This thesis concerns the study of electronic excitation in ion/atom-molecule collisions. An extensive review of the subject is given first. At high energies, the quasidiatomic correlation diagram, that is so useful in the interpretation of atomic inelastic collisions, can also be applied in the case of molecular collisions. The model breaks down at small impact parameters where orientation of the molecule begins to play a role, and diabatic potential energy surfaces must be calculated instead. Recent developments in this area are reviewed. At lower energies, a host of new theoretical techniques can now be used, notably the infinite order sudden approximation, and time-dependent semiclassical methods. With these recent theoretical developments, it is concluded that the experimentalist must increasingly turn to the coincidence technique to probe the orientation of molecules during collisions, and to supply state-specific data. Such an experiment on the K + CH<SUB>3</SUB>I collision system is described next. In these collisions, CH<SUB>3</SUB>I is excited to Rydberg levels and subsequently ionises and fragments. The process is known to onset at low scattering angles (500 eV<SUP>o</SUP>), and the aim here was to observe these fragment ions in coincidence with the scattered atoms. No coincidence signal was observed, from which an upper estimate of the cross section ratio igma<SUB>i</SUB>(θ)/igma_itot could be set. Previous workers have interpreted the excitation mechanism as involving excited ionic intermediate surfaces, and a discussion here using a correlation diagram confirms this view. A comparison with the analogous atomic systems suggest an alternative interpretation, in which the collision is viewed as a scattering of the potassium valence electron off the molecule. In the final part of the thesis, work on an apparatus designed specifically for photon-ion coincidence measurements is described. The apparatus features a multi-angle particle detector, that will allow 45 angular measurements to be made simultaneously. Here the performance of the apparatus is critically assessed, and suggestions are made for improvement. Two pulsing devices are described. The first generate bunching fields for pulsing the ion beam, and will allow us to perform time-of-flight measurements. The second is positioned after the collision zone, and is designed to deflect away the elastically scattered ions that would otherwise contribute to noise in the coincidence experiment.

Some aspects of the interaction of gas-atoms and solid-surfaces are presented. By using simple mathematical models, an intuitive understanding of the interaction between atoms and surfaces has been achieved. By a logical extension of such techniques, it is shown how a general theory for the desorption of atoms from surfaces may be formulated. Potential parameters for the gas-surface interaction are deduced for a number of experimental systems using the published data for the surface residence times of adsorbed atoms. The growth of thin semiconducting films by Molecular Beam Epitaxy is described and the results of kinetic studies for the interaction of Ga and As4 beams with a GaAs (100) surface are discussed. By adopting some of the previously developed atomistic concepts, a microscopic model is proposed which explains some of the fundamental aspects of the growth mechanism employed by this method. As part of the kinetic study of Molecular Beam Epitaxy, the time-of-flight technique for measuring the velocity distribution of a beam was used. This method has been studied experimentally and detailed results are presented for As4 and Pb neutral beams. It is shown that when a quadrupole mass spectrometer with a cross beam ionizer is employed as a detector, serious errors arise from inherent time delays, which are observed to be strong functions of several operating parameters. The consequence of this instrument malfunction is discussed with particular reference to the measurement of energy accommodation coefficients.

The dynamics of atoms and molecules in an intense femtosecond laser field are studied experimentally using time-of-flight spectroscopy. 10fe laser pulses have been used to resolve ultrafast vibrational and rotational wave packet dynamics in deuterium, using a pump-probe technique. The revival and subsequent de-phasing of the wave packet is observed with a view to controlling these processes. A new non-Gaussian intensity deconvolution technique is presented, which removes beam diffraction effects and defines the ionisation rate as a constant with respect to ellipticity of laser polarisation. As a consequence of this technique, ultrafast shake-up excitation during tunnel ionisation in atoms has been observed for the first time. The dependence of recollision in an atomic ion on the ellipticity of the laser field has been observed experimentally using laser pulses of 40fs. Through the use of an experimental intensity selective scanning technique, intensity resolved ion yields have been obtained. Following the removal of the laser confocal volume and diffraction effects, the experimental results have been compared with a theoretical model. This quasi-classical model simulates the recollision process in an elliptically polarised laser field.

