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.

We develop a transport formalism for interacting electrons in the presence of quenched disorder. Quantum effects on transport, due both to quantum interference and interaction effects, are incorporated through non-analytic terms in the irreducible interactions and appropriate contributions to the electron self-energy. Perturbatively, our approach recovers the standard results on quantum corrections to the Drude conductivity. We argue the strong coupling fixed point is a magnetic metal beyond perturbation theory. Extensions of the theory are outlined.

Using the RPA, we calculate the plasmon frequencies of an electron gas on a two-dimensional spherical surface in the presence of a weak magnetic field. We show that the magnetic field results in a coupling between electronic states with different angular momentum numbers. This coupling results in a blueshift of the dipolar plasmon resonance with increasing magnetic field. We also investigate how the plasmon energies vary as a function of the number of electrons and radius of the sphere.

The effect of spin injection into d-wave, p-wave, and s-wave superconductors is studied here. A theory is developed which treats the interplay between the bulk and boundary spin transport, as well as the interplay between pair and single-particle transport at the boundary. This theory is used to study the relationship between Andreev reflection and the linear-response spin-injection characteristics. Among the quantities of interest are, the amount of injected magnetization (m), the induced spin-dependent current (I s), and the induced boundary voltage (Vs). In general, Andreev reflection makes each of these three quantities depend on a different combination of the boundary and bulk contributions. The conditions are identified under which some of these quantities that depend solely on the bulk properties can be isolated. A correlation between the Andreev bound states and the spin-injection characteristics is established, which implies a strong directional dependence of spin injection into high-temperature superconductors and in p-wave superconductors.