Studies of gas-phase ionic clusters have become an integral component in understanding microsolvation and catalysis by transition metal cations. Further interest in this field is due to the possibility of bridging the gap between the condensed and gas phases by developing our understanding of clusters and the possibility that small clusters can have unique chemical and catalytic properties. Most gas phase studies have focused on singly charged ions. Electrospray allows for the production of multiply charged ions solvated by a few solvent molecules. Understanding smaller reactive species such as metal centered clusters with well-defined, gas phase conditions also allows for detailed comparison between theory and experiments. In these studies the main focus is to understand bond activation by transition metal cations and solvation of transition metal dications. The gas phase ions of interest are studied using an electrospray-ionization or laser-ablation dual time-of-flight mass spectrometer and are characterized using photofragment spectroscopy in the visible and ultraviolet regions of the spectrum. Photofragment spectroscopy is a powerful method that can be used in gas phase studies to gather a wealth of information on the ions' bond strengths, spectroscopic constants, and dissociation kinetics and dynamics. The study of TiO + (CO 2 ) spectroscopy (Chapter 3) was a result of study of CO 2 bond activation by Ti + that went on to provide a wealth of information on the spectroscopy and dissociation kinetics of this molecule. An electronic transition of the TiO + chromophore was observed, 2 Π[arrow left] 2 Δ, revealing new information about the excited state and the effect of TiO + electronic state on the metal- CO 2 ligand interaction. The photodissociation spectrum of this molecule is well resolved and shows progressions in the covalent Ti-O stretch and metal-ligand stretch and rock. The lifetime of electronically excited TiO + (CO 2 ) was measured, and depends strongly on vibrational energy. Calculations on TiO + and TiO + (CO 2 ) were combined with experimental results on TiO + (CO 2 ) to predict spectroscopic transitions of TiO + , an astrophysically interesting molecule. The photodissociation dynamics of M 2+ (CH 3 CN)n(H 2 O)m where M = Co and Ni, (Chapter 4) is important in understanding the gas phase microsolvation of metal dications. The coordination number and type of solvent affect the dissociation pathways. M 2+ (CH 3 CN)n (n>2) primarily lose a solvent molecule. Electron transfer is a minor channel for n=3 and is the only channel observed for n=2. Mixed clusters M 2+ (CH 3 CN)n(H 2 O)m preferentially lose water. Loss of acetonitrile is a minor channel, as is proton transfer. Water is the proton donor. Replacing acetonitrile with water increases the proton transfer channel. Nickel and cobalt complexes show similar dissociation dynamics, with proton transfer more likely for nickel complexes. Methane activation by transition metal catalysts is industrially important as it can be used to produce gasoline from natural gas. We studied the products and intermediates of the reaction of laser-ablated platinum atoms with methane (Chapter 5). Photoionization efficiency curves were measured for PtCH 2 and the [H-Pt-CH 3 ] insertion intermediate using tunable vacuum ultraviolet light. The resulting ionization energies were combined with bond strengths for the cations to derive bond strengths for the neutrals. These were used to construct a potential energy surface for methane activation by platinum atoms.

This thesis is concerned with weakly bound neutral and ionic clusters. Spectra of the region near the S1fS0 electronic origin of four neutral van der Waals molecules - aniline-argon, phenol-argon, chlorobenzene-argon and fluorobenzene-argon - were obtained using resonance enhanced multiphoton ionization (REMPI). These spectra indicate that Fermi resonances between van der Waals stretching and bending motions are important in these molecules. Effective Hamiltonians are constructed that describe well the low frequency vibrations. In order to better discuss the low frequency van der Waals motions of aromatics bound to one and two rare gas atoms a simple model for the vibrations is developed. The model enables expression of van der Waals frequencies in terms of fundamental molecular properties and enables facile comparison of effective force constants in a variety of van der Waals molecules. The model is successfully employed to explain van der Waals vibrational structure associated with the origin region of aniline-(argon)2 using van der Waals potential parameters derived from the aniline-(argon)1 spectrum. REMPI and emission spectra of larger clusters of aniline and argon are also reported and discussed. Using atom-atom potentials, equilibrium structures for aniline-(argon)n (n=l, 2, 3) are calculated. The calculations prove useful in the analysis of the spectra.The BfX transitions of the cation complexes fluorobenzene+-argon and chlorobenzene+-argon have been investigated. The cations were prepared by resonance enhanced multiphoton ionization of the neutral van der Waals molecules. A time delayed tunable dye laser was then used to dissociate the cations, loss of an argon atom being the dominant process. When the second laser was tuned to a cation resonance the dissociation cross section increased markedly, allowing characterization of BfX transition. The resulting spectra are presented and discussed.