This Thesis contains the results obtained using correlated <i>ab initio</i> calculations for both the ground and excited states of a series of triatomic molecules, such as ozone (O₃) and the halogen oxides (X₂O, where X = F, CI, Br and I), together with a subset of the pseudo halogens, including fluorine, chlorine, bromine and iodine azide, isocyanate and isothiocyanate. Chapter 1 presents a summary of the methods used to carry out these calculations, starting with the Schrödinger equation and the methods used to solve it, and proceeding to the discussion of differing methodologies such as SCF, multi-configuration SCF and CASSCF, CI and MP2 methods. Chapter 2 describes some of the ground state properties calculated in this work, in quantum chemical terminology, as well as discussing some of the differences in values obtained by experimental and theoretical methods. Chapters 3 and 4 are concerned with the studies on the ozone molecule, with the former covering the ground state and its properties, and the latter presenting the results for the excited states. Chapter 4 includes a comparison with the experimental UV and VUV spectra in the 0 to 12eV region, and assignments are offered for the valence and Rydberg states found in this energy range. These results suggest that the number of states presently identified by spectroscopic means are well short of those that might be expected in the energy range. Chapters 5 describes the study of a subset of the pseudo halogens. These results are concerned solely with the ground state and molecular properties relating to the wave-function as well as structural and ground state spectra such as infrared spectra; where possible these calculated results have been compared with experimental data. The final two Chapter 6 and 7, are devoted to the X₂O molecules, where X = F, Cl, Br and I.  Chapter 6 discusses the ground state properties of the molecules and compares with experiment where such data is known, and predicts the structure and properties of I₂O. The final Chapter covers the excited states of the X₂O molecules, where just the cases where X = F, Cl and Br are examined. Theoretical UV+VUV spectra produced, for each molecule, and Cl₂O has been compared with the experimental data.

In this work the results of highly correlated <i>ab initio</i> calculations on the low-lying states of propyne, furan, and the copper dimer are reported. The excitation energies obtained are used to interpret the electronic spectra of these molecules. The electronic spectra of propyne and furan have been studied using multi-reference configuration interaction techniques. For both molecules the calculations comprise the low-lying valence and <i>s, p,</i> and <i>d</i> Rydberg states, all calculations being performed at the SCF optimised geometry. A variety of basis sets were used; the inclusion of diffuse polarisation functions in the basis set is found to improve the description of the excitation processes. In contrast, the use of polarisation functions optimised for the molecular ground state has negligible effect. The electronic spectrum of propyne is interpreted on the basis of these results to be similar to that of acetylene. The computed excitation energies for the valence states of furan differ drastically from earlier theoretical results. The present results are compared to new experimental data and found to give a consistent picture for the low-lying states. The configuration interaction procedure was also utilised to determine the excitation energies of the <i>ungerade</i> states of Cu<SUB>2</SUB> arising from the <SUP>2</SUP><i>S</i>+ <SUP>2</SUP><i>D</i> asymptote. Much of the work took the form of a preliminary investigation to investigate the feasibility of the technique and applicability of a variety of basis sets. The calculations were performed at 2.20 AA, arbitrarily, chosen to be close to the experimental equilibrium geometry. The inclusion of <i>f</i>-functions was found to be important in describing excitations involving states with different numbers of <i>d</i> electrons. Large reorganisation effects are found to accompany these excitations, which are not adequately described at the CISD level.

The double resonance ionisation nozzle cooled spectroscopy technique (DRINCS) has been used to extend the existing vibrational constants for some ion-pair states of I<SUB>2</SUB> and IC1. Some new ion-pair states of IC1 have also been observed. Some ion-pair potential energy curves beyond the previously existing RKR potentials have been generated using only the vibrational Dunham coefficients and the known asymptotic behaviour of the Coulombic branch of the potentials of I<SUB>2</SUB>. These potentials have been used to simulate the DRINCS spectra. The lifetime of the f'(v'=0) ion-pair state of I<SUB>2</SUB> has been determined as 142±6 ns. The electronic transition dipole moment function has been calculated by simulation of the dispersed fluorescence from the (v'=11) level of the f' ion-pair state. Numerical modelling techniques have been used to simulate the vibronic coupling between an ion-pair and Rydberg state of I<SUB>2</SUB>. A theoretical study of some Rydberg and ion-pair electronic states of the H<SUB>2</SUB>...HF cluster have been carried out using ab initio calculations. In addition, some states of the (H<SUB>2</SUB>...HF)<SUP>+</SUP> cation have been investigated.

