Observation of spectral line splitting for parametric X-rays

The spectral distribution and energy dependence of the yield of parametric X-rays for 200 to 900 MeV electrons transmitted through a silicon crystal have been measured. The measurements were performed in the Bragg geometry at an angle θ = 19° to the electron beam direction. The effect of spectral line splitting for parametric X-rays was observed.     

Crystalline effects in backscattering and Auger production

Measurements have been made of the variation of the backscattered intensity and of the Auger signal intensity as a function of the direction of the incident electron beam using a (111) Al surface and an incidence energy of 1.5 keV. The dynamical theory of electron diffraction and backscattering is able to account for the results quantitatively to within a factor of about two in the case of the quasi elastic and first plasma loss backscattered components. The variation measured for the Auger signal is significantly greater than for backscattered electrons of similar energy.     

Generation of monoenergetic ultrashort electron pulses from a fs laser plasma

Ultrashort high-energy electron beams are generated by focusing fs Ti:sapphire laser pulses on a thin metal tape at normal incidence. At laser intensities above 10¹⁶ W/cm² , the fs laser plasma ejects copious amounts of electrons in a direction parallel to the target surface. These electrons are directly detected by means of a backside illuminated X-ray CCD, and their energy spectrum is determined with an electrostatic analyzer. The electrons were observed for two laser polarization directions, parallel and perpendicular to the observation direction. At the maximum applied intensity of 2×10¹⁷ W/cm², the energy distribution peaks at around 35 keV with a hot tail detectable up to about 300 keV. The number of electrons per shot at 35 keV is about 5×10⁸ per sterad per keV. Quasi-monoenergetic electron pulses with a relative energy spread of 1% were produced by using a 50-m slit in the beam path after the analyzer. This approach offers great potential for time-resolved studies of plasma, liquid, and surface structures with atomic-scale spatial resolution. PACS 41.75.Fr; 52.38.Kd; 52.70.Nc     

Determination of the inelastic mean free path of electrons in silver and copper by measurement and calculation of the elastic scattering coefficient

The inelastic mean free paths, λ, of electrons in polycrystalline Ag and Cu samples were determined for electron energies in the range 250–1500 eV by means of elastic peak electron spectroscopy (EPES). In this method the coefficient of elastic scattering of electrons, η_e, is measured. η_e is also calculated theoretically for different assumed values of λ. The value of λ giving the best agreement between the measured and calculated η_e is taken as the experimental one. the η_e measurements were performed for Ag and Cu samples cleaned by mechanical scraping only and for samples cleaned by ion bombardment in order to estimate the eventual influence of surface roughness on the η_e values. Additionally, different methods to calculate η_e theoretically are compared.     

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: NO desorption from and NO2 dissociation on Pt(111)

We present the first quantum-resolved angular distributions of ground-state neutral molecules which are products of electron stimulated desorption (ESD) and electron stimulated dissociation. Laser resonance-enhanced multiphoton ionization (REMPI) and two-dimensional imaging have been used to obtain angular distributions of NO desorbed by 350 eV electrons from O-precovered Pt(111). In a similar fashion, we have measured angular distributions for the NO product of NO₂ dissociation on clean and O-precovered Pt(111). In all cases, we observe narrow widths which are roughly the same as ion distributions determined by ESDIAD (ESD ion angular distributions). The angular distribution for NO ESD is sharply peaked (7° half-width at half maximum) along the surface normal for an O coverage (θ_o) of 0.25 monolayer (ML). The angular distribution of the NO product from dissociation of side-bonded NO₂ on clean Pt(111) is unexpectedly peaked about the surface normal, and thus does not reflect dissociative forces parallel to the surface or the 25° off-normal ground-state bond direction. On O-precovered Pt(111), where NO₂ is N-bonded, 6° off-normal beams are observed. When the substrate is precovered with θ_o > 0.5 ML, local disorder creates asymmetric site geometries which result in multiple peaked angular distributions with both normal and off-normal (9–10°) components; similar effects for NO ESD are observed. In all these studies, the NO angular distributions are invariant to rotational or vibrational state. This implies that the lateral translational degrees of freedom are essentially de-coupled from the internal modes of the molecule. The results are discussed in terms of desorption mechanisms, dissociative forces, site geometries, and disordered coadsorbate layers.     

