Application Of Ion Imaging And Laser Induced Gratings To Chemical Dynamics

This book provides an introduction on applications of lasers in Chemistry. It describes laser as a tool for chemistry, the consideration involved in describing a laser beam and what happens to beam as it is propagated through a gas. The book is useful for graduates and advanced undergraduates.

This book highlights recent progress in the chemistry of radicals. Developments include the growing use of lasers to generate radicals, the application of lasers to provide state, angular, polarization, energy and real-time resolution in kinetics and dynamics experiments, the development of theories for handling the reactions of radicals, and the simulation of the reaction dynamics of increasingly larger systems for direct comparison to experimental results. The book emphasizes the increasing interaction between experimental dynamics, kinetics and theory. It is appropriate for chemistry graduate students and researchers about to enter the field. However, the discussions of some topics progress to a more advanced level so that even an expert will find the book useful.

Laser Chemistry: Spectroscopy, Dynamics and Applications provides a basic introduction to the subject, written for students and other novices. It assumes little in the way of prior knowledge, and carefully guides the reader through the important theory and concepts whilst introducing key techniques and applications.

This book highlights recent progress in the chemistry of radicals. Developments include the growing use of lasers to generate radicals, the application of lasers to provide state, angular, polarization, energy and real-time resolution in kinetics and dynamics experiments, the development of theories for handling the reactions of radicals, and the simulation of the reaction dynamics of increasingly larger systems for direct comparison to experimental results. The book emphasizes the increasing interaction between experimental dynamics, kinetics and theory. It is appropriate for chemistry graduate students and researchers about to enter the field. However, the discussions of some topics progress to a more advanced level so that even an expert will find the book useful.

Out of the many tools for probing molecular dynamics, intense, ultrafast laser pulses are particularly well suited for this purpose. First, these pulses have temporal durations shorter than the typical rotational and vibrational periods of molecules and therefore allow the observation of molecular dynamics on their native timescales. Further, the broad bandwidth and high peak intensities of these laser pulses can result in the excitation of many transition pathways that may interfere and enable control of dynamics. The primary focus of this work is the ultrafast laser-induced dissociation of molecular ions. We generate these ions as "fast" beam targets and study their fragmentation using a coincidence three-dimensional (3D) momentum imaging technique, which allows the measurement of all nuclear fragments, including neutrals. This approach is employed to study laser-induced processes in a variety of molecules. The goal of these efforts is not to study specific molecules but rather to use them as testing grounds to deepen our knowledge of laser-induced molecular dynamics in general. For example, we find that permanent-dipole transitions, which are commonly overlooked in the interpretation of strong-field experiments, play a key role in laser-induced dissociation of metastable NO2+ ions. General consideration of these transitions in heteronuclear molecules is important in building our understanding towards more complex molecules. Speaking of more complex systems, we have also begun investigating the laser-induced dynamics of simple hydrocarbons. Our use of molecular ion beam targets gives us the unique ability to exercise control over the initial "configuration," i.e., geometry of these molecules. Utilizing C2H2^q ion beam targets (where q is the molecular ion charge state) prepared in various initial configurations, including acetylene (HCCH), vinylidene (H2CC), and cis/trans, we have determined that this property has an immense impact on the isomerization dynamics, a finding that we anticipate will lead to future work towards deeper understanding. More broadly, this approach of probing molecules in different initial configurations offers a unique perspective that could be complementary to mainstream methods-not just in the case of C2H2 but other chemical systems as well. We also describe some improvements to the 3D momentum imaging methods that facilitate the study of molecular dynamics. One of these developments is a method to distinguish and evaluate the momenta of neutral-neutral channels resulting from the fragmentation of negative ion beams. The second is a technique for imaging the breakup of long-lived metastable molecules decaying in flight to the detector and retrieving the lifetime(s) of the populated states. Our collaborative efforts in adaptive closed-loop control are also discussed. Here, an evolutionary learning algorithm supplied with experimental feedback obtains optimally-shaped ultrashort laser pulses for driving targeted molecular dynamics. While the complexity of the shaped pulses can make interpretation challenging, the combination of these efforts with basic experiments like those we perform using ion beams can help. In closing, the work presented in this thesis extends from diatomic to polyatomic molecules, following the natural progression of building from simpler to more complex systems. We believe that the results of these efforts aid in the advancement of understanding strong-field molecular dynamics and will stimulate future research endeavors along these directions.

The possibility of initiating chemical reactions by high-intensity laser exci tation has captured the imagination of chemists and physicists as well as of industrial scientists and the scientifically informed public in general ever since the laser first became available. Initially, great hopes were held that laser-induced chemistry would revolutionize synthetic chemistry, making possible "bond-specific" or "mode-specific" reactions that were impos sible to achieve under thermal equilibrium conditions. Indeed, some of the early work in this area, typically employing high-power continuous-wave sources, was interpreted in just this way. With further investigation, however, a more conservative picture has emerged, with the laser taking its place as one of a number of available methods for initiation of high-energy chemical transformations. Unlike a number of these methods, such as flash photolysis, shock tubes, and electron-beam radiolysis, the laser is capable of a high degree of spatial and molecular localization of deposited energy, which in turn is reflected in such applications as isotope enrichment or localized surface treatments. The use of lasers to initiate chemical processes has led to the discovery of several distinctly new molecular phenomena, foremost among which is that of multiple-photon excitation and dissociation of polyatomic molecules. This research area has received the greatest attention thus far and forms the focus of the present volume.