Thesis (Ph.D.) -- McMaster University, 2001. / Includes bibliographical references. Also available via World Wide Web.

Results are presented for Floquet calculations of photodetachment rates from a one-dimensional model atom irradiated by intense laser light. Light-induced quasibound states are found to originate from the movement of poles of the multichannel scattering matrix on the Riemann energy surface. The appearance of new bound states of the negative Hydrogen ion, recently predicted, is related to the motion of resonance poles that correspond to autoionising states in the absence of the field. A number of pole trajectories, leading to light-induced states, are discussed for the one-dimensional model atom. The Floquet method allows one to represent the wave function of a quantum system in a laser field, as an infinite sum of harmonic basis functions. In any practical calculation this infinite sum must be truncated. The consequences of representing the wave function, via the Floquet method, by a finite sum of harmonics is addressed. An illustration of these consequences is made by way of a number of representative calculations performed on a one-dimensional model atom. Results are presented of calculations performed to determine the influence of a laser field, of low to moderate intensity, upon the partial and total photodetachment rates of the negative Hydrogen ion, H(^-). Using the R-matrix Floquet method, a study is undertaken into the detachment of an electron from the ion, via multiphoton transitions through one of several autodetaching resonances of the ion. The discussion focuses on the influence of the laser field upon auto detaching pathways. It is found that the laser may induce structure into the continuum that does not exist in the absence of the laser field, or, conversely, may suppress field-free structure. In the latter case, the suppression of structure is related to the appearance of laser-induced degeneracies.

Thesis (Ph.D.)--University of Nevada, Reno, 2006. / "August, 2006." Includes bibliographical references (leaves 103-110). Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2006]. 1 microfilm reel ; 35 mm. Online version available on the World Wide Web.

