Spin-polarized electron energy loss spectroscopy (SPEELS) has been used to investigate several paramagnetic surfaces. In this technique, a low energy beam of spin-polarized electrons from a GaAs source is directed at the surface to be studied and the spin-polarization of the emitted electrons, as well as the kinetic energy distribution, is measured via a micro-Mott polarimeter equipped with a retarding potential energy analyzer. The near-elastic-energy electrons contain information on the inelastic scattering channels available in the solid. The spin-polarization of these same electrons is sensitive to the shape of the final state manifold, i.e., the density of unoccupied states. The low energy and behavior of the spin-polarization spectrum mainly reflects the high number of unpolarized electrons present near zero kinetic energy. Results from Cu(100), polycrystalline Au, GaAs(110), GaAs(100) and Mo(110) surfaces suggest that exchange scattering with spin-flip is ubiquitous for the lowest energy electron beam energies studied ($\sim$14 eV). A simple convolution of the empty and occupied densities of states correctly predicts the shape of the energy dependent spin-flip rate.

Metastable (Atom) De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with extreme surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface under study 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 of the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Energy resolve electron spin-polarization measurements are reported for a variety of sub-monolayer coverages of cesium on a Cu(100) surface, and for oxygen and cesium co-adsorption on a Cu(100) surface. The Cs/Cu(100) system exhibits large ($\sim$2.8 eV) change in the surface work function. The results of the current work suggest that MDS interactions in both high and low work function regimes are more complex than has previously been supposed. Several additional interactions are suggested to explain the data acquired. The question of the occupancy of the adsorbed cesium valence level at various coverages is also addressed.

Spin sensitive electron spectroscopies were used to study low energy electron inelastic scattering from metal surfaces and thin films. In these experiments, a beam of spin polarized electrons from a GaAs source is directed on the sample surface, and the spin polarization and intensity are measured as a function of energy loss and scattering angle by a Mott electron polarimeter coupled with a concentric hemispherical energy analyzer. Systematic studies of the angular dependence of inelastically scattered electrons were conducted on a Cu(100) surface, and Mo/Cu(100), non-magnetized Fe/Cu(100), and Co/Cu(100) films. The polarization and intensity of scattered electrons were measured as function of energy loss and scattering angle. Further studies were also conducted on Ag(100) surface and amorphous Cu/Ag(100) films. From the experimental results, the angular distributions of dipole and impact scattered electrons can be determined individually and both are found to peak in the specular scattering direction. Preliminary studies were conducted on magnetized Co/Cu(100) films. The spin dependent scattering intensity asymmetry was measured, with a clearly observable peak at energy loss of $\sim$1 eV, which coincides with the band splitting. The polarizations of secondary electrons produced by an unpolarized primary beam were also measured. The polarizations can be related to the band polarization of magnetized cobalt films.

We have investigated the dynamics of the isotropic N-vector model with long-range exchange couplings in random magnetic fields using a 1/N expansion. The leading order is exactly solved, showing the existence of a ferromagnetic phase separated from the disordered paramagnetic phase by a line. The critical behaviour of the system has been examined in the next-to-leading order of the 1/N expansion, showing that the critical exponents are by no means related to the ones of the pure system in d-2 dimensions. The ordered phase has been also investigated in the next-to-leading order, revealing a typical Goldstone behaviour of the non time-persistent part of the transverse fluctuations. For the longitudinal fluctuations, two different types of coexistence singularities emerge, one from the non time-persistent (as in the pure systems), vanishing with the temperature, and a more divergent one from the time-persistent part of the correlation.

Spin-polarized electron energy-loss spectroscopy has been used to investigate the probing depth of low energy ($\sim$30 eV) electrons in metals. A beam of spin-polarized electrons is directed at the surface of the sample and the polarization of the scattered electrons is measured as the function of inelastic energy loss. Different polarization loss features were observed from Cu(100) and Mo(110). By depositing thin molybdenum (copper) films on a Cu(100) (Mo(110)) substrate and observing the appearance (disappearance) of the molybdenum polarization loss feature, it is found that the probing depth in molybdenum is small ($\sim$1 monolayer), but is significantly larger in copper ($\sim$3 monolayers). This difference can be explained by the differences of the joint density of states available for electron-hole pair excitation. The growth mode of Fe/Cu(100) has also been studied by using this method.

