Observation of the image force for fast electrons near an MgO surface

The image force for fast (80 kV) electrons has been observed using the small electron microprobe available in a scanning transmission electron microscope, and imaging the surface plasmon losses. Preliminary results for the field at grazing incidence to an MgO surface qualitatively agree with the classical theory. The long range decay is found to be exponential.     

Interaction of conduction electrons with local excitations. The infrared divergencies

Using the renormalization group method the higher orders of perturbation theory in the interaction of conduction electrons in metals with local paulions (pseudospins), e.g. two-level systems, crystal-field excitations, and bosons, e.g. phonons, are considered. For the paulions, the lowest order logarithmic singularities in the electron self-energy at =E–E _F±, being the splitting, become weaker, at least in the commutative model. It is shown that the singularities of the type ln are absent. This justifies the applicability of the second order perturbation theory resultm ^{* –1} for the electron effective mass even atm ^*m. For the phonons, the singularities at ±₀, ₀ being the phonon frequency, may become stronger or weaker depending on the conduction band filling and the anharmonic contribution to the deformation potential. The singular contributions to the local excitation Green's function are calculated. They result in the change of the line shape of the local level (the orthogonality catastrophe). The singular terms in the ground state energy and average pseudospin are considered.     

Collisions of fast electrons with positronium atoms

Summary Elastic and inelastic collisions of fast non-relativistic electrons with positronium atoms have been studies in the Born-Ochkur approximation. It has been shown that exchange plays an important role for transitions between states with the same parity as in this case the direct scattering amplitude vanishes. Numerical results for the total and ortho-para conversion cross-sections for the 1s→1s, 1s→2s and 1s→2p transitions have been presented for projectile energies varying from 0.1 to 10 keV. According to charge symmetry, presented results apply also to positron projectiles.     

Sputtering by fast electrons

Calculations are presented of collision cascade and knockon contributions to sputtering induced by fast electrons. The sputtering yield for MeV electrons bombarding carbon and iron is found to be ≈10^?7 to 10^?6. Important erosion effects on astrophysical grains are possible.     

The characteristics of the grazing interaction of fast electrons with dielectric surfaces

Experimental data on the mechanism of the grazing interaction of fast electron beams with planar dielectric surfaces of various materials are analyzed. New experimental results regarding the grazing interaction between electrons with an energy of 10 keV and metalized structured surfaces are reported. The difference between the processes of the interaction of fast electron beams with planar and structured surfaces is demonstrated.     

Excitation of surface plasmons in metal particles by fast electrons and x rays

The cross sections for excitation of surface plasmons by fast electrons and x rays are calculated within the random phase approximation for a spherical metal particle. The results are compared with those obtained within the hydrodynamic model of a free electron gas.     

Electronic excitations of CO adsorbed on MgO(001)

We report ab initio calculations of the quasiparticle band structure and the optical excitation spectrum of bulk MgO, the MgO(001) surface, and CO molecules adsorbed on MgO(001). Many-body exchange and correlation effects are included within the GW approximation of the electron self-energy operator and the corresponding electron–hole interaction. The excited electron–hole states are obtained from the Bethe–Salpeter equation. At the clean MgO(001) surface exciton states are found with binding energies that are significantly stronger than in the bulk. The exciton spectrum of the adsorbate system CO:MgO is dominated by charge-transfer excitons, which couple strongly to the molecular excitations of CO. PACS 73.20.At; 73.20.Hb; 34.70.+e     

Collective excitations in fast ion conductor superlattices

Within the framework of the electromagnetic theory, the collective modes in the superlattice system composed of superionic conductors and ionic crystals are studied. The superionic conductor is described by the hydrodynamical model in which the anion cage is immersed in the cation liquid. The behavior of the modes are analysed in terms of the coupling strength between excitations pertaining to different layers. The coupling strength is controlled by varying the slab thicknesses. An interesting behavior in which the diffusion mode transforms to the relaxation mode when the coupling strength is varied from strong to weak is obtained. Also, the effect of the coupling strength on the acoustical and optical modes are shown.     

Evidence for localized excitations in MgO from electron energy-loss spectroscopy

The excited electronic states in the ionic crystal MgO have been studied by electron energy-loss spectroscopy. Structure observed above the thresholds for excitation from the Mg 2p, 2s and 1s core levels shows the final states to be predominantly excitonic levels of the Mg²⁺ ion. Comparison with the known states of the Mg²⁺ free ion indicates that solid-state shifts and splittings are small.     

