STRONG-FIELD IONIZATION STUDIES OF HOMO- AND HETEROGENEOUS TRANSITION METAL CLUSTERS

Open Access
Author:
Blumling, Daniel Edward
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 05, 2009
Committee Members:
  • Albert Welford Castleman Jr., Dissertation Advisor
  • Albert Welford Castleman Jr., Committee Chair
  • J B Anderson, Committee Member
  • K E Mueller, Committee Member
  • R Santoro, Committee Member
Keywords:
  • Coulomb exposion
  • strong-field ionization
  • transition metal clusters
  • time-of-flight mass spectrometry
Abstract:
The interaction of intense electric fields with clusters has been an active area of research following the observation of the first laser-induced Coulomb explosion of a cluster in 1994. The research reported in this dissertation focuses on the strong-field ionization behavior of small clusters composed of early transition metals, carbon, and oxygen. Specifically, several Group IV, V, and VI transition metals have been bonded either with themselves or in combination with sufficient oxygen or carbon atoms to form a variety of small (fewer than 40 atoms) cluster species. Following the ionization of these clusters via ultrashort laser pulses, observations are made regarding the ion products, their energies, and the mechanisms which led to their creation. Time-of-flight mass spectrometry is used to obtain data on the resulting species. A general overview of laser-matter interactions and strong-field ionization is provided in Chapter 1. The experimental apparati, including a colliding-pulse, mode-locked dye laser, a laser ablation cluster source, and a reflectron time-of-flight mass spectrometer, are described in Chapter 2. In Chapter 3, strong-field ionization studies of transition metal (Ti, V, Cr, Nb, or Ta) oxide clusters are presented. Trends relating the reported ionization energies of the component atoms and the observed maximum charge states of the ions are reported. Discussion is offered relating the observed ionization behavior to the most commonly considered enhanced ionization mechanisms from the literature. The results of the strong-field ionization of pure transition metal clusters are then reported in Chapter 4 and this data is compared to that obtained for the transition metal oxide species. The maximum ionization states for the metal atoms in both the homo- and heteronuclear systems were identical and the ramifications of this phenomenon with regard to ionization dynamics are discussed. Finally, Chapter 5 contains data and analysis of the strong-field ionization and subsequent Coulomb explosion of transition metal carbide clusters. Remarkably, the maximum charge states for each constituent transition metal atom in both types of heteronuclear system, as well as the pure metal clusters, were identical following ultrashort laser ionization. Studies of these systems satisfy several specific goals in laser-induced Coulomb explosion research. First, the theory regarding strong-field ionization of clusters in this size regime is somewhat lacking, and the reported ionization mechanisms are complex and not unambiguous. The additional information regarding experimental values for maximum charge states with respect to not only cluster composition but also the ionizing laser conditions should prove beneficial to the advancement of theoretical models. In that same vein, studies of heteronuclear, covalently-bound clusters have never been reported in the literature, and thus the information garnered from these experiments provides a perspective as yet unavailable. Further, by systematically controlling the elemental composition of our cluster distributions, we have been able to observe trends in the ionization behavior with respect to the overall cluster composition and its effects on the individual atomic species contained with these species.