Nonlocal theory of dissociative electron attachment to H\u3csub\u3e2\u3c/sub\u3e and HF molecules

We develop a method for calculation of dissociative electron attachment cross sections based on the Fano- Feshbach projection-operator approach. The coupling between the diabatic state and electron continuum is calculated with the inclusion of orthogonality scattering and long-range electron-molecule interaction. The dynamics of nuclear motion in the non-local complex potential is treated by semiclassical theory. We apply the theory to the calculation of dissociative attachment to the H2 and HF molecules. Our results for attachment to ground-state hydrogen molecules and the near-threshold vibrational enhancement of H2 are essentially the same as previous non-local results. However, the shape of the energy dependence of the cross section for attachment to vibrationally excited states of H2 is slightly different. The calculated value for the total attachment cross section to the ground state of HF is consistent with the little experimental data available; more definite conclusions are difficult because of the approximate nature of the experimental results. The results for the vibrational enhancement are in very good agreement with experiment

Electron collisions with octafluorocyclobutane, c-C_4F_8

We present calculated cross sections for elastic and inelastic collisions of low-energy electrons with octafluorocyclobutane, c-C_4F_8. The integral elastic cross section displays a rich resonance structure, which we analyze in terms of temporary trapping in virtual valence orbitals. The differential elastic cross sections compare well with recent measurements at energies where the approximations used in the calculations are expected to be valid. Integral and differential cross sections for electron-impact excitation of the lowest singlet and triplet excited states were obtained. We relate the small magnitude of the inelastic integral cross sections and the unusual form of the inelastic differential cross sections to the symmetries of the electronic states involved in the transition

Reactive electron scattering from biomolecules and technologically relevant molecules

The role of a set of gases relevant within the context of biomolecules and technologically relevant molecules under the interaction of low-energy electrons was studied in an effort to contribute to the understanding of the underlying processes yielding negative ion formation. The results are relevant within the context of damage to living material exposed to energetic radiation, to the role of dopants in the ion-molecule chemistry processes, to Electron Beam Induced Deposition (EBID) and Ion Beam Induced Deposition (IBID) techniques. The research described in this thesis addresses dissociative electron attachment (DEA) and electron transfer studies involving experimental setups from the University of Innsbruck, Austria and Universidade Nova de Lisboa, Portugal, respectively. This thesis presents DEA studies, obtained by a double focusing mass spectrometer, of dimethyl disulphide (C2H6S2), two isomers, enflurane and isoflurane (C3F5Cl5) and two chlorinated ethanes, pentachloroethane (C2HCl5) and hexachloroethane (C2Cl6), along with quantum chemical calculations providing information on the molecular orbitals as well as thermochemical thresholds of anion formation for enflurane, isoflurane, pentachloroethane and hexachloroethane. The experiments represent the most accurate DEA studies to these molecules, with significant differences from previous work reported in the literature. As far as electron transfer studies are concerned, negative ion formation in collisions of neutral potassium atoms with N1 and N3 methylated pyrimidine molecules were obtained by time-of-flight mass spectrometry (TOF). The results obtained allowed to propose concerted mechanisms for site and bond selective excision of bonds

Atomic excitation and molecular dissociation by low energy electron collisions

In this work, momentum imaging experiments have been conducted for the electron impact excitation of metastable states in noble gases and for dissociative electron attachment (DEA) in polyatomic molecules. For the electron impact excitation study a new experimental technique has been developed which is able to measure the scattering angle distribution of the electrons by detection of the momentum transfer to the atoms. Momentum transfer images have been recorded for helium and neon at fixed electron impact energy close to the excitation threshold and good agreement with current R-matrix theory calculations was found. A new momentum imaging apparatus for negative ions has been built for the purpose of studying DEA in biologically relevant molecules. During this work, DEA was investigated in the molecules ammonia, water, formic acid, furan, pyridine and in two chlorofluorocarbons. Furthermore, the change of DEA resonance energies when molecules form clusters compared to monomers was investigated in ammonia and formic acid. The experimental results of most studied molecules could be compared to recent theoretical calculations and they support further development in the theoretical description of DEA. The new apparatus built in this work also delivered a superior momentum resolution compared to existing setups. This allows the momentum imaging of heavier fragments and fragments with lower kinetic energy

