Metastable Atom De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with unparalleled surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface to be studied and the kinetic energy distribution of ejected electrons that result from metastable atom de-excitation is measured. Although the measured distribution contains information about the electronic structure of the outermost surface layer, its detailed analysis requires knowledge of the dynamics of the metastable atom-surface interaction. In the present work, these dynamics have been investigated directly by use of spin-labeling techniques. The electron spins on the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Energy-integrated electron spin-polarization (ESP) measurements for Pd(110) and energy-resolved ESP measurements for Cu(100) surfaces under a variety of conditions indicate that metastable atom-surface interactions are more complex than has been assumed in past MDS studies. Spin-polarized MDS has also proven useful in the study of surface magnetism. The present work includes a study of the surface electronic and magnetic structure of the Fe(110) surface. Results indicate an interesting reversal of surface electron spin-polarization for Fe(110) surfaces exposed to oxygen.

Development of a low-energy electron-spin-polarized He$\sp+$ ion source is described. The ion source uses an optically pumped rf discharge. Metastable He(2$\sp3$S) atoms are produced by electron impact in the discharge and then are optically pumped into the M$\sb{\rm j}$(M$\sb{\rm s}$) = +1 or $-$1 magnetic sublevels by circularly polarized radiation from a Ti:sapphire laser operating at 1.083 $\mu$m. He$\sp+$ ions are produced by Penning ionization in metastable-metastable collisions and should be spin-polarized because of spin conservation in such processes. A method for accurately determining the net polarization of He(2$\sp3$S) atoms in the discharge by measuring the absorption of light from a tunable diode laser is described. This method can be used to optimize source performance. Optically pumping the discharge on the D0, D1, or D2 transitions gave polarizations of 31.2%, 47.1%, and 18.2%, respectively. It is expected that optically pumping the discharge on the D1 transition will produce $\sim$80+% polarization of the metastable atoms after suitable optimization of operating parameters.

We present calculations of the broadening and the shifts of the Rydberg electron levels of a Xenon atom near a metal surface. The Xenon atom is described using a pseudopotenial. The energies and widths of the Xenon states, computed from the Schrodinger equation using the complex scaling method, exhibit a complicated distance dependence. We show that some of the orbitals hybridize, when the atom approaches the surface. This effect even causes the widths of some states to decrease with decreasing atom-surface separation.

A spin polarized He$\sp+$ ion source has been developed to study the dynamics of ion-surface interactions. The He$\sp+$ ions are produced by Penning ionization in collisions between electron-spin-polarized He(2$\sp3$S) metastable atoms contained in a weak rf-excited discharge. The ions are extracted from the discharge and focused onto clean or adsorbate-covered surfaces using a series of electrostatic lenses. The impact energy can be varied from $\rm{<}10eV$ to $\rm{\ge}650eV$ and typical beam polarizations are 10-15%. Information about ion/surface interactions, and the dependence of those interactions on the incident ion energy, is obtained by measuring the energy distribution and polarization of electrons ejected from the surface as a result of Auger neutralization.

An apparatus has been developed for studying the interaction of Rydberg atoms with surfaces, particularly the perturbation of Rydberg states by a nearby surface and the distance from the surface at which resonance ionization occurs. For this purpose, a source of metastable Xe(3P 0,2) atoms was constructed and characterized. Using single photon laser excitation from the 6s'[1/2]0 state, the nf[3/2]1 Rydberg series for n = 16--20 has been observed with signals well above the background. Measurements of the energy levels of these Rydberg states has identified systematic errors in quoted energy levels from earlier work. The isotopic structure of the 20f Rydberg state was resolved, and the effect of Rydberg excitation in an electric field was investigated. Studies are underway to better understand the behavior of Xe(nf) Rydberg atoms in an electric field. This information will be used in subsequent experiments to study Rydberg-surface interactions.

Spin-polarized ion neutralization spectroscopy (SPINS), in which a beam of electron-spin-polarized He+ ions is directed onto a surface and analysis is done of the electrons ejected from the surface as a result of ion neutralization, is a unique tool for surface studies that has unparalleled surface specificity. Using SPINS, we have discovered that the polarization of electrons ejected from Au(100) and Cu(100) is strongly correlated with incident ion polarization. The incident ion, in essence, locally magnetizes the surface, creating a net polarization in the local occupied density of states on the surface. Recent experiments on a CO2-covered surface are also described. The results from these experiments cannot be explained using the traditional Auger or Penning processes. A model is proposed in which the He+ forms a collision complex with one or more CO 2 molecules in the surface layer, making it energetically possible for an electron to be ejected.

