6 Two-Electron Spectroscopy Versus Single-ElectronSpectroscopy for Studying Secondary Emission fromSurfaces

Sergey Samarin, Oleg Mihailovich Artamonov, Anthony David Sergeant, and James FrancisWilliams

Experimental results on the (e,2e) reaction on surfaces of a dielectric (LiF film), a metal(W(110) crystal) and a semiconductor (Si(001) crystal) are presented and discussed. A com-bined analysis of secondary emission spectra together with the (e,2e) spectra of LiF film allowsone to establish a link between a “true secondary emission feature” and an energy loss processin the film. A comparison of the (e,2e) spectra of tungsten and silicon shows that the sec-ondary emission mechanisms are different in these materials. The oxygen adsorption stronglymodifies the distribution of correlated electron pairs from W(110).

6.1 Introduction

The energy distribution of electrons scattered by a solid surface consists of an elastic (quasi-elastic) maximum and a secondary electron spectrum. The secondary electron spectrum is di-vided conventionally into two parts: true secondary emission features and energy losses. Theorigin of the first part is a cascade process of inelastic electron scattering and de-excitationof collective excitations via electron ejection. The energy loss part consists of features dueto single-electron excitations (interband transitions) or collective excitations (plasmons, exci-tons). Techniques based on the analysis of the secondary electron energy distribution (withangular and spin resolution) have become powerful tools for studying the electronic structureof surfaces and electron scattering dynamics.

Inelastic electron scattering from a crystalline surface is usually treated theoretically as asuperposition of dipole scattering and impact (or binary) scattering [1, 2]. In the dipolar limitthe momentum transfer vanishes and the excitation mechanism is equivalent to electronicexcitation by a photon, whereas the impact (binary) limit corresponds to a large momentumtransfer of the order of the incident electron momentum. Dipole scattering arises as a resultof the Coulomb interaction between an incoming electron and the electric field fluctuationsset up in the vacuum outside the target by oscillating surface- and near-surface atoms (charge)[1, 2]. The dipole process occurs most likely outside the target surface, where the electronsundergo small-angle inelastic scattering either preceded or followed by elastic scattering fromthe surface. However, incident electrons that penetrate the target may be inelastically scatteredin the near-surface region by short-range interactions and produce electron–hole pairs. Littleis known about the angular distribution of such impact-scattered electrons as they emerge from

Correlation Spectroscopy of Surfaces, Thin Films, and Nanostructures. Edited by Jamal Berakdar, Jürgen Kirschner

Copyright c© 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-40477-5

6.1 Introduction 69

the target. There have been only a few attempts to study the relative contributions of dipole andimpact scattering to inelastic electron scattering from surfaces. In one study [3] the electron-spin-labeling technique was used. It was found for Ag and Cu, that have filled 3d-shells, thatdipole scattering was dominant, whereas for targets like Mo, Fe, and Co with large densitiesof unoccupied states the rate for impact scattering was greater than, or comparable to, that fordipole scattering. It was also observed that impact-scattered electrons tend to be concentratednear the specular direction, but not so strongly as dipole-scattered electrons [3]. In the caseof highly oriented pyrolitic grafite (HOPG) the scattering mechanism of the electron-energy-loss process in specular reflection geometry was studied [4]. It was shown that an elasticcollision always accompanies the inelastic one and two independent channels contribute tothe inelastic cross-section, depending on whether the inelastic event precedes or follows theelastic one [4]. The presence of the elastic event associated with the inelastic one implies thatin specular reflection geometry the momentum transfer in the inelastic event is minimized andthen it can be treated in the dipole approximation [4]. It can be summarized that the fairlycomplicated phenomenon of electron interaction with a solid surface resulting in a secondaryelectron emission has been studied extensively but is still not completely understood. Onewould think that, if both the electrons resulting from an individual electron–electron collisionare detected, it would provide insight into the mechanism of secondary emission.

In the recently developed, two-electron-coincidence–spectroscopy-in-reflection mode (re-ferred to as (e,2e) spectroscopy) [5–8], two electrons generated by a single incident electronare detected in coincidence, and the momenta of both electrons are measured. For two elec-trons generated in a single electron–electron collision, both momentum and energy are con-served and their detection within time intervals of the order of nanoseconds identifies theirtime correlation and ensures that a minimal number of scattering events is observed [9]. Thecharacteristics of the detected electron pairs contain information on the electron–electron scat-tering potential and the correlated behavior of electrons inside the solid. This information isrelated to a near-surface layer of the solid because of the high surface sensitivity of the tech-nique [10]. The spin-polarized version of the (e,2e) spectroscopy has been shown to be a veryefficient approach to visualize the spin-dependent scattering dynamics in a ferromagnetic sur-face [11].

