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We present results of secondary electron emission (SEE) induced by UV photons and low energy He ions interacting with a pristine and ion damaged surface of highly-oriented pyrolytic graphite (HOPG). A critical examination of the changes in the SEE spectra produced by the radiation damage allows us to identify the origin of the different features of the SEE spectra. In this way,we conclude that the lowenergy peak at ~3 eV is strongly determined by the structure of the (empty) HOPG density of states above vacuum level. In contrast the highenergy structure at ~14 eV could not be explained by such an argument but rather by exciton autoionization. Fil: Ferron, Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Física del Litoral; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Vidal, Ricardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Física del Litoral; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Bajales Luna, Noelia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Córdoba. Instituto de Física "enrique Gaviola"; Argentina Fil: Cristina, Lucila Josefina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina Fil: Baragiola, R. A.. University Of Virginia; Estados Unidos

The intensity asymmetry of the secondary emission spectra of thin iron films on W(110) excited by spin-polarized electrons was measured at various geometrical arrangements and various primary energies. The magnitude and shape of the Stoner excitation asymmetry provide information on the magnetic state of the film. The geometrical structure of the film influences its magnetism and this is reflected in the asymmetry of the spin polarized electron energy loss spectra (SPEELS). Oxygen adsorption reduces the asymmetry of Stoner excitations, but not as dramatically as in studies of Ni surface. The kinematics of the scattering process influences the asymmetry of the spectra and should be taken into account while using SPEELS for studying surface magnetism. Theoretical analysis of asymmetry spectra measured at various scattering conditions is required.

The primary ion directional effects observed in secondary electron yields induced by ion bombardment [5 keV Ar+ → Cu(100)] are simulated using a semi-empirical molecular dynamics model. The directional effects are presumed to arise from inelastic energy transfers that take place in close binary atomic encounters. The latter are estimated using the Oen-Robinson model, in combination with a critical apsidal distance (Rc). The connection between the measured kinetic electron emission (KEE) yields (γKEE) and the predicted inelastic energy loss in a binary atomic collision (ΔEi) is established through a semi-empirical fitting procedure involving Rc and other parameters in the following model: γe = γ0 + γKEE = γ0 + 〈ΔEi(z)exp(− z/λ)〉, where z is the collision depth. The directional effects are best reproduced by fitting the model to Ar–Cu inelastic collisions for two azimuthal incident directions: Rc is estimated to be 0.47 ± 0.03 A; the parameter, λ (an effective electron attenuation length), is estimated to be 18 ± 2 A. The same model also describes the γKEE energy dependence for 5–10 keV Ar+ normally incident on low-index Cu crystal targets [Phys. Rev. 129 (1963) 2409]. The spatial and temporal distributions of the hard collisions that initiate KEE are discussed on the basis of the model.

The yield of secondary electrons emitted from an epitaxial three monolayer (3 ML) NiO(1 0 0)/Ag(1 0 0) film excited by soft X-ray linearly polarized synchrotron radiation at the Ni L 2,3 absorption threshold has been measured for different values of the thickness of a MgO(1 0 0) capping layer. Compared with the as grown 3 ML NiO(1 0 0)/Ag(1 0 0) film, we observe a significant enhancement by about a factor 1.2 of the secondary electron emission for the capped 8 ML MgO(1 0 0)/3 ML NiO(1 0 0)/Ag(1 0 0) sample. A further substantial yield enhancement by a factor 1.6 with respect to the uncapped NiO sample is observed after deposition of an additional 8 ML MgO(1 0 0) film, for a total capping layer thickness of 16 ML. The observed secondary electron yield enhancement is discussed in terms of modified electronic structure, surface work function changes, and characteristic electron propagation lengths.

Emission of MCs n + (n = 1, 2, … etc.) molecular ions has been studied under the joint influence of electropositive (cesium) and electronegative (oxygen) elements in the secondary ion mass spectrometry (SIMS) process. The kinetic energy distributions, measured for Cs+, Cs 2 + , AgCs+ and AgCs 2 + ions at different oxygen pressures exhibited changing slopes in their leading parts that hinted at appreciable change in the surface work function. The maximum observed change in the surface work function Δϕmax was ∼0.44 eV. The measured integrated counts of all ionic species showed a strong variation with the changing oxygen environment. The observations are explained in the light of surface work function changes at the sputtering site. Formation mechanisms of Cs 2 + , AgCs+ and AgCs 2 + ions are explained in the framework of sputter-ion emission models.

Scanning Kelvin-probe microscopy is applied in ultra-high vacuum to characterise local changes in the work function of the hydrogen-terminated diamond surface following modification by thermal annealing and ion-implantation. The effect on the work function of hydrogen desorption was monitored during annealing. Ion irradiation was found to give rise to a different response, inconsistent with hydrogen removal. Increased ion fluence causes a rapid enhancement in the work function, each impinging ion causing band structure modification which extends over length scales of at least 20 nm beyond the impact site, considerably larger than the damage cylinder in the vicinity of each ion impact.