Molecular and cluster anions have been investigated using photoelectron velocity-map imaging spectroscopy to study the nature of electrons in radical species. We report a negative-ion photoelectron imaging study of benzonitrile and several of its hydrated, oxygenated, and homo-molecularly solvated cluster anions. The photodetachment transition from the unsolvated benzonitrile anion to the X̃¹A₁ state of the neutral peaks at 58 ± 5 meV. The electron affinity (EA) of the lowest excited electronic state of benzonitrile, ã³A₁, is determined as 3.41 ± 0.01 eV. The next excited state, the open-shell singlet ùA₁, is found about an electron-volt above the triplet, corresponding to a vertical detachment energy of 4.45 ± 0.01 eV. The step-wise and cumulative solvation energies of benzonitrile anions by several types of species were determined, including homo-molecular solvation by benzonitrile, hydration by 1–3 waters, oxygenation by 1–3 oxygen molecules, and mixed solvation by various combinations of O₂, H₂O, and benzonitrile. Ethylene has been shown to be a degradation product following the 1-e⁻ attachment to ethylene carbonate. As a solvent molecule for (O₂)^(□), our photoelectron imaging study shows a relatively small solvation energy of ≤0.24 eV for the expected 𝜋-𝜋 interaction in the ((O₂)^(□))(C₂H₄) cluster anion. The EA of the O₂(C₂H₄) cluster was measured at 0.69 ± 0.01 eV, while the 𝑋³A″ ← 𝑋²A″ photodetachment transition shows a 1400 ± 100 cm⁻¹ vibrational progression in the 1064 nm spectrum. Negative-ion photoelectron imaging was used to investigate the substituted carbene derivative of fluoroacetonitrile. We report a closed-shell singlet ground state for the cyanofluorocarbene, FCCN, with an adiabatic electron affinity EA = 2.081 ± 0.002 eV and a singlet-triplet gap of ΔEₛ₋ₜ = 0.42 ± 0.04 eV. The open-shell singlet ¹A″ state was also observed experimentally. We find that the experimentally measured ΔEₛ₋ₜ of FCCN agrees well with the general trend of similar carbenes. We report preliminary results on the photoelectron imaging of phenylcarbene, cyanophenylcarbene, and chlorophenylcarbene anions. Triplet phenylcarbene is observed to have an EA of ≤0.83 eV, considerably lower than the previously indirectly-determined value. Transitions to the singlet and triplet ground state of both cyanophenylcarbene and chlorophenylcarbene are observable, though unidentified bands make full assignment difficult. Cyanophenylcarbene is found to have a triplet ground-state, with a tentative EA of 2.04 eV. Chlorophenylcarbene is found to have a singlet ground-state. The phenyl-group is found to favor the singlet state slightly. The cyanofluoromethyl radical, FC(H)CN, was estimated to have an EA of 1.53 ± 0.08 eV, by a combination of experimental and theoretical results.. With similar methodology, we report the adiabatic electron affinity of the cyanobenzyl radical, EA(PhCHCN) = 1.90 ± 0.01 eV, and assign an upper limit of the EA for the chlorobenzyl radical, EA(PhCHCl) ≤ 1.12 eV. These values were used to estimate the C-H bond dissociation energy (BDE)s for these substituted methanes. Fluoroacetonitrile was found to have a BDE of D𝐻₁₉₈ = 90.7 ± 2.8 kcal mol^(□1). The C-H bond dissociation energies at the benzyl-α sites of the phenylmethanes are determined as 80.9 ± 2.3 kcal mol⁻¹ for benzyl nitrile and an upper limit of 84.2 kcal mol⁻¹ for benzyl chloride. These results are discussed in terms of substituent interactions in a simple MO framework and in relation to other similar molecules, including recently reported results for chloroacetonitrile. The 532 nm photoelectron spectrum of glyoxal provides the first direct spectroscopic determination of the adiabatic electron affinity, EA = 1.10(2) eV. This assignment is supported by a Franck-Condon simulation of the experimental spectrum that successfully reproduces the observed spectral features. The vertical detachment energy (VDE) of the glyoxal radical anion is determined as VDE = 1.30(4) eV. The EA of methylglyoxal is determined as ≤0.8 eV based on the signal-to-noise ratio of the 𝑋¹A′←𝑋²A″ transition, with a VDE = 1.28(4) eV. The EA of the a³A″ ← X²A″ and 𝐴¹A″ ← 𝑋²A″ transitions are determined as 3.28(3) eV and 3.614(5) eV respectively. The intrinsically short-lived ethylenedione molecule (OCCO) was observed and investigated using anion photoelectron spectroscopy. The adiabatic electron affinity of its ³Σg^(□) ground state is 1.936(8) eV. The vibrational progression with a 417(15) cm⁻¹ frequency observed within the triplet band corresponds to a trans-bending mode. Several dissociative singlet states are also observed, corresponding to two components of the ¹Δg state and the ¹Σg⁺ state. The experimental results are in agreement with the theory predictions and constitute the first spectroscopic observation and characterization of the elusive ethylenedione molecule. Two glyoxal derivatives related to the ethylenedione anion (OCCO⁻), ethynediolide (HOCCO⁻) and glyoxalide (OHCCO⁻), were studied. These anions provide access to the corresponding neutral reactive intermediates: the HOCCO and OHCCO radicals. In the HOCCO/OHCCO anion photoelectron spectrum, we identify several electronic states of this radical system and determine the adiabatic electron affinity of HOCCO as 1.763(6) eV. This result is compared to the corresponding 1.936(8) eV value for ethylenedione (OCCO). Initial attempts were made to detect and observe the dicyanoacetylene anion, NCCCCN⁻, by photoelectron imaging. While it is believed the experimental design path of H₂⁺ abstraction from fumaronitrile is sound, no spectral signature can be assigned to NCCCCN⁻. Calculations targeting the low-lying transitions from the anion indicate that the molecule should have a significantly positive electron affinity and at least the ground state should be accessible with the currently available laser sources. The cluster ion O₂(N₂O)⁻ of the same nominal mass as NCCCCN⁻ is identified as an interfering ion and ideas have been proposed for resolving this difficulty.

