Plasmon photoelectric effect from alkali halides

The existence of the plasmon photoelectric effect was postulated elsewhere by one of the authors. In the plasmon photoelectric effect, incident photons excite plasmons in the solid; the decay of plasmons excites electrons. The emergence of these electrons from the solid is responsible for a group of photoelectrons of definite energy. The existence of the plasmon photoelectric effect is confirmed by comparison of spectral photoelectric yield data with electron energy loss spectra in 16 alkali halides. The presence of the plasmon photoelectric effect explains additional peaks observed with some alkali halides in the energy spectra of photoelectrons. These peaks were not understood previously.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–16, October, 1974.     

Plasmonic secondary electron emission from alkali metals

One of the authors (Gornyi) has demonstrated earlier the possibility of plasmonic secondary electron emission (PSEE). Here electrons striking a solid substance excite in it plasmons, which then break up and in turn excite electrons. These electrons, upon being emitted into vacuum, form clusters of secondary electrons with definite energy levels. When energy-loss spectra of electrons are compared with energy spectra of secondary electrons and with work-function curves for secondary electrons, all of them plotted for lithium, sodium, and potassium, then the occurrence of the PSEE phenomenon in these metals can be established. The PSEE explains the peaks along the energy spectra and the work-function curves for secondary electrons. These peaks were not understood before.     

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.     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

Plasmaschwingungen in Al,Mg, Li,Na und K angeregt durch schnelle Elektronen

We report on energy loss experiments with 50 keV electrons transmitted through thin films of the metals Al, Mg, Li, Na, and K. The valence electrons of these materials behave like a gas of free electrons to a good approximation and, in particular, give rise to significant collective effects. Since these substances oxidize rapidly they were evaporated and studied in ultra high vacuum. The apparatus and the measurement technique are described. Measurements of loss spectra for different electron scattering angles yield the volume plasmon dispersion curve and the dependence of the damping of the volume plasmon on the wave vector. New experimental values of the energy and the half width of the plasma oscillations and the coefficients of dispersion and damping are given. In addition, the results of some measurements on surface plasmons are presented. There are significant deviations from the predictions of the simple electron gas model.     

Angle- and energy-dependent core-level photoelectron energy loss studies in Al and In

Electron energy loss structures of Al and In core-level photoemission spectra, in particular surface and bulk plasmon losses, have been investigated as functions of photon energy (i.e., photoelectron kinetic energy). These studies utilized synchrotron radiation to provide a variable photon source in the ultra-soft X-ray region, thus allowing these loss processes to be studied at photoelectron kinetic energies for which the mean free path of the electrons is minimal. The Al plasmon loss structure was also studied with soft X-ray radiation in an angle-resolved mode, allowing the variation of effective photoelectron sampling depth with different electron take-off (collection) angles. These results for the relative intensity of the bulk and surface plasmons as a function of electron kinetic energy and electron exit angle are in qualitative agreement with the predictions of ?unji? and ?ok?evi?. The core-level binding energies of surface atoms have also been studied with the result that no significant shift has been observed with respect to bulk-atom core levels.     

Assignment of quantum number for plasmon energies in carbon layer systems

Electron-emission distribution curves of carbon layer surfaces excited by primary electrons of energies in the 118-534 eV range have been measured. The first four peaks in the plasmon spectrum are observed. It is concluded that the oscillator energies are presented to explain the assignment of the quantum number (n = 0,1,2,3) for internal plasmons in carbon layer systems. The preliminary assignment is in good agreement with the experimental results. It is also shown that the existence of limit between internal and surface plasmons. It is pointed out that the plasmon energy does not depend on both the external electrostatic voltage and the sample temperature. Moreover, the quantum number was adopted to the names of internal plasmons in the observed spectra.     