Clusters of silicon and germanium exhibit multiple isomers that can be annealed to single unique structural forms. The "magic number" reactivity trend observed for positive silicon clusters in the size range from 10 to 45 has also been observed in reactions with ammonia, and has also been observed with ethylene, water and trimethylamine (TMA). It was observed that Si$\sb{21\sp+}$ anneals to a reactive form, so is no longer a "magic number". Negative silicon ions also display the same pattern. Additionally, mass-selected germanium clusters were levitated in the FT-ICR, and probed in chemisorption reactions with nitric oxide, TMA, and water. Clusters in the 10-51 atom size range were effectively annealed to unique structural forms by collisional excitation and cooling through infrared radiation and collisions with argon. For clusters over 33 atoms in size the reactivity pattern for the two elements is the same, suggesting the structures of these clusters is identical and common to tetravalently bonded clusters. Fragmentation studies were also performed to elucidate the difference in relative stability of the clusters and a correlation between stability and reactivity was found.

Vanadium ultra-thin (1-7 monolayers) films are deposited epitaxially on well-defined single crystalline Ag(100) substrate. The topmost layers of the films are studied by electron capture spectroscopy(ECS). ECS is a surface-sensitive technique for the investigation of magnetic order existing at surfaces. It is found that the topmost atomic layer of V(100) films are ferromagnetic for all film thicknesses, in contrast to bulk vanadium which is paramagnetic at all temperatures. The films of thickness of 5 monolayers possess a surface Curie temperature T$\sb{\rm cs}$ = 475.1 K, and the critical behavior of the magnetization of the films is identical to that predicted by the well-known two-dimensional Ising Model of ferromagnet.

A spin-polarized low-energy electron gun with high current transmission is described here and has been applied to SPEELS studies. Current transmission in excess of 40% is routinely achieved. This is a dramatic improvement compared with the $\sim$1% transmission obtained using an earlier electron gun. The high current transmission has allowed SPEELS studies of energy loss mechanisms in low-energy electron scattering from atomically clean metallic surfaces. There are two dominant energy loss mechanisms: dipole scattering which occurs in the vacuum outside the target and impact scattering via electron-hole pair excitation occurring inside the target surface. Preliminary results of SPEELS experiments on Cu(100) and Ag(100) surfaces and on thin films of Mo, Co, and Fe epitaxially grown on a Cu(100) are presented.

Neutron resonant scattering in the epithermal regime provides a potential tool for studying molecular dynamics. The theory is established by expressing the scattering function in terms of the Fourier and Laplace transforms of correlation functions which relate to the different positions of two coherent scatterers at different times. The correlation between resonant and non-resonant scatterers is especially useful in solving the scattering "phase" problem and obtaining further dynamic information. The theory is applied to the dynamic study of polyhedron and macro molecules in the linear regime (harmonic).

A new technique, Spin-Polarized Electron Emission Spectroscopy (SPEES), capable of investigating surface magnetism, has been successfully developed. SPEES allows us to energy- and spin-analyze electrons emitted during the interaction of a grazing-incidence ion beam with a solid surface. The energy and spin information obtained from the emitted electrons helps us to unravel the processes involved in ion-surface interactions at grazing angles. SPEES data obtained at Ni(110) picture-frame single crystals exhibit new characteristic features in the electron energy distribution that are strikingly different from that of electron-induced spectra. For the first time, two electron capture measurements, which are sensitive to "local" magnetic order existing on an atomic scale, have been performed at low energies (15-30 keV) at surfaces of the amorphous ferromagnet Fe$\sb{80}$B$\sb{20}$; the non-zero electron spin polarization amounts to 55%. These two new techniques open the way to study surface electronic and magnetic properties with unprecedented sensitivity.