Some general problems of fast electrons transport

A generalization is given of the segments method in the form of a multistep method with generalized time for computing the transport of fast particles. The integral equation for a flow with generalized time in the phase space of variables is written under the assumption that the flow cuts the generalized time surface at right angles. The Green's function for the differential flow operator is the kernel of the integral equation. It is also shown that such an integral equation which can be obtained from a nonstationary kinetic equation provides a uniform consistent algorithm for solving either nonstationary or stationary problems. Examples of Green's functions are given for an operator of differential flow of fast electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 110–114, August, 1974.The author would like to express his thanks to A. A. Vorob'ev and B. A. Kononov for their encouragement, to A. P. Yalovets for discussing the work with him, and to A. M. Kol'chuzhkin for going through the text.     

Loss of electrons from fast heavy structure ions colliding with atoms

Nonperturbative theory is developed for the multiple ionization of fast heavy structure ions colliding with neutral complex atoms. The cross sections for multiple loss of electrons by structure uranium ions U¹⁰⁺ (loss of up to 82 electrons) and U²⁸⁺ (loss of up to 64 electrons) colliding with argon atoms are calculated. The results are compared with available experimental data.     

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.     

Experimental studies of optical surface excitations

Experimental methods for optical excitation of nonradiative surface waves, such as surface plasmons and surface phonons, are reviewed. Results for surface plasmons in InSb obtained by using samples with line gratings in the surface and by using the technique of attenuated total reflection (ATR) are compared. Best agreement between the theoretical and experimental surface plasmon dispersion curve ω(k) of InSb is found when using excitation by ATR, since the sample surface need not be disturbed in this case. Surface phonons have been excited optically by applying ATR to alkali halide crystals and to the polar semiconductor GaP. While agreement with the theoretical dispersion curve is extremely good for GaP and also for CaF₂, there are systematic deviations to lower frequencies for NaF and other alkali halides. In polar semiconductors the free carrier plasmon couples to the LO-phonon when ω_p ≈ ω_LO. This behaviour has been investigated theoretically and experimentally for the surface modes. For an interesting application of the ATR-technique, one can use the fact that dispersion and linewidth are a direct measure of the complex dielectric function ε(ω) in the frequency range of the optical surface waves.     

Experimental investigation of plasma excitations in asymmetric stripes of two-dimensional electrons

The plasmon microwave response of symmetric and asymmetric stripes of two-dimensional electrons with different boundary conditions has been studied. The symmetric case corresponds to stripes either without ohmic contacts or with ohmic contacts on both sides, while the contact in asymmetric stripes is present only on one side. It has been shown that there is a frequency shift of the observed plasma modes in asymmetric stripes. The lowest-frequency mode becomes a previously unobserved mode for which the quarter wavelength of the plasmon coincides with the length of the stripe. The behavior of the lowest mode and its multiple frequencies in asymmetric stripes has been studied under variation of the magnetic field, electron density, and temperature and size of the stripes. The results indicate that all plasma modes in symmetric and asymmetric stripes have the same physical nature.     

Dephasing of multiparticle Rydberg excitations for fast entanglement generation

An approach to fast entanglement generation based on Rydberg dephasing of collective excitations (spin waves) in large, optically thick atomic ensembles is proposed. Long-range 1/r(3) atomic interactions are induced by microwave mixing of opposite-parity Rydberg states. The required long coherence times are achieved via four-photon excitation and readout of long wavelength spin waves. The dephasing mechanism is shown to have favorable, approximately exponential, scaling for entanglement generation.     

Relaxation of fast electrons in laser fusion targets

Collisional relaxation of a low density beam of non-thermal electrons injected into a high density Maxwellian plasma is investigated by computational treatment of Fokker-Planck type equation, with respect to the fast electron energy deposition and geometry configuration effects.     

A fast solver for the gyrokinetic field equation with adiabatic electrons

Describing turbulence and microinstabilities in fusion devices is often modelled with the gyrokinetic equation. During the time evolution of the distribution function a field equation for the electrostatic potential needs to be solved. In the case of adiabatic electrons it contains a flux-surface-average term resulting in an integro-differential equation. Its numerical solution is time and memory intensive for three-dimensional configurations. Here a new algorithm is presented which only requires the numerical inversion of a simpler differential operator and a subsequent addition of a correction term. This new procedure is as fast as solving the equation without the surface average.