Normal skin effect in the polycrystals

The surface impedance γ and the penetration depth δ of the electromagnetic field was calculated for a polycrystalline conductor under the conditions of the normal skin effect. The polycrystal is composed from the single-phase slightly anisotropic single-crystalline grains of arbitrary symmetry. Corrections (γ-γ₀)/γ₀ and (δ-δ₀)/δ₀ due to the polycrystallinity are functions of the parameter ζ=δ₀/a, where γ₀ and δ₀ are the impedance and the penetration depth of the isotropic conductor with the conductivity σ₀=Sp σ_ik/3, σ_ik is the separate grain conductivity tensor, and a is the mean size of a grain.

The effective conductivity of a polycrystal can be obtained either from the impedance γ (σ_ef^(γ)) or from the penetration depth δ (σ_ef^(δ)) It was found that σ_ef^(γ)≠σ_ef^(δ) The corrections (σ-σ_ef^((γ))/σ₀ and (σ₀-σ_ef^(δ))/σ₀ depend on the frequency of the incident wave.

The obtained results are also applicable in the infrared spectrum region. It was shown that, just as for the isotropic conductor, there is the total reflection of the electromagnetic wave from the polycrystal surface.

It was necessary to examine the oblique incidence of an electromagnetic wave onto the surface of an isotropic metal for the substantiation of the set of the boundary conditions used.     

Influence of electromagnetic fluctuations on the resonant tunneling of electrons

We have considered the influence of electromagnetic fluctuations on electron tunneling via one non-degenerate resonant level, the problem that is relevant for electron transport through quantum dots in the Coulomb blockade regime. We show that the overall effect of the fluctuations depends on whether the electron bands in external electrodes are empty or filled. In the empty band case, depending on the relation between the tunneling rate Γ and characteristic frequency Ω of the fluctuations, the field either simply shifts the conductance peak (for rapid tunneling, Γ Ω) or broadens it (for Γ Ω). In the latter case, the system can be in three different regimes for different values of the coupling g between electrons and the field. Increasing interaction strength in the region g < 1 leads to gradual suppression of the conductance peak at the bare energy of the resonant level ε₀, while at g 1 it leads to the formation of a new peak of width at the energy ε₀ + E_cis a charging energy. For intermediate values of g the conductance is non-vanishing in the entire energy range from ε₀ to ε₀ + E_c. For filled bands the problem is essentially multi-electron in character. One consequence of this is that, in contrast to the situation with the empty band, the fluctuations of the resonant level do not suppress conductance at resonance for g < 1. At g> 1 a Coulomb gap appears in the position of the resonant level as a function of its bare energy which leads to suppression of conductance.     

Trinal Nature of the Excitation of Bulk Plasmons by a Fast Charged Particle at Grazing Incidence on the Interface between Vacuum and a Metal with Strong Spatial Dispersion

The excitation and propagation of bulk and surface (surface waves of transition radiation of plasmons at frequencies above the plasma frequency) plasma waves by incident electrons moving both in vacuum toward the surface of a metal and inside the metal whose boundary elastically and spectacularly reflects internal nonequilibrium electrons have been analyzed. In contrast to work [B. N. Libenson, J. Exp. Theor. Phys. 113, 553 (2011)], attention in this work is focused on the influence of surface effects on bulk-plasmon excitation by incident electrons. The probabilities and spectra of single characteristic energy loss of an intermediate- energy electron (50–500 eV) moving at an angle to the surface of the medium in three regions: in vacuum, in the medium, and again in vacuum after the electron leaves the medium are calculated. The kinetic approximation is used for the dielectric function, where the entire range of the plasmon spectrum is taken into account correctly for the problem under consideration. In the indicated energy range of incident electrons, surface effects, on the one hand, significantly reduces the probability of excitation of bulk plasma waves in the medium with strong spatial dispersion, in particular, as compared to the results obtained in [B.N. Libenson, J. Exp. Theor. Phys. 113, 553 (2011)], where surface effects were disregarded and the probability of bulkplasmon excitation by a 200-eV electron incident and emitted perpendicularly to the boundary is about one third of that in the unbounded medium. On the other hand, at grazing incidence from vacuum, the probability of transition radiation of bulk plasmons increases significantly and can lead to a change in the character of the angular dependence of the intensity of bulk plasma energy loss. Thus, the main result of this work is that a decrease in the glancing angle of the fast electron with respect to the vacuum–metal interface is accompanied both by an increase in the contribution from the transition mechanism to the probability of bulk-plasmon excitation in the vacuum region and by a decrease in the contribution from Cherenkov and bremsstrahlung mechanisms of excitation in the medium. The probability of bulk-plasmon excitation in the vacuum region exceeds the probability of excitation at the further motion of the electron in metallic aluminum at angles of incidence larger than 65°, 70°, and 75° at the energies E = 200, 350, and 500 eV, respectively.     

