Probing the Surface-Vacuum Interface with Spin-Sensitive Metastable Atom Deexcitation, Electron Capture and Electron Emi
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PROBING THE SURFACE-VACUUM INTERFACE WITH SPIN-SENSITIVE METASTABLE ATOM DEEXCITATION, ELECTRON CAPTURE AND ELECTRON EMISSION SPECTROSCOPIES G. K. WALTERS AND C. RAU Physics Department and Rice Quantum Institute, Rice University, Houston, TX 77251-1892 ABSTRACT Spin-Polarized Metastable Atom Deexcitation (SPMDS) and Electron Capture (ECS) Spectroscopies probe the exponential tails of electronic wavefunctions extending from the surface into the vacuum, and are consequently extremely sensitive to the surface-vacuum interface. The use of SPMDS to probe the near-surface vacuum magnetization of Ni(1 10) and Fe(1 10) and the dramatic changes that result upon exposure to ambient gases is discussed, as is the use of ECS and Spin-Polarized Electron Emission Spectroscopy (SPEES) to determine the ferromagnetic and critical behavior of surfaces and ultra-thin epitaxial systems. INTRODUCTION Particle (atoms, ions)-surface scattering experiments provide a powerful means to study the topmost surface-layer electronic and magnetic properties of magnetic materials. This can be achieved by keeping the energy component E_L of the incident particles normal to the probed surface below 10 - 20 eV, thus preventing their penetration into the surface and assuring top layer specificity in their interaction with target surfaces. Spin-polarized metastable (atom) deexcitation spectroscopy (SPMDS), electron capture spectroscopy (ECS), and spin-polarized electron emission spectroscopy (SPEES) have emerged as extremely surface specific probes of magnetic properties at the surface-vacuum interface. 1 The physical processes underlying each of these spectroscopies are briefly described below and selected experimental results are presented to illustrate the insights into surface magnetic behavior that they provide. SPIN-POLARIZED METASTABLE DEEXCITATION SPECTROSCOPY (SPMDS) In SPMDS surface electronic and magnetic structure and the near-surface magnetic environment are probed by investigating spin dependences in the interaction of thermal-energy (-0.03 eV) electron-spin-polarized He(2 3S) metastable atoms with a magnetized surface. The energy distributions and polarization of electrons ejected from the surface as a result of metastable-atom deexcitation are measured as is (for a magnetized surface) any spin dependence 2 in the total ejected-electron signal. ,3 2 The apparatus is shown schematically in Fig. 1 and is described in detail elsewhere. -4 Briefly, a fraction of the atoms contained in a ground-state helium atom beam are collisionally excited to the 2 1,3S levels by a coaxial electron beam. The 21S atoms are removed from the beam by illuminating it with 2.06-gm radiation from a helium discharge which excites 2 1 S ---> 2 1P - 11S transitions. A weak (-0.5 G) magnetic field is applied perpendicular to the beam to preserve a well-defined quantization axis. Circularly polarized 1.08-tm 23S --- 2 3P resonance radiation from a high-power rf-excited helium lamp is incident along the magnetic field direction and is used to optically pump the 23S atom
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