Crossed-beam chemical reaction dynamics probed with universal and state resolved ion imaging

Dr. Arthur G. Suits, Thesis Supervisor.Field of study: Chemistry.Includes vita."July 2018."The main goal of chemical reaction dynamics is to unravel the intimate motions of individual atoms during a chemical transformation. This information must generally be inferred from indirect macroscopic measurement. Very important information such as translational energy dependence of the reaction cross-section, vibrational mode-specific promotion of reactivity, product angular and velocity distributions are normally extracted. Understanding how these chemical reactions occur at the microscopic level gives us a better insight in understanding reactive intermediates and products of reaction. For a better understanding of the elementary chemical reactions, it is imperative that the studies are performed under well-defined laboratory conditions. Over the last few decades, the field has witnessed unprecedented advances in both experiment and theory. Advancements in generating reactants, state selection, improvement of crossed-molecular beam machines and products detection have gone a long way to improve our ability in studying chemical reactions in the gas phase. In 1986, Hershbach,[1] Lee[2] , and Polayni[3] together shared the Nobel Prize in Chemistry for their work on the dynamics of gas phase reactions.Includes bibliographical references (pages 87-101)

Radiation Induced Processes in Biomolecules and Clusters in Controlled Beams

The fundamental nanoscale processes that initiate radiation damage in biological material have not yet been fully elucidated. This represents a significant barrier to developing multidimensional simulations of radiation effects that can lead to advances in radiotherapy and radioprotection. This thesis explores UV- and electron-induced processes in DNA and RNA bases. Pure and hydrated clusters are studied in order to better understand the effects of the chemical environment on the radiation response of these important biomolecules. Although extensive research has been carried out on the relaxation pathways of UV-excited nucleobases, no previous experiments have investigated bond breaking in neutral electronic excited states. This thesis reveals a new fragment ion from uracil (C3H4N2O+) that can be accessed by multi-photon ionization (MPI) but not by electron impact ionization (ElI). This provides the first experimental demonstration that neutral excited state dynamics in a nucleobase can lead to bond breaking in the aromatic ring, as predicted in recent theoretical studies. The specific excited state dynamics have not yet been identified definitively and are the subject of on-going ultrafast pump-probe experiments in collaboration with Townsend and co-workers (Heriot-Watt University). The time-resolved measurements provide new evidence supporting a theoretically predicted relaxation pathway into long-lived triplet states. Dissociative ionization of hydrated nucleobases and uracil-adenine clusters has been studied experimentally for the first time. Evidence for deamination reactions is observed in hydrated adenine complexes. The production of C3H4N2O+ fragments from uracil is strongly suppressed by clustering with water whereas the channel remains open in uracil-adenine complexes. To unravel the specific cluster-mediated dynamics and reactions responsible for these effects, further experiments are required with greater control over the cluster targets. Indeed the range of monomers and cluster configurations in neutral beams currently limits interpretations and direct comparisons with calculations. In response to this challenge, a new experiment has been built that enables radiation effects to be studied on molecules and clusters in Stark-deflected beams (MPI, ElI, and future electron attachment measurements). Early results on nitromethane beams include a demonstration that studying ElI as a function of the Stark deflector voltage can be used to deduce whether certain product ions came from monomers or from clusters

Role of diffusion on molecular tagging velocimetry technique for rarefied gas flow analysis

The molecular tagging velocimetry (MTV) is a well-suited technique for velocity field measurement in gas flows. Typically, a line is tagged by a laser beam within the gas flow seeded with light emitting acetone molecules. Positions of the luminescent molecules are then observed at successive times and the velocity field is deduced from the analysis of the tagged line displacement and deformation. However, the displacement evolution is expected to be affected by molecular diffusion, when the gas is rarefied. Therefore, there is no direct and simple relationship between the velocity field and the measured displacement of the initial tagged line. This paper addresses the study of tracer molecules diffusion through a background gas flowing in a channel delimited by planar walls. Tracer and background species are supposed to be governed by a system of coupled Boltzmann equations, numerically solved by the direct simulation Monte Carlo (DSMC) method. Simulations confirm that the diffusion of tracer species becomes significant as the degree of rarefaction of the gas flow increases. It is shown that a simple advection–diffusion equation provides an accurate description of tracer molecules behavior, in spite of the non-equilibrium state of the background gas. A simple reconstruction algorithm based on the advection–diffusion equation has been developed to obtain the velocity profile from the displacement field. This reconstruction algorithm has been numerically tested on DSMC generated data. Results help estimating an upper bound on the flow rarefaction degree, above which MTV measurements might become problematic

Velocity Augmentation of a Supersonic Source and The Production of Slow, Cold, Molecular Beams

In this thesis I describe the second generation of a rotating supersonic beam source. The purpose of this device is to produce velocity augmented molecular beams for use with scattering experiments or subsequent slowing methods. The beam emerges from a nozzle inserted at the tip of a hollow aluminum rotor which can be spun at high speeds in either the forward or backwards direction. The forward direction mode increases the laboratory frame velocity distribution of the emitted beam and the backward direction mode decreases this velocity distribution. Both rotor modes are analyzed theoretically and experimentally within the text. I introduce a pulsed gas inlet system for the rotating source as well as cryocooling of the vacuum chamber. This new version provides moderately intense beams of slow molecules, containing ∼1012 molecules at lab speeds as low as 35 m/s, and very intense beams of fast molecules, containing ∼1015 molecules at 400 m/s. Beams of any molecule available in gas phase can be produced utilizing this system. For collision experiments, the ability to scan the velocity utilizing the rotating source is very advantageous when using two merged beams. If the two velocities can be closely matched, very low relative collision energies can be produced without making either beam slow