Nonadiabatic dynamical processes are ubiquitous in chemistry and biology. Such events are directly connected to the treatment of energetically close lying states which gives rise to strong vibronic interactions in which case the Born-Oppenheimer approximation tends to break down. In case of biradicals, nonadiabatic events are facilitated by conical intersections, as a result of symmetry lowering of degenerate electronic states due to Jahn-Teller distortion. A central problem in the treatment of the nonadiabatic molecular dynamics is posed by the representation of potential energy surfaces. A point by point calculation of a potential energy surface on a multi-dimensional grid is very cumbersome and in general does not provide with an analytical functional form of the potential. This becomes even more complicated when the adiabatic surfaces have cusps, where the function becomes non-differentiable. Vibronic model Hamiltonians, which represent the potential in the form of a potential matrix which contains the electronic energies as well as the couplings in a diabatic basis. A Taylor series expansion of the potential matrix can be done to get a smooth analytical functional form of the potential matrix elements. These models can then be used to perform nuclear dynamics using either exact diagonalization time-independent method or the wavepacket propagation based time-dependent methods. Thus, vibronic models provide a compact representation of complicated coupled potential energy surfaces, which can be used in conjunction with non-adiabatic nuclear dynamics Vibronic models have been constructed for selected biradicals, for which photodetachment spectra have been simulated using the time-independent (VIBRON) as well as time-dependent (MCTDH) methods. Consistent results have been obtained with both the approaches for small systems. This also assures the use of MCTDH program for larger systems, where the time-independent methods are not applicable. Moreover, for biradicals, the parent anionic state also undergoes a Jahn-Teller distortion, or often the ground state potential energy surface is highly anharmonic in nature. This requires the description of anionic ground state by a vibronic model. Therefore, in order to simulate the photodetachment spectra of biradicals, three vibronic models are constructed for each simulation. The first model describes the ground and excited states of the parent anionic (neutral) species. Two other vibronic models describe singlet and triplet states of the target neutral (cation) species, and the spectrum is simulated using the vibronic ground state(s) of the anion (neutral) as the absorbing state in VIBRON/MCTDH. The electronic states and vibronic model parameters are obtained using the IP-EOM-CCSD and DIP-STEOM-CCSD methodology as coded in the ACESII quantum chemistry program package. The photodetachment spectra of nitrate radical, cyclobutadiene negative ion and trimethylene negative ion have been studied using this methodology.

Nonadiabatic dynamical processes are ubiquitous in chemistry and biology. Such events are directly connected to the treatment of energetically close lying states which gives rise to strong vibronic interactions in which case the Born-Oppenheimer approximation tends to break down. In case of biradicals, nonadiabatic events are facilitated by conical intersections, as a result of symmetry lowering of degenerate electronic states due to Jahn-Teller distortion. A central problem in the treatment of the nonadiabatic molecular dynamics is posed by the representation of potential energy surfaces. A point by point calculation of a potential energy surface on a multi-dimensional grid is very cumbersome and in general does not provide with an analytical functional form of the potential. This becomes even more complicated when the adiabatic surfaces have cusps, where the function becomes non-differentiable. Vibronic model Hamiltonians, which represent the potential in the form of a potential matrix which contains the electronic energies as well as the couplings in a diabatic basis. A Taylor series expansion of the potential matrix can be done to get a smooth analytical functional form of the potential matrix elements. These models can then be used to perform nuclear dynamics using either exact diagonalization time-independent method or the wavepacket propagation based time-dependent methods. Thus, vibronic models provide a compact representation of complicated coupled potential energy surfaces, which can be used in conjunction with non-adiabatic nuclear dynamics Vibronic models have been constructed for selected biradicals, for which photodetachment spectra have been simulated using the time-independent (VIBRON) as well as time-dependent (MCTDH) methods. Consistent results have been obtained with both the approaches for small systems. This also assures the use of MCTDH program for larger systems, where the time-independent methods are not applicable. Moreover, for biradicals, the parent anionic state also undergoes a Jahn-Teller distortion, or often the ground state potential energy surface is highly anharmonic in nature. This requires the description of anionic ground state by a vibronic model. Therefore, in order to simulate the photodetachment spectra of biradicals, three vibronic models are constructed for each simulation. The first model describes the ground and excited states of the parent anionic (neutral) species. Two other vibronic models describe singlet and triplet states of the target neutral (cation) species, and the spectrum is simulated using the vibronic ground state(s) of the anion (neutral) as the absorbing state in VIBRON/MCTDH. The electronic states and vibronic model parameters are obtained using the IP-EOM-CCSD and DIP-STEOM-CCSD methodology as coded in the ACESII quantum chemistry program package. The photodetachment spectra of nitrate radical, cyclobutadiene negative ion and trimethylene negative ion have been studied using this methodology.