Spin-polarized ion neutralization spectroscopy (SPINS), in which a beam of electron-spin-polarized He+ ions is directed onto a surface and analysis is done of the electrons ejected from the surface as a result of ion neutralization, is a unique tool for surface studies that has unparalleled surface specificity. This technique has been used in studies described in this thesis of Mg surfaces and rare gas van der Waals solids. The possibility of plasmon excitation in He+ ion neutralization at Mg has been investigated, but this process is found to be unlikely. Instead, the mechanism for ion neutralization appears to be resonance neutralization followed by electron ejection through Auger deexcitation. Experiments at frozen Xe surfaces found that ion neutralization proceeds by an Auger neutralization-type process above the surface, while experiments at frozen Kr are explained by the formation of two types of collision complexes at the surface, one ejecting an electron by double ionization of a surface atom, the other by single ionization of two surface atoms.

Spin-labeling techniques, specifically using electron-spin-polarized 4He+ ions coupled with energy-resolved measurements of the polarization of ejected electrons, are providing significant insight into surface electronic states and the dynamics of the neutralization of charged particles at clean and adsorbate-covered metal surfaces. The electronic structure of surfaces and the process by which charged particles electrically interact with such surfaces are of fundamental interest, yet only partially understood. A powerful technique for studying surface states and interactions has been Ion Neutralization Spectroscopy, in which noble gas ions are directed into a surface, where they are neutralized by electrons from the surface. Energy conservation causes other electrons to be emitted, which are collected and their energy distributions analyzed. Because neutralization takes place outside the surface, this technique serves as a sensitive probe of that part of the material. The current studies expand this technique by analyzing spin-aspects of the interaction. Spin-polarized He+ ions are produced in a radio-frequency driven discharge and directed at selected surfaces. Emitted electrons are analyzed with a retarding grid energy analyzer to determine their energy distributions and a mott polarimeter to measure their spin polarization. Correlating the spins of the outgoing electrons and incoming ions provides previously unavailable information about the dynamics of this reaction. Analysis of the energy distributions and polarization of electrons emitted from Au(100), Cu(100), and Al(100) indicate that neutralization occurs at distances closer to the surface than previously believed, and that for the period during which the ion is close to the surface, its presence causes a spin-dependent perturbation in the local density of electronic states---in essence, it locally magnetizes the surface. Further, the data indicate that surface plasmon excitation, a prominent feature in several theoretical models, does not appear to play a significant role in ion neutralization. Polarization data collected on alkali-covered surfaces clarify the dynamics by which the neutralization process takes place, while CO2 surface studies reveal that the ions undergo a previously unexamined neutralization mechanism which should apply to a broad range of van der Waals solids.

The buffer gas cooling process and the properties of the cooled beam in a novel rf quadrupole ion beam guide with axial field was studied. The operation principle of the axial field quadrupole ion beam guide is similar to the quadrupole mass filter operating in AC only mode but the rods were segmented and differential DC biases were applied to produce the axial field. It was operated in buffer gas. / A Monte Carlo simulation was developed and the simulation results were compared with the experimental results. The beam distribution in a thermal equilibrium was assumed as the thermal equilibrium distribution in a simple harmonic motion with rf distortion and the assumption was confirmed by the Monte Carlo simulation. / The temperature of the cooled beam was measured with a beam profile, measurement system which consists of a miniature quadrupole ion beam guide aid a rotating semi-circular Faraday plate. The temperature of the cooled beam was measured to be 0.0424 +/- 0.0026 eV in the experiment while that of the simulated beam was 0.0372 +/- 0.0009 eV. The variation of beam temperature with beam current was 0.0153 +/- 0.0026 eV/nA. The maximum achieved ion transmission was 80%. The transmitted ion beam's energy spread was measured with a retarding potential energy analyzer to be less than 0.1 eV. The cooling time was 0.7 ms with 1V/ cm axial field and was observed to be very dependent on the ion current when there was no axial field.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1058. Includes bibliographical references (leaves 78-81) Available on microfilm from Pro Quest Information and Learning.