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.

We introduce a new method for calculating the broadening of atomic levels as a function of the atom's position outside the surface. The surface is studied using a cluster model, and the adsorbate-cluster eigenproblem is solved using quantum chemistry codes. The resulting density of states is projected on the adsorbate orbitals, revealing the broadening of adsorbate energy levels into resonances. We extract the width of these resonances from the projected density of states to calculate the broadening. Arbitrary lateral adsorbate positions and surface geometries can be explored by specifying different atom-cluster configurations.

As the size of a physical system decreases toward the nanoscale, quantum mechanical effects such as the discretization of energy levels and the interactions of the electronic spins become readily observable. To understand what happens when an isolated quantum mechanical object, such as an individual molecule, is coupled to a classical object, such as a macroscopic piece of metal, is one of the goals of modern condensed matter physics. The central question of our research is: How do the degrees of freedom of a single molecule (both electronic and mechanical) interact with an electrostatic environment under a constrained geometry? We have chosen to answer this question by looking at the electronic transport through single molecule transistors (SMTs), nanometer-scale transistors in which charge transport occurs through individual molecular states. We use an electromigration technique to fabricate SMTs based on C60 and transition metal coordination complexes (TMCCs). In these devices, the molecule of interest is constrained between two metallic electrodes which act as reservoirs of electrons and energy. At low temperatures, each transistor acts as a single-electron device in the Coulomb blockade regime. Our experimental results suggest that the vibrational modes of the molecules contribute to the transport characteristics of the SMTs. From measurements of the differential conductance of these devices, we observe direct tunneling features that are consistent with vibrational excitations of the molecules. In the TMCC-based SMTs, we also observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of gate-modulated inelastic tunneling spectroscopy. In some of the SMTs we observe conductance features characteristic of the Kondo effect, a coherent many-body state comprising an unpaired spin on the molecule coupled by electronic correlation effects to the conduction electrons of the leads. The inferred Kondo temperature in these devices typically exceeds 50 K. In TMCC-based SMTs that exhibit the Kondo effect we observe unusual transport characteristics that deviate from the simplest model of Kondo physics in single electron devices. We suggest possible mechanisms, including strong intramolecular exchange and electron-phonon interaction, that may explain the observed deviation.