True secondary emission features and energy losses in the secondary emission spectrumare usually considered and discussed separately. In contrast, (e,2e) spectroscopy allows, insome cases, the energy loss structure and the true secondary emission features to be related.Indeed, it may happen that one of the detected electrons of the correlated electron pair is aninelastically scattered primary electron, which has lost part of its energy for plasmon exci-tation, for example, and the second electron of the pair is an ejected electron resulting fromthe plasmon decay. If this scattering mechanism is dominant then, in the two-dimensionalenergy distribution of correlated electron-pairs, a maximum will be observed at electron ener-gies E1 = Ee and E2 = Ep − hωp, where Ee is the energy of the ejected electron and hωp isthe plasmon energy.

In this chapter we present experimental results on the (e,2e) reaction on surfaces of a di-electric, a metal and a semiconductor. The paper is structured as follows. In Section 6.2the experimental details of low-energy time-of-flight (e,2e) spectroscopy are described. Sec-tion 6.3 contains experimental results on the (e,2e) reaction on surfaces of a LiF film, a singlecrystal of W(110) and a single crystal of Si(100) and their discussion, followed by conclusionsin Section 6.4.

70 6 Two-Electron Spectroscopy Versus Single-Electron Spectroscopy

6.2 Experimental Details of the Time-of-Flight (e,2e)Spectroscopy in Reflection Mode

6.2.1 Experimental Set-Up

The geometry and electronics of the two-electron coincidence experiment are shown inFigure 6.1. Experiments were carried out in UHV conditions with a base pressure in the10−10−10−11 Torr range. The residual magnetic field within the vacuum chamber was re-duced to less than 5 mG using Helmholtz coils. A sample was mounted on a movable holderand was cleaned in the vacuum prior to measurements. A Faraday cup (FC) was placed onthe axis of the electron gun behind the sample and was used for incident current measurementwhen the sample was moved off-axis.

Figure 6.1: Geometry and schematics of the time-of-flight coincidence spectrometer.

A time-of-flight (TOF) technique was used for measuring the energies of both correlatedelectrons. A reference point on the time scale was obtained by pulsing the incident electronbeam. An electron gun produced a pulsed electron beam with a pulse width of less than 1 nsand repetition rate of 4 × 106 Hz.

Position-sensitive detectors with resistive anodes were used for electron detection. Eachof the two detectors, 40 mm in diameter, consists of two micro-channel plates in a Chevronarrangement with a resistive anode (Quantar Technology, Model 3394).

6.2 Experimental Details of the Time-of-Flight (e,2e) Spectroscopy in Reflection Mode 71

6.2.2 Combination of Time-of-Flight Energy Measurements andCoincidence Technique

When an incident electron generates a correlated pair of electrons and they are detected,two pulses from constant fraction discriminators (CF) start two time-to-amplitude convert-ers (TAC). A stop pulse to both TACs comes from a logic unit that delivers a stop pulse onlywhen two delayed and shaped (200 ns width) pulses from the detectors, and a delayed shorttrigger pulse from the generator, coincide. The width of the two pulses from the two detectorswas chosen to be 200 ns to ensure that for a given flight distance of 100 mm all the electronsin the range from Ep (20−40 eV) down to (0.5−0.6) eV are detected. The combination ofcoincidence technique with the time-of-flight energy analysis provides the energy distributionmeasurements “in parallel”. This means that whatever the electron energy the electron is de-tected and its energy is determined. This is the advantage of this technique, which enables anoverview of two-dimensional energy distribution of correlated electron-pairs. In the presentexperiment the average incident current was in the 10−13 A to 10−14 A range, that implies,on average, less than one electron per incident pulse.

