Electron and positron work functions of Cr(100)

We report measurements of the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum. The positron work function ø₊ is obtained by measuring the spectrum of slow positrons reemitted by the Cr(100) surface when it is bombarded with keV energy positrons. The electron work function ø_- is measured relative to Al(100) by comparing the target biases at which the slowest emitted positrons are recollected by the target. We obtain ø₊ = ?1.76(10) eV and ø_- = 4.46(6) eV for our Cr(100) surface using the value ø_- = 4.41(3) eV for Al(100) reported by Grepstad, Gartland and Slagsvold. The ø₊ value is in agreement with the ?2.2 eV calculated by Nieminen and Hodges. The positronium work function for Cr implied by these results is ?4.10(10) eV; the positronium negative ion (Ps^-) work function for this surface is calculated to be + 0.37(7) eV. A search for Ps^- showed that at a 90% confidence level less than one in 10³ thermalized positrons reaching the Cr surface are emitted as Ps^-. The slow positron emission spectrum was observed not to change over the 70–300 K range in contrast to recent theoretical predictions.     

The Observation of Positron Surface State of Germanium

The formation yield of positronium (Ps) as a function of target temperature has been measured when positron implanting energy was 20eV.By thermal dynamic analyising we obtained the binding energy E_b of positron surface state of germanium to be 2.2eV and the thermal activation energy E_a of Ps to be 0.2±0.01eV at vacuum 1.33×10^－4Pa.The different of E_b and E_a at the condition of high and ultra-high vacuum was discussed.     

Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

Electroabsorption in TlInS2

Electroabsorption spectra of single crystals have been studied near the fundamental absorption edge at 77 and 300 K. At 300 K two positive peaks (2.34 and 2.42 eV) and a negative peak (2.38 eV) are observed in the electroabsorption spectrum. At liquid-nitrogen temperature a fine structure corresponding to the formation of a parabolic exciton (2.503 eV) is observed.Values of the width of the forbidden gap E_g, the n = 1 exciton positions, the exciton activation energy ΔE_b, the effective Bohr radius a_exc, the reduced effective mass of an electron-hole pair μ, and the exciton ionization field F(E_g = 2.535 eV, E_exc = 2.503 eV, E_b = 32 meV, $\text{a}{}_{\mathrm{<mtext>exc</mtext>}}\text{=}\text{28}\text{A}\text{A;}\text{;}$;, μ = 0.15 m₀, and F = 1.2 × 10⁵ V cm^-1) have been determined from the electroabsorption spectrum.     

Variations of the local density of electron states and short-range order in amorphous hydrated silicon films

Ultrasoft x-ray spectroscopy methods have been used to observed a change in the energy distribution of the silicon valence states after annealing a-Si:H films at 500 °C. This change appears as three distinct maxima in the density of states 3.5, 7.2, and 10.2 eV above the top of the valence band, which indicates ordering of the a-Si:H structural network. The energy distance between the latter two maxima (E?E _v=7.2 and 10.2 eV) supports electron-diffraction data indicating a decrease in the silicon-silicon interatomic distance by 0.2 Å in comparison with the crystal. The presence of a third maximum (E?E _v=3.5 eV) is connected with the change in the hybridization of the s-p-functions of silicon with decrease of the coordination number.     

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni₃Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E _sp, and adatom creation, E _ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E _sp of 6.5?eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0?eV for a perfect M surface. High values of E _sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E _sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E _ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E _sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1?eV different from that obtained for a perfect surface. Temperature up to 300?K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E _sp studied here. Possible reasons for this are discussed.     

Electronic structure and fundamental absorption edges of KPb2Br5, K0.5Rb0.5Pb2Br5, and RbPb2Br5 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion irradiated (001) surfaces of KPb₂Br₅, K_0.5Rb_0.5Pb₂Br₅, and RbPb₂Br₅ single crystals grown by the Bridgman method have been measured and fundamental absorption edges of the ternary bromides have been recorded in the polarized light at 300 K and 80 K. The present X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (001) surfaces of K_xRb_1?xPb₂Br₅ (x=0, 0.5, and 1.0) single crystals. Substitution of potassium for rubidium in K_xRb_1?xPb₂Br₅ does not cause any changes of binding energy values and shapes of the XPS constituent element core-level spectra. Measurements of the fundamental absorption edges indicate that band gap energy, E_g, increases by about 0.14 and 0.19 eV when temperature decreases from 300 K to 80 K in KPb₂Br₅ and RbPb₂Br₅, respectively. Furthermore, there is no dependence of the E_g value for KPb₂Br₅ upon the light polarization, whilst the band gap energy value for RbPb₂Br₅ is bigger by 0.03–0.05 eV in the case of E6c compared to those in the cases of E6a and E6b.     

