The areas of greatest progress and activity in science are usually associated with recent developments of powerful experimental techniques. The field of clusters and the studies described here are prime examples of this, owing much of their progress to the combined developments of the laser, vacuum technology, and relatively inexpensive electronic components for data collection that are especially useful on time-of-flight mass spectrometers. Although clusters represent the basic step from atoms to bulk materials, their study had to wait for certain technological developments since most bare clusters are extremely reactive and typically cannot be observed in preserved form outside a vacuum.¹ Although it is possible to deposit some clusters on surfaces or into inert-gas matrices, molecular beam techniques are the main tool of study for clusters. Metal clusters without ligands are reactive enough that deposition or solvation changes their structure and thus the common techniques of x-ray structure determination and spectroscopic absorption techniques are not applicable. The majority of bare clusters have been studied by generating a molecular beam with an oven or laser vaporization source and then probing this beam with a laser while detecting ions with a quadrupole or time-of-flight mass spectrometer.

The study of metal clusters in particular has often been a cooperative venture of experimental and theoretical work. There has often been "an immediate and nearly perfect coupling between experiment and theory"² in the study of metal clusters that has made their study enjoyable and successful. Metal clusters have aroused interest mainly because they are novel species, but also because an understanding of their chemical and electrical properties can lend insight into a theoretical understanding of the optical and electronic properties of small particles and a continuation of what is known from surface science. In the past ten years rapid progress has been made on generating and characterizing these species.

The interaction of a photon with a specific size cluster can lend insight into its optical and bonding properties, and has often been used to determine cluster geometry. However, the metallic cluster absorption spectrum is unlike those encountered with small molecules in that it is typically a confluence of electronic states with rovibronic bands that are difficult to resolve. In addition the number of geometric isomers present in the molecular beam increases rapidly with cluster size. Deducing cluster geometry through vibrationally resolved spectroscopy has only been possible for the smallest metal clusters and even then often requires collaboration with high level theoretical calculations. As cluster size increases a brute force approach that ascertains vibrational spectroscopic features rapidly fails and approximate models are soon required. In the chapters that follow an approximate model of cluster geometry is developed that assigns a 'shape' rather than an exact geometry to the cluster to explain optical properties.

Following that progression of thought, this thesis documents the design and use of an apparatus intended to probe transition metal clusters. The apparatus has now been used to detect a number of phenomena indicative of the basic properties of some transition metal clusters that were hitherto unknown. In the initial stages it was used to deposit platinum particles and clusters onto surfaces in an attempt to observe catalytic properties.⁵ It has also been used to observe the physical structure of deposited gold particles through electron microscopy.⁶ However, almost all the work in this thesis regards silver clusters.⁷ Exploratory work in the initial phases of development indicated that silver was particularly easy to vaporize in this device – a phenomenon that can probably be assigned to the near resonance between the Ag bulk plasmon and the 308nm XeCl laser used for vaporization. Pragmatically, this is the impetus for our emphasis on silver clusters.

There are essentially two key experiments that compose the main section of this thesis. The first experiment establishes the ionization potentials of a range of silver clusters and the second observes photofragmentation patterns in silver cluster cations. Associated with each experiment is a theoretical model that attempts to explain the observed properties. In addition there are two chapters whose contents are not directly related to experimental observations but are of interest as separate topics.

Chapter 2 is a presentation of the experimental apparatus and techniques that were used for both ionization potential determination and for photofragmentation. The device was modified for the latter experiment so that there are actually two separate designs referred to as Phase I and Phase II. Although this chapter is only a small part of the thesis, most of the work performed over the past few years is represented here.

Chapters 3 and Chapter 4 represent the core of the thesis. Here, the experimental determination of the ionization potentials of silver clusters is described – Chapter 3 – and a theoretical model that explains most of the observed features is documented. The entirety of Chapter 4 on the distorted sphere model is original research that has not been presented previously.

Chapter 5 is a status report on research regarding the photofragmentation of silver cluster cations – an experiment intended to investigate both photoabsorption cross sections and fragmentation pathways. This chapter includes a statistical model that is derived extensively from ideas already in the literature. Although the model is not tested extensively by the experimental findings, photofragmentation appears to agree with the model and follows statistical behavior. Since the research is still in progress, there is a certain pedagogical tone to this section with the intent that it will be extended and applied to future results.

Chapter 6 documents briefly an icosahedral metal cluster model that was also developed to explain the behavior of silver clusters. This model should be applicable to larger silver clusters than those observed experimentally in Chapter 3 and can be used to understand the unique electronic properties of icosahedrally shaped metal clusters.

Finally, the computer programs and routines that are referred to throughout this dissertation have been documented separately by the author. This documentation and the original source code are available from the author upon request. This also includes a large C++ library of mathematical objects. These are largely of interest only to individuals who are working in the laboratory or who have an interest in scientific, particularly C++, programming.

In addition to fulfilling program requirements, it is also hoped that this thesis will be of help to future researchers designing models of cluster behavior or analyzing and collecting laboratory results.^{8 9 10}

As is common for experiments in physical chemistry, a significant amout of work, not directly representable or available in the published material, has been devoted to the development, optimization, and understanding of the operating characterstics of the apparatus used in these experiments. A significant portion of this experience has already been conveyed to the next generation of scientists, however, it stands as one purpose of this document to record in complete form both the practical and theoretical knowledge of these types of devices so that this can be a tool employed by future workers to proceed rapidly beyond what has already been acomplished. It is hoped that this information will be read and useful to those researchers.