Resonance enhanced multiphoton ionization (REMPI) utilizes pulsed laser radiation to prepare a molecule in an excited state via absorption of one or more photons and to subsequently ionize that level before it decays. A remarkable feature of REMPI is that the very narrow bandwidth of laser radiation makes it possible to select a specific rotational level in the initial (ground) state and to prepare the excited state of interest in a single rotational level. Thus, by suitable choice of the excitation step, it is possible to selectively ionize a species that may be present. The key objective of the effort is to carry out quantitative studies of REMPI of molecules and molecular fragments, as well as of single-photon ionization of these species by coherent VUV radiation, in order to provide a robust description of significant spectral features of interest in related experiments and needed insight into the underlying dynamics of these spectra. A major focus of the effort is joint theoretical and experimental studies of these ion rotational distributions which are being widely studied by the zero-kinetic-energy (ZEKE) technique. This technique, which is based on the detection of photoelectrons resulting from pulsed-field ionization of very high Rydberg states lying just below an ion threshold, makes it possible to obtain cation distributions with subwavenumber resolution. The unprecedented resolution of this ZEKE technique is opening up entirely new vistas in studies of photoionization dynamics, ion spectroscopy, and state-selected ion-molecule reactions. Emerging applications built on the ultra-high resolution of this technique include its use for accurate determination of thermochemically important ionization energies, for characterization of ion rovibrational level structure of large organic molecules, of elemental clusters, and of weakly bound molecular complexes, for probing reactive fragments, and for pump-probe photoelectron studies of wavepacket dynamics. This surge of experimental activity in ultra-high resolution studies of molecular photoelectron spectra continues to raise new theoretical challenges and has provided the stimulus for several of the collaborations with experimental groups in North America and Europe.

We have completed studies of ion rotational distributions produced by resonance enhanced multiphoton ionization of excited states of molecules and by single-photon ionization of ground states of jet-cooled molecules by coherent VUV radiation. The objective of this effort was to provide a robust analysis and prediction of key spectral features of interest in related experimental studies and technological applications of these laser-driven ionization techniques. Specific achievements include: identification of underlying mechanisms for anomalous behavior of ion rotational distributions in laser ionization of molecules and molecular fragments, development of schemes for exploiting such anomalous behavior to achieve state-selective production of ions, and providing needed insight into the underlying dynamics of state-resolved molecular photoionization.

The overall objective of this work is to carry out quantitative theoretical studies of these laser-driven ionization processes in molecules so as to provide both a robust description of key spectral features of interest in applications and related experiments and needed insight into these spectra. A major focus of this effort is combined theoretical-experimental studies of molecular ion spectra which are being widely studied by the zero-kinetic-energy (ZEKE) technique. This ZEKE technique, which is base3d on pulsed-field ionization (PFI) of very high Rydberg states, makes it possible to obtain ion distributions with sub-wavenumber resolution and is clearly opening up entirely new vistas in studies of molecular ionization. Some highlights of the progress include: (1) The author has extended the theoretical formulation and computational procedures used in these studies of molecular ionization spectra to general polyatomic systems; (2) He has completed combined theoretical-experimental studies of the molecular ion distributions for photoionization of H[sub 2]S, H[sub 2]CO, and CH[sub 3] by coherent VUV radiation; (3) He has carried out the first calculations of the molecular ion rotational distributions for electronically excited states of NO[sup+] (a[sup 3][Sigma][sup+]) and CO[sup+] (A[sup 2][Pi]); (4) he has also completed calculations of the ion rotational distributions for laser ionization of the small prototypical radicals OH, NH, and CH; and (5) Extensions of the studies of molecular photoionization processes of interest here to large polyatomic molecules are computationally quite demanding. These computational demands arise primarily from complexities associated with the quantum mechanical equations which must be solved to obtain the photoelectron wavefunctions required in these studies. To meet these computational needs the author is currently developing strategies for carrying out these calculations on massively parallel computers such as the Intel Paragon and Cray T3D.

The work presented in this thesis was carried out with the ultimate aim of learning about the photoionization dynamics of polyatomic molecules. This is a complex problem; in order to obtain sufficient experimental data to shed light on the dynamics careful measurement of photoelectron angular distributions (PADs) is required. Ideally these measurements are rotationally-resolved, and the angular distributions measured correspond to the formation of the molecular ion in a single rotational state. The ionization event, in the dipole approximation, can be completely described by the dipole matrix elements. If sufficient experimental data to determine the radial components of the matrix elements and associated phases, the dynamical parameters, can be obtained the photoionization experiment may be said to be complete. Analysis of such experiments requires that the initial state of the molecular system is also known, to this end resonance-enhanced multi-photon ionization (REMPI) schemes can be used in order to populate a single quantum state prior to ionization. The experiments presented here follow this methodology, with various REMPI schemes used to prepare (pump) and ionize (probe) the molecule under study, and the velocity-map imaging (VMI) technique used to (simultaneously) record the photoelectron spectra and angular distributions. Two molecules have been studied experimentally, acetylene (C2H2) and ammonia (NH3). In both cases dynamical parameters pertaining to the formation of specific states (vibronic or vibrational) of the molecular ion have been determined from experimental data. Additionally, in the ammonia work, rotationally-resolved photoelectron images were obtained.

This monograph reviews the recent progress in vacuum ultraviolet (VUV) photoionization and photodissociation processes. Photoionization, photoelectron, and fluorescence spectroscopic techniques have played an important role in revealing the photoionization and photodissociation dynamics of molecules in the vacuum ultraviolet region and in providing accurate energetic and spectroscopic information of ions as well as neutral molecules. The book represents the first detailed review of major experimental developments in the studies of single vacuum ultraviolet photon ionization and dissociation processes of gaseous molecules and clusters.

State-Selected and State-to-State Ion-Molecules Reaction Dynamics details the recent experimental and theoretical accomplishments in the field to date by some of its foremost researchers and theorists. Divided into two parts, each of which separately describe the experimental and theoretical aspects of the field, State-Selected and State-to-State Ion-Molecule Reaction Dynamics is an accessible, well organized look at a highly useful and emerging chemical specialty. Part 1, "Experiment," contains eight in-depth studies, which illustrate the key experimental work being done in the field today: Chapter 1 provide a comprehensive review of the theory and application of inhomogeneous rf fields for the study of the dynamics of low-energy ion-molecules processes Chapter 2 describes the application of multiphoton ionization (MPI) for the preparation of reactant ion states Chapter 3 reviews the application of MPI schemes for state specific cross-section measurements involving transition metal cations Chapter 4 describes the development of the threshold photoelectron secondary ion coincidence (TESICO) method Chapter 5 presents the conceptual and practical aspects of a multicoincidence technique Chapter 6 details the experimental results obtained using the photoionization and differential reactivity methods Chapter 7 reviews the several recent crossed beam studies of charge transfer and collision-induced dissociation systems involving atomic and molecular ions Chapter 8 is a survey of 15 years of high resolution crossed beam scattering of protons with atoms, diatoms, and poly-atomic molecules State-Selected and State-to-State Ion-Molecule Reaction Dynamics, Part 1: Experiment offers professionals a true state-of-the-science look at this fascinating and increasingly influential subject.