Besides the timing pulses, the electron arrival positions on the detectors were measured.The position sensitivity of the detectors allowed: measurement “in parallel” of the angulardistribution of the electrons, observation of electron diffraction patterns, estimation of theelectron beam size and measurement of the position-dependent flight time that takes into ac-count the difference in flight distances for electrons arriving, for example, at the center orat the edge of the detector. Three analogue pulses from one detector and three pulses fromthe other representing the electrons’ arrival times T1 and T2 and positions on the detectors(x1, y1, x2, y2), are processed by the ADCs and stored in a list-mode file in a computer. Themeasured distribution of the correlated electron pairs is then a six-dimensional array, whichcan be projected on any two-dimensional or three-dimensional distributions such as the num-ber of pairs as a function of E1 and E2, for example. An MPA-3 multi-parameter acquisitionsystem (FAST ComTec) was used for data collection.

In addition, the above-described two-electron coincidence spectrometer can be used formeasuring low energy electron energy loss spectra by switching off the coincidence conditionsin the electronic set-up. The characteristics of the spectrometer are described in Ref. [13].

6.2.3 Data Processing

As an example of data processing we present a description of how the six-dimensional arrayof a measured (e,2e) spectrum is projected onto a two-dimensional energy distribution of thecorrelated electron-pairs.

As mentioned above, each correlated electron-pair is represented by six numbers:T1, x1, y1, T2, x2, y2. Three numbers (Ti, xi, yi) define the energy of each electron. To con-vert these numbers to electron energy we determine first the exact flight distance from thesample to the impact point on the detector and calculate t0, the time when the primary elec-tron hits the sample (and scattered electrons leave the sample). For an electron detected at timeT and at position (x, y) on the detector, the flight time is t = T − t0 and its flight distance is:

L = (L20 + x2 + y2)1/2, (6.1)

where L0 is the distance from the sample to the center of the detector.

72 6 Two-Electron Spectroscopy Versus Single-Electron Spectroscopy

The elastically scattered electrons, with well-defined kinetic energy E0, are used to cal-culate t0. Using the time position T0 of the elastically scattered electrons, the distance L0

between the sample and the detector center, and the incident electron energy E0, we can cal-culate t0 using:

t0 = T0 − L0C−1E

−1/20 , where C = (2/m)1/2.

Then the energy E of an electron detected with coordinates (x, y) on the detector at timeT is calculated as follows:

E = L2(tC)−2 = (L20 + x2 + y2)

[(T − T0)C + L0E

−1/20

]−2

. (6.2)

The use of position sensitive detectors allows measurements of the angular distributionsof correlated electron-pairs and, consequently, scanning of component of electron momentumparallel to the surface. For the case of a single crystal sample, this component of electron mo-mentum is conserved. Therefore the measurement of the outgoing electron momenta enablesthe determination of the parallel component of the valence electron. Figure 6.2 shows howthe momenta of the incident electron k0 and the two outgoing electrons k1, k2 are related tothe parallel component of the valence electron q||. In the experimental geometry shown inFigure 6.1 the accessible range of q|| depends on the primary electron energy. For example,for 30 eV primary energy and normal incidence the accessible range of q|| is −1.5 Å−1 to1.5 Å−1. This range can be extended by increasing the primary electron energy or by usingoff-normal incidence.

6.3 Experimental Results and Discussion

The experimental results on low-energy (e,2e) scattering from an insulator (LiF film), a metal(W(110)) and a semiconductor (Si(001)) are presented and discussed in this section.

6.3.1 LiF Film on Si(100)

LiF is a typical insulator with a wide band gap of about 13 eV. It is characterized by a highsecondary electron emission yield. Due to the large band gap one can expect a small contribu-tion from a cascade process to the coincidence electron spectrum. Indeed, an electron insidethe solid can undergo a collision with a valence electron only if its energy is sufficient to excitethe valence electron over the band gap. Given that the electron affinity of LiF is very small,much smaller than the band gap, the electrons with energy below (10 to 11) eV can probablyescape from the solid if their momentum points to the vacuum. It is known that the secondaryemission spectrum of a LiF film exhibits “true secondary emission” features at about 7 eV and11 eV [13]. We analyzed one of these features (7 eV) using a combination of single-electronand two-electron spectroscopies.

