determines its kinetic energy. This information allows for the determination of copper’s photoelectron spectrum, which provides insight into its iv electron affinity that can be used as a calibrant to aide the study of clusters in future experiments. The second theme of this thesis regards a new protocol that is proposed providing a pathway to producing materials with clusters as their building blocks. Through a synergistic effort by Professor Castleman’s research group, Professor S. N. Khanna’s research group, and Professor A. Sen’s research group that combined results of gas phase, theoretical and synthetic research, the feasibility of this procedure is demonstrated by making a solid via assembly and comprised of As7K3 units identified as being a stable magic cluster in the gas phase. X­ray diffraction and theoretical studies show the material to have rings consisting of As7 and K units as its building block. To our knowledge, the example presented herein offers the first viable protocol for accomplishing successful passage from free gas­phase clusters to cluster­crystals.