Angular-resolved secondary-electron-emission spectroscopy of clean and adsorbate covered tungsten single-crystal surfaces

Energy-distribution measurements are reported for secondary electrons back-scattered into a narrow angle about the normal direction to three low-index tungsten single-crystal surfaces, viz. (100), (110) and (111). Improved spectral resolution provides unambiguous evidence for scattering out of excited “final” states located above the vacuum level; the results for all three faces correlate closely with high-energy states of a calculated energy band structure, the intensity of emission being directly related to features in the one-dimensional density of unfilled states along the corresponding low-index symmetry directions. In the presence of ordered adsorbate monolayers, additional SEE spectral fine-structure is observed at energies which lie within finalstate band gaps of the crystal. Results are presented for the specific case of CO adsorption on W(110), which shows a distinct disorder-order structural transition after exposure of the clean surface to 10 L of gas at 300 K and subsequent annealing to temperatures ?1000 K. We interpret these adsorbate surface resonances to be due to two-dimensional Bloch-like surface states produced by the periodicity of the adsorbate layer, which manifest themselves as a direct consequence of the special circumstances associated with “band-gap emission”.     

Correlations of high transverse momentum π0 pairs produced at the CERN ISR

Correlations of two π⁰ mesons with transverse momenta up to 10 GeV/c have been measured utilizing apparatus with large azimuthal acceptance. The data are analysed in the context of constituent scattering models, and also compared with a simple “back-ground” hypothesis.     

Infrared vibration spectroscopy of molecular adsorbates on tungsten using reflection inelastic electron scattering

The observation of adsorbate vibrational energies in the range, 30 ?, hv_vib ? 1000 meV, by electron-energy-loss spectroscopy, provides detailed information on the geometry of atomic and molecular complexes. The “surface normal dipole selection rule”, is discussed and illustrated with results obtained for CO and C₂H₂ adsorption on the principal low-index faces of tungsten, viz.: W(100), W(110) and W(111) using a high-resolution electron reflection spectrometer. Specifically, the behaviour of chemisorbedd diatomic carbon monoxide and polyatomic acetylene is compared as a function of coverage and surface crystallography. Comparison is made with the spectral information obtained by reflection infrared spectroscopy and recent ultraviolet photoelectron spectroscopy studies of the chemisorption binding energies. The energy loss spectra are discussed in terms of current adsorbate models and the possible formation of “distorted rehybridized surface molecular complexes” based on molecular orbital theories of organometallic compounds.     

Observation of polarization in bottomonium production at square root of s = 38.8 GeV

We present a measurement of the polarization observed for bottomonium states produced in p-Cu collisions at square root of s = 38.8 GeV. The angular distribution of the decay dimuons of the Upsilon(1S) state shows no polarization at small values of the fractional longitudinal momentum x(F) and transverse momentum p(T) but significant positive transverse production polarization for either p(T)>1.8 GeV/c or for x(F)>0.35. The Upsilon(2S+3S) (unresolved) states show a large transverse production polarization at all values of x(F) and p(T) measured. These observations challenge NRQCD calculations of the polarization expected in the hadronic production of bottomonium states.     

The first sideways-bonded peroxo complex for a tetraaminecobalt(III) species

Cobalt(II) salts react with H2O2 in the presence of 2 equiv of tetramethylethylenediamine (tmen) to produce a stable sideways bonded mononuclear peroxo complex [Co(tmen)2O2]ClO4 which has been characterized chemically and by a single crystal X-ray determination. This is the first such tetraaminecobalt(III) complex, indeed, the first for any tetraaminemetal ion complex. The mononuclear peroxo complex can also be synthesized by O2 2- anation of cis-[Co(tmen)2(OH2)2]3+. This reaction is reversed in acid, and this offers the potential to develop Co(III) catalyzed oxidation reactions.     

Determination of the complex Young and shear dynamic moduli of viscoelastic materials

The Young and shear dynamic moduli of viscoelastic materials are determined from laser vibrometric measurements of the surface motion of a three-dimensional sample excited by a piezoelectric actuator inside a chamber with controllable temperature and static pressure. The moduli are estimated from an inversion code that minimizes the difference between the data and the predictions from a finite element model in which the elastic moduli are the adjustable parameters. The technique is first used to measure the dynamic properties of homogeneous samples and the results are compared with those obtained by the standard rod resonance technique. Results are then obtained with microvoided samples in the 0.5-3 kHz frequency range, at temperatures ranging from 7 to 40 degrees C, and static pressures ranging from ambient to 34 atm (3.45 MPa or 500 psi). The limitations of the technique are discussed.     

Effect of magnetic doping on the electronic states of Ni

Angle-resolved photoemission is used to determine the change in the electronic states of Ni induced by doping with Fe and Cr. Well-defined spin and k states are selected using high energy and k resolution combined with single crystal alloys. Iron suppresses the mean free path of minority spins only, while chromium suppresses both spins and decreases the magnetic splitting. The strong variation of these effects from one impurity to the other supports the concept of magnetic doping.