A LiF film was deposited on a clean Si(001) surface by thermal evaporation from a molyb-denum crucible. The thickness of the film was estimated to be 100 Å to 150 Å. Low energysecondary emission spectra from the LiF film at various primary energies were recorded using

6.3 Experimental Results and Discussion 73

Figure 6.2: Relationship between parallel to the surface components of momenta of the scat-tered electrons K1 and K2 and the momentum of the bound electron q; a) accessible range ofq|| is determined by the combination of K1 and K2 that can be measured; b) combination ofK1 and K2 that corresponds to the excitation of the valence electron with q|| = 0, assumingconservation of the components parallel to the surface.

one of the arms of the time-of-flight (e,2e) spectrometer when coincidence conditions wereswitched off [13]. A set of (e,2e) spectra was measured for the same primary energies. Forcomparison a secondary emission spectrum for 26.3 eV primary electron energy is shown(Figure 6.3(a)) along with the projection of the two-dimensional energy distribution of corre-lated electron-pairs on the E1 axis. In the secondary emission spectrum there are two energyloss features at about 16 eV and 10 eV as well as a prominent true secondary emission featureat about 7.2 eV. The first two correspond to the thresholds of excitation of an excitonic leveland the interband transition over the band gap. The maximum at 7.2 eV does not depend on theprimary electron energy and therefore is called a “true secondary emission” maximum. Theprojection of the coincidence spectrum shows the onset at about 13.6 eV and the maximum(7 eV) that coincides with the maximum of the SE spectrum. In the two-dimensional energydistribution (Figure 6.3(b)) there are two maxima, one of which corresponds to the combina-tion of energies E1 = (2.6 ± 0.3) eV and E2 = (7.2 ± 0.3) eV. The second corresponds tothe combination: E1 = (7.3 ± 0.3) eV and E2 = (2.5 ± 0.3) eV. Systematic measurementsof the 2D energy distributions of electron-pairs from LiF film for various primary energiesshow that above 25 eV incident energy, one electron of the pair is preferentially emitted withE1 = (7.2 ± 0.3) eV energy and the second with energy E2 = (Ep − 23.3) ± 0.5 eV,where Ep is the incident electron energy. This allows the establishment of a link betweenthe true secondary emission feature and the energy loss feature. Indeed, one of the electronsin the maximum of the 2D energy distribution is the one which lost a fixed amount of energy(23.3 eV) for a collective excitation. The second one (7.2 eV) is the electron ejected as a resultof the de-excitation process. A detailed discussion of the secondary emission mechanism in aLiF film is the subject of a forthcoming paper [14].

74 6 Two-Electron Spectroscopy Versus Single-Electron Spectroscopy

Figure 6.3: a) Electron energy loss spectrum (EELS) (open circles) and projection on E1 axis(solid circles) of the (e,2e) spectrum of LiF film excited by 26.3 eV primary electrons; b) two-dimensional energy distribution of correlated electron-pairs excited from LiF film by 26.3 eVprimary electrons.

The combination of the conventional EELS with the (e,2e) spectroscopy allowed the mea-surement of the basic parameters of the LiF film: band gap, valence band width, excitoniclevel position and electron affinity. Such analysis is presented in Ref. [15].

6.3.2 Single Crystal of W(110)

A joint experimental and theoretical study of the (e,2e) reaction on a clean W(001) surface forlow energy primary electrons has been carried out recently [16]. In the theoretical treatment,

6.3 Experimental Results and Discussion 75

the elastic multiple scattering by the ion cores was taken into account for the primary electron,the valence electron and two detected electrons. The importance of specific elastic events (forexample, specular or nonspecular reflection of the incident electron, specular or nonspecularreflection of one or both outgoing electrons) was outlined by additional calculations in whichelastic scattering amplitudes were selectively switched off. Generally, elastic reflection wasfound to be very important in the (e,2e) reaction. Certain features in the (e,2e) spectrummainly require elastic reflections in the primary electron state, while for others reflection inone of the ejected electron states is required. For valence electron energies a few eV belowthe Fermi energy, good overall agreement between experiment and theory was found. Sincethe theory involves only a single direct collision between the projectile and a target electron,this agreement implies that such direct collisions are the dominant origin of the two electronsobserved experimentally [16].

Comparison between the calculated (e,2e) spectrum and the density of states in momen-tum space (k-DOS) shows that the main (e,2e) features occur in the regions of high k-DOS,whereas in regions of vanishing k-DOS the (e,2e) intensity also vanishes. There is, however,no detailed correspondence between (e,2e) spectrum and k-DOS. This implies that the inci-dent and ejected electron states play an important role because of their strong elastic multiplescattering by the ion cores. In contrast, at high incident electron energy [17], the primary elec-tron and two outgoing electrons are, to a good approximation, represented by plane waves.Therefore the scattering cross section is proportional to the momentum density of the valenceelectrons [17].