The role of halouracils in radiotherapy studied by electron transfer in atom-molecule collisions experiments

Dissertação para obtenção do Grau de Doutor em Engenharia FísicaThe role of ionising radiation as a source of damage to living tissues and cells has been recognized as a key issue regarding cellular DNA integrity and, ultimately, mutagenesis. The lethal effect of radiation, despite being most of the time undesired, can sometimes be useful, as is the case of radiation therapy. However, still the major concern in medicine is that only the cancerous cell material should be destroyed, keeping as much as possible healthy tissue unaffected. One way to control this damage seems to be the application of radiosensitizers that are incorporated into cancer cells. The cancer tissue doped with these radiation sensitizing molecules may be destroyed preferentially under radiation exposure, in very well defined places and even with radiation doses which may be low enough to prevent healthy cell material to be affected in the surrounding medium. This leads to nanodosimetry and so the sorts of interactions have now to be described at the molecular level. Upon irradiation, the most abundant secondary species produced along the radiation track are low energy electrons and so the study of electron induced damage to biological relevant molecules seems indubitably relevant. The research described in this thesis covers for the first time the study of electron transfer on two halouracils (5-chlorouracil and 5-fluorouracil) and isolated DNA/RNA basis (thymine and uracil)by atom-molecule collisions. In order to investigate such molecules, a crossed beam experiment, comprising a neutral potassium beam and a biolomecular effusive beam, was improved and a time-offlight mass spectrometer implemented allowing for the detection of negative ion formation following electron transfer processes in atom-molecule collisions. In these experiments the anionic fragmentation patterns and formation yields were obtained. These results are shown to be significantly different from the dissociative electron attachment (free electrons) results, unveiling that the damaging potential of secondary electrons to biomolecules can be somewhat underestimated. In addition, the halouracils sensibility to electron induced damage appears to be enhanced with respect to thymine and uracil, which may be extremely relevant as it reinforces their effectiveness as radiosensitizer molecules.Portuguese Foundation for Science and Technology - SFRH/BD/32271/200

Studies of negative ions formed by low energy electron impact

In this thesis the processes responsible for the formation of negative ions by the interaction of low energy electrons (0 to 15eV) with molecules in the gas phase have been investigated. Particular attention has been paid to the processes known as associative resonance capture and dissociative resonance capture. For a molecule AB, associative resonance capture is described by the equation AB + e → AB⁻, where the metastable molecular negative ion AB⁻ is formed by the capture of slow electrons. Dissociative resonance capture, described by the equation AB⁻ → A⁻ + B, results in the formation of a stable negative ion and can occur throughout the energy range studied. A historical review of the theoretical approach to electron-attachment is followed by detailed accounts of the most recent theoretical treatments of associative and dissociative resonance capture. The time-of-flight mass spectrometer used for this study has been described in some detail as have the experimental procedures developed. The various devices used to overcome the problems created by the broad electron energy distribution, which is due to the use of thermionically emitted electron beams, have been critically reviewed and the analytical deconvolution procedure adopted in this study has been described in detail. Autodetachment lifetimes and capture cross-sections for the associative attachment of electrons by several groups of organic and inorganic molecules have been measured and comparisons made with the predictions of the statistical theory for associative electron capture. Attempts to calculate electron affinities from this theory, using the lifetimes and cross -sections measured, met with some success for simple molecules and enabled conclusions to be made concerning the adequacy and limitations of the theoretical treatment. From studies of the electron energy dependence of negative ion formation for several groups of inorganic and organic molecules, various ionisation processes have been identified. Deconvolution of the ionisation curves has enabled accurate appearance potential data to be determined and, in many cases, allowed bond dissociation energies, electron affinities and heats of formation of various species to be evaluated