[pt] Um espectrômetro de massa tipo tempo-de-vôo, montado no Laboratório Nacional de Luz Síncrotron (Campinas - SP), foi utilizado para analisar os íons dessorvidos de uma amostra de amônia condensada (temperaturas de análise: 25 a 150 K) ao ser impactada por fragmentos de fissão do 252Cf. O espectrômetro permite identificar e determinar as abundâncias das espécies iônicas dessorvidas. Quanto à parte teórica, foram feitos cálculos para determinar as estruturas mais estáveis dos agregados de NH3 e para determinar suas dinâmicas de emissão de íons secundários observados na parte experimental. A amônia foi escolhida por sua semelhança com a água, uma molécula muito bem estudada. Outra razão é o atual interesse em determinar a formação de compostos orgânicos nas superfícies de corpos interestelares, uma vez que está comprovada a presença da amônia naquelas superfícies. Nos espectros obtidos observa-se a formação de agregados de amônia que podem ser representados por (NH3)nNHm ± com n = 0-30 e m = 0-5, para íons positivos, e com n = 0- 3 para íons negativos. Uma forma de evidenciar a possibilidade de formação de novos compostos foi realizar experiências com a mistura NH3-CO, com a mesma montagem experimental utilizada para a amônia. Observa-se no espectro obtido (antes da sublimação do CO a 30 K) linhas de massa resultantes de reações primárias que correspondem a íons moleculares híbridos com estrutura CnOmHl+.Os cálculos teóricos referentes às estruturas dos agregados foram realizados través do programa Jaguar 5.5 e Jaguar 6.0. O objetivo é determinar as estruturas mais estáveis dos agregados iônicos da amônia através da teoria DFT (Teoria do Funcional de Densidade) por meio da minimização da energia. Encontrou-se uma relação direta entre as estabilidades determinadas e as abundâncias relativas no espectro de massa. Finalmente foram realizados cálculos com o modelo teórico de dessorção iônica induzida por elétrons. Os resultados de distribuição de velocidades e energias foram comparados com os dados experimentais dos agregados da amônia (n = 0, 4), apresentando uma concordância razoável em valor absoluto, mas moderada em forma. / [en] A time-of-flight mass spectrometer, mounted at the Laboratório National de Luz Síncrotron (Campinas - SP), was used to analyze desorbed ions of a condensed ammonia sample (analyzing temperatures: 25 - 150 K) being impacted by 252Cf fission fragments . The spectrometer allows identifying and determining the relative yields of ionic desorbed species. Besides, it had been made theoretical calculations to determine the most stable cluster structures as well as to determine the emission dynamics of NH3 clusters observed in the experimental part. The ammonia was chosen because of its similarity with the water molecule (very well studied). Another reason is the current interest in determining the organic compounds formation in the interstellar surfaces, now that it is proven the presence of ammonia in those surfaces. The measured spectra show the formation of ammonia clusters that can be represented by (NH3)nNHm ± with n = 0 - 30 and m = 0 - 5 for positive ions, and n = 0 - 3 for negative ones. One way to evidence the formation possibility of new compound is to perform experiments with CO-NH3 mixture samples, using the same experimental set up used for the ammonia. In the spectrum measured before CO sublimation (30 K), mass lines, product of primary reactions, corresponding to hybrid molecular ions having the CnOmHl + structure were observed. Theoretical calculations referring to cluster structures had been carried out using the programs Jaguar 5,5 and Jaguar 6.0. The objective is to determine the most stable structures of the ammonia clusters through the Density Functional Theory (DFT) by means of energy minimizations. A direct relation between the computed stabilities and the relative abundances in the mass spectra was found. Finally calculations with the Secondary Electron Induced Desorption (SEID) model had been carried out. Results of velocity and energy distributions had been compared with the experimental data of ammonia clusters (n = 0, 4), presenting a good agreement in absolute values but moderate agreement in shape.