Electronic structure of solid pyridine and pyridine adsorbed on polycrystalline silver

The electronic structure of the valence bands of polycrystalline films of pyridine (C₅H₅N), including all valence levels extending from initial energies of 4 down to 30 eV (E_VAC = 0), has been determined from photoelectron energy distribution measurements for photon energies 20eV ? hν ? 170 eV, using synchrotron radiation. The valence bands showing a one-to-one correspondence to the gas phase, a rigid relaxation shift of δe_r = 0.3 eV for the vertical binding energies, and a considerable solid state broadening (? 0.5 eV) are assigned in comparison to recent MO calculations. By tuning the photon energy and thereby achieving high surface sensitivity for hν around 45 eV, we have also studied pyridine adsorbed at 120 K (6 Langmuir) on in situ prepared polycrystalline Ag-substrates. Thus, we were able to study in detail the surface electronic structure in the range of the Ag 4d bands. Due to a strong mixing of substrate 4d and pyridine 2b₁ (π), 1a₂ (π) and 7a₁ (n) energy levels, the surface electronic structure is strongly modified in the upper part of the 4d bands, and a strong and sharp (0.4 eV FWHM) surface resonance at an energy of 3.7 e V below E^AG_F is observed, which we attribute to a 4d-7a₁ (n) bonding of the nitrogen lone-pair orbital.     

Crystal structure of and evidence of a lattice distortion in (Me2ø2P)(TCNQ)2

(Dimethyldiphenylphosphonium)⁺(7,7,8,8-tetracyanoquinodimethanide)^?₂ is monoclinic, space group C_c, with a = 32.01(2), b = 6.56(1), $\text{c = 15.72(2)}\text{A}\text{?}$, β = 107.4(8)°. The TCNQ's stack plane-to-plane in columns parallel to b with (i) a mean interplanar spacing of 3.28 Å along the conducting chains and (ii) an exocyclic bond to quinonoid ring overlap of adjacent molecules. The conductivity along b, the needle axis, varies as $\text{σ = σ}{}_0\text{exp}\text{(}\text{?E}{}_{\mathrm{a}}\text{kT}\text{)}$ where σ_{300 K} = 0.05 S cm^?1 and E_a = 0.20 eV (Diethyldiphenylphosphonium)⁺(7,7,8,8-tetracyanoquinodimethanide)^?₂ is similarly monoclinic, space group C_c, with a = 31.48(2), b = 6.51(1), $\text{c = 15.48(2)}\text{A}\text{?}$, β = 104.2(8)°. The conductivity at 300 K and activation energy, both determined along b, are 1–10 S cm^?1 and 0.05 eV respectively. There is evidence of a lattice distortion in the dimethyl analogue only.     

Angular and energy dependences of the surface excitation parameter for electrons crossing a solid surface

When fast electrons cross a solid surface, surface plasmons may be generated. Surface plasmon excitations induced by electrons moving in the vacuum are generally characterized by the surface excitation parameter. This parameter was calculated for 200-1000 eV electrons crossing the surfaces of Au, Cu, Ag, Fe, Si, Ni, Pd, MgO and SiO₂ with various crossing angles. Such calculations were performed based on the dielectric response theory for both incident (from vacuum to solid) and escaping (from solid to vacuum) electrons. Calculated results showed that the surface excitation parameter increased with crossing angle but decreased with electron energy. This was due to the longer time for electron-surface interaction by glancing incident or escaping electrons and by slow moving electrons. The results were fitted very well to a simple formula, i.e. , where P_s is the surface excitation parameter, E is the electron energy, α is the angle between the electron trajectory and the surface normal, and a, b and c are material dependent constants.     

Degenerate Landau levels for tripod-type cold atoms in U(2) Abelian gauge field

New data for positronium beam production efficiency are presented. As a converter of positrons to forward-going positronium, O₂ has been found to be as good as CO₂ from ～250 to 400 eV. Preliminary data is also presented for the production efficiency from Ar at 2.8 eV.     

Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

Appearance of the dielectric instability and its coexistence with the superconductivity in ultrathin metallic filaments with decreasing diameter

Some dielectric instability appears before the superconducting (s/c) transition of Hg, In, Ga, Sn filaments when their diameters are decreased to 20–30 Å. Above the s/c transition the differential conductivity of the filaments may be approximated by the formula σ = σ_a + σ_b exp [-E₀(T)/E], where the electric field E₀ is equal toabout 10–60 mV cm^-1, and falls abruptly before the s/c transition. There are conductivity oscillations along E in the s/c transition region. The results are interpreted in terms of Peierls instability and charge-density wave (CDW) conductivity.     

Intracellular potential relief and size effects in the lattice of a LaF3 superionic conductor

The potential relief in the lattice of a LaF₃ crystal is calculated by quantum-mechanical methods for clusters containing from 24 to 1200 ions. For the dielectric phase, formation energy E _a for defects of the vacancy-interstitial fluorine ion type and potential barrier E _d preventing the motion of fluorine ions are found to grow from minimal values E _a = 0.12 eV and E _d = 0.22 eV for a cluster of 24 ions to maximal values E _a = 0.16 eV and E _d = 0.26 eV for clusters of 576 and 1200 ions. The values of E _a and E _d obtained in quasi-mechanical calculations are in good agreement with those obtained from Raman and quasi-elastic light scattering data.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

<Superscript>1</Superscript>H Spin–Lattice Relaxation Study of Dynamical Inequivalence of Methyl Groups in Solid 1,2-<Emphasis Type="Italic">O</Emphasis>-(1-Ethylpropylidene)-α-<Emphasis Type="SmallCaps">d</Emphasis>-Glucofuranose

We investigated the dynamics of methyl groups in organic polycrystalline 1,2-O-(1-ethylpropylidene)-α-d-glucofuranose by the proton spin–lattice relaxation method. The temperature and nuclear magnetic resonance Larmor frequency dependence of relaxation time is presented and interpreted in terms of simple possible dynamical model for the reorientation of methyl groups: the random hopping for methyl groups, which are in a, b, and c sites in the crystal. The energy E _a of 13.5 kJ mol⁻¹ for the a-type methyl groups is typical for methyl groups in ethyl groups. In contrast, the b- and c-methyl groups characterized by the lower E _a values of 9.5 and 6.5 kJ mol⁻¹ are located in the crystal structure where the intermolecular interactions significantly influence the potential, leading to a decrease in the total energy.     

Resonances in positron-atom scattering

Theoretical studies of atomic resonances involving positrons will be discussed in this talk. Investigations on resonances in positron-hydrogen scattering below various hydrogen and positronium thresholds are reviewed, as well as resonances in positronalkali and e⁺-He⁺ scattering. Resonance phenomena in other atomic systems involving positrons will also be discussed. These systems include positronium ions Ps⁻, positronium molecules Ps₂, and positronium hydride PsH.     

Image potential eigenstates and resonances on the (110) surfaces of noble metals: Energies and lifetimes

The energies and lifetimes of the image potential resonances at the ? point on the Cu(110), Ag(110), and Au(110) surfaces are studied, and the energies of the image potential states on the Pd(110) surface are analyzed. It is shown that every quantum number n corresponds to a pair of image potential states (resonances) n ⁺ and n ^? at the ? point. The average energy of a pair of eigenstates (resonances) at n ≥ 2 is well described by the formula ē _n = (E _n+ + E _n?) = E ₀ ? (0.85 eV)/(n + δ)², where δ is the quantum defect, E ₀ = (1/2m _e )(?π/a)², and a is the lattice parameter. The splitting energy of a pair of eigenstates (resonances) obeys the law ΔE _n = E _n+ — E _n? ∝ n ^?3. The lifetimes τ _n± of image potential resonances are proportional to n ³.     

An interpretation of dlts spectra of Al/Si3N4/ultrathin SI/GaAs structures — Effect of quantum well or interface states?

We discuss DLTS andC-V measurements on Al/Si₃N₄/Si(2nm)/n-GaAs (≈ 5×10¹⁷ cm^?3) structures. Three discrete deep traps superimposed on a U-shaped interface-state continuum have been identified, with respective thermal energies:E _c?0.53 eV,E _c?0.64 eV, andE _v+0.69 eV. The second one (0.64 eV) is presented as an electric field sensitive level, its enhanced phonon-assisted emission resulting in a rapid shift of the corresponding DLTS peak to lower temperatures, as the applied (negative) reverse bias voltage increases. An interpretation through emission from the quantum well, introduced by means of the intermediate ultrathin Si layer, has failed.