We present here experimental results on the (e,2e) spectroscopy of W(110). Although thecrystallographic face is different from that used in Ref. [16], we believe that the theoreticalmodel and general conclusions are valid here as well.

A clean W(110) surface was obtained by a two-step cleaning procedure [18]. First, thecrystal was heated at 1500 K in an oxygen atmosphere at 10−7 mbar pressure for several hoursto remove carbon impurities. Subsequent flashing to high temperatures (approximately 2500K) removed the excess oxygen remaining on the surface from the carbon-removal procedure.

The (e,2e) spectra were measured for normal and off-normal incidence at primary energy25 eV. For off-normal incidence the sample was rotated around the vertical axis by ±α (theangle is indicated in the figures). Energy sharing distributions in the total energy band of∆E = 2 eV below the Fermi level, recorded at normal and off-normal incidence (Figure 6.4),indicate that, for off-normal incidence, the energy sharing distribution becomes asymmetricwith respect to the zero point where E1 = E2. If we take the average energy of 19 eV alongthe total energy band ∆E, then the maximum A in the sharing distribution of Figure 6.4(b)corresponds to the combination of energies E1 = 13.2 eV, E2 = 5.8 eV and, consequently,to momenta K1 = 1.86 Å−1 and K2 = 1.24 Å−1. If we assume that these momenta arepointing towards the centers of the detectors, then the summed momentum would run closeto the direction of specular reflection. On the basis of this result we can draw the followingconclusions: (a) the maximum in the sharing distribution is consistent with the picture inwhich a correlated electron-pair is generated by the specular beam and the electron with thesmaller scattering angle has larger energy; (b) the contribution to the (e,2e) spectrum frombound states with a small component of momentum parallel to the surface is dominant; (c) thekinematics of scattering indicates that the “clean knock out” is the dominant mechanism ofthe electron-pair generation.

76 6 Two-Electron Spectroscopy Versus Single-Electron Spectroscopy

Figure 6.4: Energy sharing distributions of correlated electron-pairs excited from clean W(110)surface by 25 eV primary electrons; a) normal incidence; b) and c) sample tilted by ±10◦, asshown in right column.

Using the momentum conservation law for the parallel components of electron momenta:qx + k0x = k1x + k2x, where q is the momentum of the valence electron and k0, k1, k2

are the momenta of the incident and two outgoing electrons, one can plot the number of de-tected electron-pairs as a function of qx (Figure 6.5(a)) for two positions of the sample: ±12◦

between the sample normal and the bisector between the detectors. We recall here that thesecurves do not represent directly the momentum density distribution of the valence electrons, aspointed out in Refs. [6, 16]. On the other hand, the momentum density does contribute to the

6.3 Experimental Results and Discussion 77

Figure 6.5: “Projections” of (e,2e) spectra on the qx component of the valence electron (leftcolumn) and total energy distributions (right column) of correlated electron-pairs excited by25 eV primary electron from: a) clean W(110) surface, b) oxygen covered surface, c) oxidizedsurface. Sample position is indicated on figures (±12◦).

(e,2e) cross-section with the main (e,2e) features occurring in the regions of high k-DOS [16].Figure 6.5(a) shows that the contribution from electron states with small qx(±0.5 Å−1) isdominant in the case of a clean surface. We tried to change the electronic structure of thesurface layer by adsorbing oxygen atoms on the surface and looking at how the distribution ofelectron-pairs changes as a function of the bound electron momentum. In Figures 6.5(b) and6.5(c) the corresponding distributions are plotted for oxygen covered (b) and oxidized (c) sur-faces. Curve (b) corresponds to the saturated p(1×1) oxygen layer on the W(110) surface that

78 6 Two-Electron Spectroscopy Versus Single-Electron Spectroscopy

is confirmed by LEED patterns. The presence of oxygen on the surface decreases the contri-bution of the valence electrons from the near center of the surface Brillouin zone but increasesthe relative contribution from electron states with larger parallel momentum. It may indicatethat the momentum density distribution in the energy band 2 eV below the Fermi level changesupon oxygen adsorption. Indeed, the 2p-oxygen state is “responsible” for the oxygen–metalbonding and can contribute to the (e,2e) spectrum in this momentum range. (We recall thatmomentum density distribution of atomic oxygen is centered at 1.5 Å−1 [19]). In the energybinding spectrum the contribution from an oxygen–adsorbate state shows up at about −6 eV(see right column of Figure 6.5) that is consistent with other measurements [20]. For an ox-idized surface (3 min at 10−7 Torr pressure of oxygen and 1500 K sample temperature) therelative contribution from bound states with small momenta decreases further (Figure 6.5(c)).