[pt] O fenômeno de dessorção iônica a partir da incidência de pulsos de radiação laser ultravioleta sobre superfícies em vácuo foi estudada. A dessorção de três tipos diferentes de sólidos foi analisada: insulina, carbono (amorfo e grafite) e policristais de haletos alcalinos. Os processos fundamentais da interação da radiação laser com sólido e o vapor formado, assim como a evolução do plasma gerado, foram descritos satisfatoriamente através de um modelo térmico e da simulação de espectros de têmpo-de-vôo para os primeiros instante da expansão ao vácuo. Um novo método foi proposto para determinar as velocidades iniciais e o início da expansão livre em função da intensidade do laser para LDI (Laser Desorption Ionization) e MALDI (Matrix Assisted Laser Desorption Ionization). Embora o estudo da expansão do plasma gerado não tenha sido tratada dinamicamente pela sua complexidade, a análise utilizando varias combinações do sólido irradiado permitiram concluir que a dessorção induzida por laser pode ser caracterizada por dois processos fundamentais: i) a atomização seguida de recombinação dos constituintes do alvo formando aglomerados e ii) a emissão de aglomerados pré-formados do material. As estruturas geométricas mais estáveis das espécies detectadas foram caracterizadas utilizando a Teoria do Funcional da Densidade (DFT) e classificadas taxonomicamente em função de sua energia (método D-plot); determinou-se a influência da estabilidade dessas estruturas nas abundâncias relativas no espectro de massa. / [en] Ion desorption induced by ultraviolet laser radiation pulses was studied in surfaces in vacuum. The ion desorption from three different solids was analyzed: insulin, carbon (amorphous and graphite) and polycrystals of alkali halides. The main processes involved in the laser-solid and laser-vapor interactions, as well as in the plasma evolution, were well described by a thermal model and by the simulation of the time-of-flight spectra for the first moments of the plasma expansion to vacuum. A new method to determine the initial velocity and the beginning of the free expansion regime as a function of the laser intensity was proposed for LDI (Laser Desorption Ionization) and MALDI (Matrix Assisted Laser Desorption Ionization). Considering the complexity of the dynamical treatment of the expansion of the laser-generated plasma, an analysis by using several combinations of irradiated solids was performed. It was established that the desorption process is characterized by two main mechanisms: i) the atomization followed by recombination of the target elements in clusters and ii) the emission of preformed clusters of the target material. The most stable geometric structures of the measured species were characterized using Density Functional Theory (DFT) and classified taxonomically as a function of their internal total energy (D-plot method); the influence of the structure`s stability on the relative mass abundances was also determined. / [fr] Le phénomène de la désorption ionique induite par des pulses laser ultraviolets dans la surface de solides est étudié. Trois types différents de solides ont été analysés: insuline, carbone (amorphe, graphite et CO condensé) et halogénures alcalins polycristallins (CsI, KI, KBr). La dynamique des ions secondaires émis est analysée par la comparaison des résultats de modélisation avec leur distributions de vitesse mesurées. Un modèle thermique est proposé pour décrire l´interaction entre la radiation laser avec une pellicule de CsI polycristallin et aussi avec la plume émise dans ce processus. Dans le cadre de ce modèle, une nouvelle méthode est utilisée pour caractériser le régime de collision dans lê plasma, soit dans le cas du LDI (Laser Desorption Ionization), fait sur le CsI, soit dans le cas du MALDI (Matrix Assisted Laser Desorption Ionization), fait sur l´insuline dissoute dans une solution solide d´ ACHC. Il est rappelé que le ions CsI émis peuvent être reconstitués après une atomisation complète de la cible mais ceux des de l´insuline difficilement le peuvent. Pour mieux comprendre l´émission des agrégats ioniques Cn + et des halogénures alcalins, leurs structures moléculaires ont été étudiées théoriquement par DFT (Density Functional Theory). L´énergie totale de chaque isomère a été calculée et transformée dans une nouvelle quantité nommée déviation énergétique (D). Le graphique D-plot, oú D est présenté en fonction du nombre de monomères, n, s´est montré très utile pour classer les agrégats en familles et pour estimer leur stabilité, laquelle est liée vraisemblablement à ses abondances de désorption.