This dissertation explores the use of anion photoelectron imaging to interrogate electronic dynamics in small chemical systems with an emphasis on photoelectron angular distributions. Experimental ion generation, mass selection, laser photodetachment and photoelectron imaging were performed in a negative-ion photoelectron imaging spectrometer described in detail. Results for photodetachment from the simplest anion, H⁻, are used to illustrate fundamental principles of quantum mechanics and provide basic insight into the physics behind photoelectron imaging from a pedagogical perspective. This perspective is expanded by introducing imaging results for additional, representative atomic and small molecular anions (O⁻, NH₂⁻ and N₃⁻) obtained at multiple photon energies to address the energy-dependence of photoelectron angular distributions both conceptually and semi-quantitatively in terms of interfering partial photoelectron waves. The effect of solvation on several of these species (H⁻, O⁻, and NH₂⁻) is addressed in photoelectron imaging of several series of cluster anions. The 532 and 355 nm energy spectra for H⁻(NH₃)n and NH₂⁻(NH₃)n (n = 0-5) reveal that these species are accurately described as the core anion solute stabilized electrostatically by n loosely coordinated NH3 molecules. The photoelectron angular distributions for solvated H⁻ deviate strongly from those predicted for unsolvated H⁻ as the electron kinetic energy approaches zero, indicating a shift in the partial-wave balance consistent with both solvation-induced perturbation (and symmetry-breaking) of the H⁻ parent orbital and photoelectron-solvent scattering. The photoelectron energy spectra obtained for the cluster series [O(N₂O)n]⁻ and [NO(N₂O)n]⁻ indicate the presence of multiple structural isomers of the anion cores, the former displaying sharp core-switching at n = 4, the latter isomer coexistence over the entire range studied. The photoelectron angular distributions for detachment from the O⁻(N₂O)n and NO⁻(N₂O)n isomers deviate strongly from those expected for bare O⁻ and NO⁻, respectively, in the region of an anionic shape resonance of N₂O, suggesting resonant photoelectron-solvent scattering. Partial-wave models for two-centered photoelectron interference in photodetachment from dissociating I₂⁻ is presented and discussed in the context of previous results. New time-resolved photoelectron imaging results for I₂⁻, for both parallel and perpendicular pump and probe beam polarizations, are presented and briefly discussed. Finally, new ideas and directions are proposed.

Le cœur du travail de notre groupe est l'étude de la réaction de photodétachement, qui consiste en l'expulsion de l'électron excédentaire d'un ion négatif lors de l'absorption d'un photon. Ce travail de thèse s'articule autour de deux projets : la microscopie de photodétachement, technique d'interférométrie électronique permettant de produire des données spectroscopiques sur les ions négatifs ; et le projet SIPHORE qui envisage la neutralisation d'un jet rapide d'ions négatifs à partir de la réaction de photodétachement, dans le but de servir la maîtrise de la fusion thermonucléaire contrôlée. Les évolutions de ces deux projets se recoupent dans la nécessité d'augmenter le nombre d'événements de photodétachement produits en un temps donné. Ce travail a permis d'étudier et de mettre en place différentes techniques expérimentales pour réaliser l'amplification de la réaction de photodétachement. Notre montage nous permet de produire cette réaction dans une zone d'interaction formée par l'intersection d'un jet d'ions et d'un faisceau laser. Nous envisageons d'une part la modification de la section efficace de photodétachement lorsque la réaction est produite en présence d'un champ magnétique, d'autre part l'amplification du flux de photons dans la zone d'interaction par stockage de lumière en cavité optique. Les avancées réalisées ouvrent de nouvelles perspectives sur les études fondamentales et les applications techniques liées aux ions négatifs. / The core of the work of our group is the photodetachment reaction, which consists in the expulsion of the extra electron of a negative ion by the absorption of a photon. This thesis work is organised around two projects: the photodetachment microscopy, an electron interferometric technique which produces spectroscopic data on negative ions; and the SIPHORE project which considers the neutralization of a fast negative ions beam by the help of the photodetachment process, for the purpose of controlled thermonuclear fusion. The evolutions of these two projects are overlapping in the need of increasing the number of photodetachment events produced per unit of time. This work has led to the study and the implementation of several experimental techniques to realise the amplification of the photodetachment reaction. Our setup permits to produce this reaction in an interaction area formed by the intersection of a negative ions beam with a laser beam. On the one hand we investigate the modification of the photodetachment cross section when the reaction is produced under a magnetic field. On the other hand we consider the amplification of the photon flux inside the interaction region using light storage with optical cavities. The results obtained pave the way towards new prospects for the fundamental studies and the technical applications affiliated with negative ions.