Change of the surface induced optical anisotropy of the clean Si(110) surface by oxidation

Normal incidence ellipsometry has been used to measure the change in the complex anisotropic reflectance ratio γ upon oxidation of the clean Si(110)16 × 2 surface. The spectroscopic change in the amplitude of γ (tan(Ψ)) shows a broad maximum of height 1.4 × 10⁻³ in the high energy region above 2.5 eV. No phase shift difference for the reflectance coefficients belonging to the surface principal optical axes has been measured. A Kramers-Kronig transformation of the amplitude ratio showed that a change in the phase is not expected. The change in tan(Ψ) indicates that the change in reflection upon oxidation in the optical region ismainly in the ( 10) direction.     

Experimental observation of a copious yield of electrons with small transverse momenta in pp collisions at high energies

Inclusive electron and positron emission have been observed for θ_cm = 30° and S = 2800 GeV² at the CERN Intersecting Storage Rings (ISR). Over the transverse momentum interval 0.2 GeV/c < p_T < 1.5 GeV/c, electrons and positrons, which are equal in number within the experimental accuracies, appear to grow with respect to other particles (pions) approximately like 1/p_T. We are unable to explain their number and p_T-dependence in terms of “conventional” mechanisms.     

Infrared reflection-absorption spectroscopy of dipole-forbidden adsorbate vibrations

Formally dipole-forbidden adsorbate vibrational modes have recently been detected using infrared reflection-absorption spectroscopy (IRAS). In an earlier publication we have proposed that the excitation of these modes is indirect, mediated by the metal electrons and closely related to the concept of surface resistivity. Here we extend that theory to lower frequencies; for parallel adsorbate vibrations the theory predicts an anti-absorption resonance with an unique asymmetry which only depends on the ratio ω₀/ω₁ between the adsorbate vibrational frequency ω₀ and ω₁ = v_F/δ where v_F is the Fermi velocity and δ = c/ω_p the skin depth. The theory is in very good agreement with the IRAS measurements of Hirschmugl et al. [Phys. Rev. Lett. 65 (1990) 480] for CO on Cu(100) and in qualitative agreement with the measurements of Lin et al. [Phys. Rev. B 48 (1993) 2791] for CO on Ni(100).     

Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO₃ crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO₃ at 544°C, where LaAlO₃ changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}_pseudocubic planes of twin domains were measured by SFM on the surface of a macroscopic (100)_cubic cut crystal plate. The misorientation angle ω₁₀₀ between {100} twins is 0.195(8)°, while {110} twinning gives an angle of ω₁₁₀=0.276(7)°. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the phase as ₁=90.0973(40)° and ₂=90.0975(30)°, respectively. The XRD result for this rhombohedral angle is =90.096(1)°. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T_c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps.     