Figure 6.6: Total energy distributions of correlated electron-pairs excited by 25 eV primaryelectrons from clean W(110) surface at normal (open circles) and 10 degrees off-normal (solidcircles) incidence.

The comparison of the total energy distributions of correlated pairs from clean W(110),for normal and off-normal incidence shows, in Figure 6.6 that for off-normal incidence the rel-ative contribution of the pairs with the highest total energy (that corresponds to the excitationof the valence electrons from the vicinity of the Fermi level) increases compared to the normalincidence and forms the maximum at about 19.4 eV. For normal incidence the distribution isbroader with a maximum at about (12−13) eV. Given that the time-correlated pairs with smalltotal energy (4 eV below the Fermi edge and lower) result mostly from the multi-step scatter-ing [9], one can conclude that the off-normal incidence increases the probability of single-stepelectron–electron collisions. The physical reason for this is that at off-normal incidence themomentum transfer, in the electron–electron collision that follows elastic scattering from theion core, is lower than at normal incidence and consequently leads to the asymmetric sharingdistribution.

6.3 Experimental Results and Discussion 79

6.3.3 Single Crystal of Si(001)

Silicon is a typical semiconductor with a band gap of 1.12 eV. A silicon single crystal, withcrystallographic plane (001) parallel to the wafer surface, was chosen as a sample. It waschemically cleaned before placing into the vacuum chamber, as described elsewhere [13].The final stage of the sample cleaning by heating was monitored by LEED and recorded(e,2e) spectra. The evolution of a total energy distribution of correlated electron-pairs duringthe sample cleaning is shown in Figure 6.7. The maximum at about 15 eV in spectra 1 to 3(−6.5 eV binding energy) is most likely due to the presence of oxygen (silicon dioxide) thatwas removed from the surface after heating to 1200◦C (curve 4 in Figure 6.7). The maximumin the total energy distribution of the clean sample in the 18 to 22 eV energy range probablycontains a contribution from the surface states on Si(100) because it is sensitive to surfacecontamination.

Figure 6.7: Evolution (from curve 1 to 4) of the total energy distribution of correlated electron-pairs excited by 26 eV primary electrons from Si(100) while cleaning the sample.

In contrast to the case of W(110) the energy sharing distribution of pairs excited by 26 eVprimary electrons from Si(100) does not show the asymmetry with respect to the zero pointwhen the sample is rotated by 5◦ or by 25◦ (Figure 6.8). This means that the scatteringmechanism in this case is different from the case of W(110). The reason for this might be a lowdensity of unoccupied electronic states in Si and, consequently, a low joint density of occupiedand unoccupied electronic states, which favors dipole scattering [3]. In addition, the screeninglength in tungsten λ = 0.48 Å (Thomas–Fermi) is much smaller than in semiconductors(λ = 1−1000 nm, Debye). This means that the correlation effects are quite different in metalsand semiconductors, is in turn can modify the distribution of correlated electron-pairs [21].

80 6 Two-Electron Spectroscopy Versus Single-Electron Spectroscopy

Figure 6.8: Energy sharing distributions of correlated electron-pairs excited by 26 eV primaryelectron from clean Si(100) surface at: a) normal, b) 5◦ tilted sample, c) 25◦ tilted sample.

6.4 Conclusions

We have presented examples of the application of (e,2e) spectroscopy for studying the electronscattering dynamics and the electronic structure of a dielectric, a metal and a semiconductor.We have shown that the true secondary electron features in the secondary emission spectrumof LiF film can be identified using (e,2e) spectroscopy. The link between secondary emissionand energy loss process was established.

References 81

The combination of (e,2e) spectroscopy and EELS allows the basic band parameters of aLiF film to be measured. A detailed description is presented in Ref. [15].

The kinematics of (e,2e) scattering on W(110) suggest that binary collisions prevailwhereas in the case of silicon the scattering mechanism is different.

Oxygen adsorption on a clean W(110) surface changes dramatically the “projection” ofthe (e,2e) distribution on the qx component of the valence electron. This change is thought tobe partially due to the change in the momentum density distribution in the surface layer uponoxygen adsorption.

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