No description available.

This thesis concerns the study of state-specific collision-induced energy transfer processes between the SiF C<SUP>2</SUP> Δ and <I>B<SUP>2</SUP></I>Σ<SUP>+ </SUP>and the analogous SiC1 <I>B'<SUP>2</SUP> </I>Σ<SUP>+</SUP> states. Laser excitation spectra established the wavelengths at which the SiF <I>B<SUP>2</SUP></I>Σ<SUP>+</SUP> (v' = 0,1,2,4 and 5) and C<SUP>2</SUP> Δ (v' = 0,1) levels could be populated selectively. The vibrational transition probabilities of the C - X and B - X systems were measured by observations of the resultant dispersed fluorescence. Comparison with the calculated Franck-Condon factors allowed an assessment of the behaviour of the transition dipole moment functions. The B - X transition moment was essentially invariant with internuclear separation. In contrast, the C - X transition moment was found to be strongly decreasing with increasing internuclear separation. These observations were justified on the basis of simple linear combination of atomic orbitals arguments. The radiative lifetimes of the C and B states were shown to be 94 ± 2 ns and ≤ 10 ns respectively by time resolved measurements of the C - X and B - X decays. Total quenching cross sections for removal of the SiF C (v' = 0,1) state were found to be large for the molecular quenchers H<SUB>2</SUB>, N<SUB>2</SUB>, CH<SUB>4</SUB> and CO<SUB>2</SUB>, although no quenching was observed for Ar and He. A small but quencher dependent fraction of the removed C state molecules was transferred to the <I>B<SUP>2</SUP></I> Σ<SUP>+</SUP> state except for CO<SUB>2</SUB> as a quencher. The product B state vibrational populations were shown to correlate well with the Franck-Condon overlap between the initial and product vibronic states. The study of the isoelectronic SiC1 transfer system produced results in good agreement with, but of improved precision to those of previous studies. The B' state quenching was efficient for all collision partners with crosssections shown to correlate with long range attractive forces between the collision pair.

The research papers submitted in this thesis describe experimental and theoretical investigations of particle collisions in which the projectiles have been electrons, ions and atoms, and the targets have been atoms and molecules. Non-reactive and reactive collisions have been studied in order to explore the fundamental nature of the collision event, to understand the dynamics, and to facilitate the determination of thermochemical parameters and reaction properties. The formation of positive and negative ions under single collision conditions as a function of electron impact energy has been investigated for small molecules and for molecular clusters. The measurement of accurate ionization efficiency curves and ionization thresholds has been achieved using custom designed near-monochromatic electron sources or analytical deconvolution. In many cases, detailed energy balancing has been attempted through the measurement of the recoil energies of fragment ions using retarding electric fields. Ionization mechanisms for associative and dissociative resonance electron capture and the formation of isomeric positive ions have been deduced. Thermochemical parameters, including electron affinities, ionization potentials, enthalpies of formation and bond dissociation energies, have been determined. Experiments in which the molecular targets were spatially oriented have shown, for the first time, that the mass spectrum and the ionization efficiency are orientation dependent. A theoretical model has been developed which accounts for the experimental measurements. Investigations of ion-molecule chemistry and non-reactive ion-molecule interactions have been carried out using a custom designed drift-tube mass spectrometer. It has been shown that isomeric ions can be distinguished by their ion transport properties and that the isomeric form of an ion-molecule reaction product ion can be directly measured. A theoretical model based on a generalised ion-helium interaction potential was developed which quantitatively accounted for the relative ion mobilities of a wide range of ions according to their physical properties.

No description available.