Dans cette thèse, nous initions la démonstration, à échelle réduite, de la faisabilité du photodétachement presque total, par laser, d’un jet d'ions négatifs d'hydrogène en cavité optique Fabry-Perot pour les futurs injecteurs de neutres destinés au chauffage des plasmas des réacteurs de fusion nucléaire.Nous élaborons une nouvelle méthode de mesure d’une section efficace de photodétachement, dont la connaissance à la longueur d’onde d’excitation est requise pour le dimensionnement de la cavité Fabry-Perot, basée sur l’observation de la saturation en régime d’éclairement impulsionnel. Le calcul analytique de l’accroissement du signal de détachement produit lors de l’éclairement d’un jet d’ions par une impulsion laser supposée gaussienne, fait apparaître une contrainte mathématique sur le flux requis pour transiter vers le régime saturé. Cette contrainte est une caractéristique de la transition vers la saturation pour toutes les expériences réalisées en faisceau gaussien et pour tous les processus d’interaction lumière-matière linéaires. Avec cette méthode, nous déduisons une section efficace de photodétachement de H- à 1064 nm – longueur d’onde sélectionnée pour les futurs injecteurs de neutres – en léger désaccord avec les prédictions théoriques.Pour réduire les exigences technologiques sur la cavité et le laser, nous étudions les résonances de Landau qui apparaissent dans le spectre de photodétachement en champ magnétique. S’asservir sur l’une de ces résonances permettrait d’augmenter la probabilité de photodétachement à un flux donné.Par ailleurs, nous présentons nos mesures des affinités électroniques du phosphore, du sélénium et de l’étain, réalisées avec le microscope de photodétachement. L'expérience de microscopie de photodétachement du phosphore est la première où l’atome neutre est laissé dans un terme excité. / In this thesis, we initiate the demonstration, at a reduced scale, of the feasibility of the almost-complete laser photodetachment of negative hydrogen ion beams in a Fabry-Perot optical cavity for future neutral beam injectors for the heating of fusion power plants plasmas.We develop a new method to measure a photodetachement cross section, the knowledge of which at the exciting wavelength is needed to scale the Fabry-Perot cavity, based on the observation of the saturation in a pulsed lighting regime. The analytical calculus of the detachment signal growth produced while illuminating a negative ion beam with a Gaussian laser pulse bring out a mathematical constraint on the required flux to pass through the saturated regime. This constraint is the signature of the transition toward the saturation for all experiment carried out in Gaussian beam and for all linear light-matter interaction processes. With this method, we measure the photodetachement cross section of H- at 1064 nm – selected wavelength for future neutral beam injectors – in slight disagreement with theoretical predictions.To reduce the technological requirement on the Fabry-Perot cavity and the laser, we study Landau resonances which appear in the photodetachement spectrum. Locking the laser on one of these resonances would allow increasing the photodetachment probability at a given flux.We also present our phosphorus, selenium and tin electron affinity measurements carried out with the photodetachment microscope. The photodetachment microscopy experiment of phosphorus is the first one where the neutral atom is left in an excited term.

No description available.