Electron Energy Loss Spectroscopy study of the near surface region of the ternary Co-Cr-Mo alloy

Nature of the characteristic electron energy losses in the second electron emission spectra from the ternary Co-Cr-Mo alloy surface are studied in the low energy range of the primary electron energy E₀. The main types of losses were found: surface and bulk plasmons and their hybrid modes, interband transitions and ionization losses. For Co, Cr, Mo and Co-Cr-Mo alloy the experimental values of the plasmon energy were established to be less than it was predicted by free-electron gas model. Excess of conductive electrons in the surface layers for Co, Cr and Mo was observed by dependence of the surface plasmon dispersion from E₀, while for Co-Cr-Mo alloy the situation is quit opposite. Such behavior is explained by the complex phase structure of the ternary alloy. The analysis of intensity lines of plasmons from E₀ showed deeply changed alloy profile. Ionization Spectroscopy was used for studying the alloy elements distributing on the depth. Mo atoms preferred segregation in the outermost layers of Co-Cr-Mo alloy and enrichment with Cr competitive atoms in underlayers is displayed.     

Inelastic mean free path, surface excitation parameter, and differential surface excitation parameter in Au for 300-3000 eV electrons

An analytical approach for simultaneously determining an inelastic mean free path (IMFP), a surface excitation parameter (SEP) and a differential SEP (DSEP) with absolute units was applied for the analysis of absolutely measured reflection electron energy loss spectra for Au. The IMFP, SEP and DSEP in Au for 300-3000 eV electrons are successfully obtained. The obtained DSEPs show a reasonable agreement with those theoretically calculated. The present SEPs were compared with those calculated by several empirical equations, revealing that the present SEPs are close to those calculated using the Oswald's equation. The IMFPs for Au determined by the present analysis were compared with those calculated by the TPP-2M predictive equation, revealing that the present IMFPs are in fairly good agreement with those calculated by the TPP-2M equation. The results confirmed that the present approach is effective for experimentally determining the SEP, DSEP, and IMFP for electrons in solids.     

Quantification of surface effects: Monte Carlo simulation of REELS spectra to obtain surface excitation parameter

The surface excitation effect is investigated by using the quantum mechanical frame work of complex self-energy of electrons which interact with a bounded semi-infinite medium. In the self-energy formalism, differential inverse inelastic mean free path (DIIMFP) has contributions from bulk and surface plasmons. Monte Carlo simulation of the interaction of electrons with a solid medium and surface has been performed. The surface excitation parameter (SEP) is then obtained from the simulated reflection electron energy loss spectroscopy (REELS) spectra. The calculated SEP results by Monte Carlo simulation are compared with the previous calculations of total surface excitation probability, which was estimated by a numerical integration of surface term of DIIMFP. The contribution merely due to surface excitations towards REELS spectra is extracted by subtracting the two Monte Carlo simulated REELS spectra that based on the two models of electron inelastic scattering, i.e. a full surface model (SM) and a pure bulk model (BM). The surface excitations found to be significant at low energy losses and diminish at higher energy losses whereas the bulk plasmon contributions show opposite behavior and are negligible at lower energy losses. The average number of surface excitations is then evaluated by the computation of ratio of the integrated surface contribution to the elastic peak. The calculated results for Ag are found to be reasonably in agreement with our previous results for total probability of surface excitations and other reported experimental data for SEP. Surface correction factor (SCF) is calculated using SEP for several metals and is compared with the reported ratio of SCF with Ni sample as reference.     

Electron spectroscopy of iron disilicide

We have reported on the results of a complex investigation of iron disilicide FeSi₂ using characteristic electron energy loss spectroscopy, inelastic electron scattering cross section spectroscopy, and X-ray photoelectron spectroscopy. It has been shown that the main peak in the spectra of inelastic electron scattering for FeSi₂ is a superposition of two unresolved peaks, viz., surface and bulk plasmons. An analysis of the fine structure of the spectra of inelastic electron scattering cross section by their decomposition into Lorentzlike Tougaard peaks has made it possible to quantitatively estimate the contributions of individual energy loss processes to the resulting spectrum and determine their origin and energy.     

The determination of electron mean free paths in solids by depth and angular dependences of inelastically scattered electron spectra

A technique is presented of electron mean free path determination in solids using the spectra of backscattered electrons. The mean free path without energy loss (λ₀) in Ge, and the mean free paths for excitation of volume (λ_v) and surface (λ_s) plasmons in Al, are determined in the electron energy range E₀ = 0.1–10 kev. The depth d_s of surface plasmon localization is estimated for Al.     