Enhanced backscattering in a magnetic field

By infinite-order perturbation theory the authors study the scattering of p-polarized light from a small-amplitude random grating ruled on the surface of a metal or an n-type semiconductor to which a constant magnetic field is applied. The surface is defined by the equation x₃=ζ(x₁), where the surface profile function ζ(x₁) is a stationary, stochastic, Gaussian process. The plane of incidence is the x₁x₃ plane, and the magnetic field is directed along the x₂ axis. We find that the position of the peak in the angular distribution of the intensity of the incoherent component of the scattered light that is observed in the retroreflection direction in the absence of the magnetic field—enhanced backscattering—is shifted in the direction of larger scattering angles from the retroreflection direction with increasing magnetic field strength. At the same time the width of the peak increases and its amplitude decreases. This is interpreted as due to the breakdown of the coherency between the contribution to backscattering from a given light/surface polariton path and from its time-reversed partner, caused by the removal of time reversal symmetry from the scattering system by the application of the external magnetic field. The latter is manifested by the non-reciprocity of the surface polariton dispersion relation in the presence of the magnetic field.     

A new type oscillatory phenomenon in the magnetotransport of θ-(BEDT-TTF)2I3

A new type oscillatory magnetotransport phenomenon has been observed in θ-type crystals of (BEDT-TTF)₂I₃ at temperatures below 6 K and in the magnetic field above 3 T. The oscillation appears when the magnetic field of a fixed strength is rotated from the direction normal to the conductive two dimensional plane to a direction parallel to the plane. The period of the oscillation is described by an equation tan (θ_min)=sN (s=0.39, N=0, 1, 2, 3,…), where θ_min is the angle giving the position of the trough of the oscillation. The amplitude of the oscillation is primarily determined by the magnetic field component normal to the conductive plane. It is related to the temperature through the change in the resistivity ₀.     

Quasi-static surface modes of plasma layer with anisotropic electron temperature

Quasi-static surface wave propagation in a plasma layer with anisotropic electron temperature is considered. The case is analyzed where the electron temperature in the direction normal to the plasma boundary is considered to be zero, while in the direction along the boundary, electrons are described by the Maxwellian velocity distribution. It is shown that the modes of such a layer are described by equations for bulk plasma waves with renormalization of the electron density affecting the surface wave dispersion and damping.     

Anisotropic profile decay on perturbed Au(111) vicinal surfaces

The anisotropy of the surface free energy has a large influence on the decay rate of periodic surface profiles at temperatures below the roughening temperature. This has been demonstrated by a study of the periodic surface profiles that were etched into vicinal Au(111) crystals in two orientations: with their wavevector parallel (ψ = 0) and perpendicular (ψ = π/2) to the intrinsic step direction of the vicinal surface. Due to this modulation, steps become either oscillatory in shape or remain straight, with their average separation modulated. Accordingly, profile decay is driven primarily by step self-energy (ψ = 0) or step interaction energy (ψ = π/2). The rate of decay was measured under ultra-high vacuum conditions at 750°C using a scanning tunneling microscope for imaging. A large anisotropy of decay was observed on two vicinal crystals of 1.5 and 5° miscut, with the ψ = 0 decay being much faster than with ψ = π/2). The results are evaluated and discussed in the framework of recent theory.     

Interaction between CO and NH3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia (¹⁵NH₃ or ND₃). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θ_CO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θ_CO = 0.5. The dissociation of ammonia in the β−(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the −(2 × 2) structure. The HREEL spectra indicated that the C_3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β−(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH₃ interaction on metal surfaces is presented.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Nonlinear phase characteristics of the dispersion interferometer at high-intensity pulses

The dependences of the second harmonic (SH) intensity output by the dispersion interferometer (DI) upon the relative phase of the SH and fundamental waves θ₀ (called phase characteristics) are studied both experimentally and theoretically. It has been found that high irradiance (when self-action effects are not negligible) makes the phase characteristics differ from the cosine type which is usual for linear interferometry. In the case of high power pulsed radiation, a strong dependence of the SH pulse shape on θ₀ was observed. The evolution of the SH pulse shape in the DI output registered as θ₀ varies is checked against the numeric simulation data. By measuring the nonlinear phase characteristics of the DI, the main influence of the slow (response time of ≈ 1 ns) cubic nonlinearity on the frequency doubling of nanosecond pulses is determined for the employed KTP crystals.