Measurement of the isotope composition of hydrogen in carbon materials using elastically scattered electron peak spectroscopy

The quantitative composition of hydrogen and helium isotopes in the surface layers of structural materials is reconstructed using the developed technique for processing the energy spectra of electrons scattered from plane-parallel layers of solids into a preset solid angle element. These are the spectra measured with a high energy resolution ΔE ∷ 0.2−0.4 eV. The change in the shape of peaks for elastically scattered electrons is analyzed depending on the probe electron beam energy and experimental geometry. The theory of electrons scattered from plane-parallel layers of solids is constructed using the solution of the boundary value problem for the transfer equation by invariant immersion methods. The analytic solution to the system of equations is found in the small-angle approximation for the reflection and transmission functions. The results of calculations are compared with experimental data on electron scattering from polyethylene. The shape of the energy spectra of electron scattering from deuterium and tritium is predicted. The sensitivity threshold of the method relative to percentage concentration of hydrogen isotopes in hydrocarbons is analyzed.     

Electron energy-loss spectroscopy of tetracyanoquinodimethane,TCNQ, tetrathiafulvalene,TTF, and the salt TTF-TCNQ

Energy loss spectra of low energy electrons (ELS) have been obtained for thin solid films of tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF), and the charge transfer salt TTF-TCNQ as a function of incident energy (20 to 100 eV). The incident energy dependence of energy loss features allows the identification of triplet and symmetry forbidden excitations in TCNQ and TTF. The ELS spectrum of TTF-TCNQ is compared with high energy electron loss data to show that the surface of evaporated films of TTF-TCNQ contains 20–50% neutral TCNQ molecules.     

Spectroscopy of charged particles elastically scattered by plane-parallel solid layers

The energy spectra of electrons elastically scattered by plane-parallel solid layers are presented. The solid surface is analyzed by a method based on the identification of similar spectra and is called electron Rutherford scattering in analogy with the well-known ion spectroscopy method. The effect of multiple scattering processes on peak intensities in the energy spectra of elastically scattered particles is analyzed. The applicability range of the strong single scattering approximation for the interpretation of the energy spectra of elastically scattered electrons is established.     

Plasma losses of electron energy and secondary electrons of discrete energies in aluminum

Discrete energy' loss is examined for electrons of energy 30–116 V. The losses due to excitation of three types of plasma wave are found; these plasmons have energies of 15.5, 9.5, and 7 eV. The energy distribution curves for the secondary electrons contain groups with discrete energies, which in aluminum coincide with the plasma energies, and so arise by decomposition of plasmons.     

Study of the order–disorder transition and martensitic transformation in a Cu–Al–Be alloy by EELS

Changes in 3d states occupancy associated with order–disorder transition and martensitic transformation in a Cu–Al–Be alloy was investigated by electron energy loss spectroscopy (EELS) in both high energy and low energy loss regions. From the high energy loss region, the Cu L_2,3 white-line intensities, which reflect the unoccupied density of states in 3d bands, was measured for three states of the alloy: disordered austenite, ordered austenite and martensite. It was found that the white-line intensity remains the same during order–disorder transition but appears slightly smaller in martensite, indicating that some electrons left Cu 3d bands or some hybridization took place during phase transformation. From the low energy loss region, the optical joint density of states (OJDS) was obtained by Kramers–Kronig analysis. As maxima observed in the OJDS spectra are assigned to interband transitions, these spectra can be used to probe changes in the electronic band structure. The analysis shows that during the martensitic transformation, the peaks positions and relative intensities in the OJDS spectra undergoes noticeable changes, which are associated with interband transitions.     

Magnetoresistance effect obtained from the band structure of a crystalline solid

This paper demonstrates that the Lorentz equation combined with the equation for electron drift in an external magnetic field gives a definite value for the relaxation time of the electron motion in a crystalline solid in the aforementioned field. The main result is that the product of the relaxation time and the strength of the magnetic field remain constant and are dependent only on the structure of the energy band of the solid. Free electrons, or nearly free electrons, result in the diagonal components of the tensor for magnetoresistance tending to zero for all electron states. For the electrons in a crystal lattice, the relaxation time considered along the cross-section line in the reciprocal space of a plane normal to the magnetic field and the surface of constant energy become highly anisotropic quantities.