We analyze the excitation of secondary electrons by hard-X-rays in subsurface layers. By studying core-excited photoelectron lines and their plasmon satellites in photoemission spectra, we show how electrons excited by hard-X-rays can carry information from deep regions deep within the sample to the surface. It is believed that the decay of high-energy photoelectrons via plasmon-loss is strongly related to the production of secondary electrons. For high-energy electrons, however, the momentum transfer to plasmons is small compared to the electron’s initial momentum, so the lateral position on the surface from which the secondary electrons are emitted is close to that of the atom initially excited by the hard-X-rays. This explains why the spatial resolution of hard-X-ray photoelectron emission microscope (HX-PEEM) images is good even if the buried interface is covered by a film with a thickness many times the inelastic mean free path of the primary electrons. This argument explains well recent HX-PEEM results.

Secondary electron imaging plays an important role in surface science, not only in imaging of surface topography and in the ability to resolve, for example, surface defects, but also in revealing the local variations in surface barrier height or work function, e.g. p–n junctions in semiconductor devices/materials. In our study, high resolution FESEM is used to qualitatively investigate the secondary electron contrast associated with band-structure variations in the semiconductor heterostructure formed by two adjacent hexagonal (4H) and (6H) silicon carbide polytype layers. In such heteropolytipic structure, effects due to different chemical constituents, defects, incoherent interfaces, and lattice mismatching can be avoided. The study of the 4H/6H heterostructure has revealed that the 6H polytype region, with a band-gap energy of 3.05 eV, exhibits higher secondary emission than the 4H region, with a larger band-gap of 3.26 eV. Reasons for the observed contrast will be discussed, with regard to the effects of the electronic structure and the theory of secondary electron emission with its stages of generation, transport to the surface and escape. Our measurements reveal that the operating “window” for maximizing SE contrast is very sensitive to the operating conditions, particularly incident electron energy. The observed secondary electron contrast can be useful in revealing electronic structure variation between different components of silicon carbide semiconductor devices.

Intensity of low energy electrons ejected by a spin polarized metastable helium (He ∗ ) beam was measured for clean and water-adsorbed Fe surfaces. A definite He ∗ spin dependence was found in the intensity of electrons with energies just above the vacuum level of the sample when an Fe surface was covered with a water multilayer. The He ∗ spin dependence was found to be associated with the He ∗ deexcitation process. The spin dependence is likely to be due to the spin-dependent secondary processes.

The effect of surface reconstruction on contrast in scanning electron microscopy of the Si(1 0 0)-2 × 1 surface is investigated. A theory of the initial secondary production rate is developed and an upper bound on the rate is shown to depend on the product of the integrated intensities of the initial and final RHEED states. These states are calculated with a reflection matrix method and their depth dependence is investigated. The results are used to analyse scanning electron microscopy contrast in images of 1 × 2- and 2 × 1-regions of the Si(1 0 0)-2 × 1 surface reported by Watanabe et al. The calculated integrated intensities are consistent with the experimental images and with the experimentally observed dependence of the contrast on the azimuth of the incident electron beam. This supports the idea that the observed contrast is caused by the effect of surface reconstruction on the RHEED states.

We investigated the structural and magnetic properties of Fe(1 1 0) for molecular oxygen adsorption at room temperature. For the c(2 × 2) and c(3 × 1) superstructures, spin-polarized secondary electron emission (SPSEE) induced by protons and electrons reveals a nearly unchanged polarization compared to clean Fe(1 1 0). An appreciable decrease in polarization is found for the disordered layer of oxygen at a coverage Θ ≈ 1. This decrease is more pronounced for the spin polarization determined by electron capture (EC) to excited atomic He levels after grazing scattering. From a comparison of data obtained by proton-induced SPSEE and spin-polarized EC we conclude that the polarization at the vacuum boundary vanishes for an oxygen coverage Θ ≈ 1 while the polarization in the underlying Fe substrate layers is hardly changed.

The structure and magnetism of thin iron oxide layers formed on a Fe(1 1 0) single crystal surface are investigated by Auger electron spectroscopy, ion beam triangulation, and spin polarized secondary electron emission. The formation of a FeO(1 1 1)-like film on top of the metal substrate is observed for oxidation at elevated temperatures. Additional oxygen exposure at room temperature is suggested to lead to a gradual conversion to Fe3O4.

A preliminary investigation of photon-stimulated ion desorption (PSID) from condensed deuterated formamide (HCOND 2 ) by using polarization-angle-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with velocity-selected time-of-flight (TOF) mass spectrometry, is presented. It was found that the PSID yield of total D + ion is enhanced by the N 1s → σ*(N D) transition at both grazing and normal photon incidence angles and its enhanced yield shows no polarization dependence despite of direct PSID, which indicates the N D bonds are not aligned with the surface-oriented CNO molecular plane of HCOND 2 . It was also found that the fastest velocity components of H + and D + ions monotonically decrease as the energy at the normal incidence angle. This finding suggests that X-ray-induced electron stimulated desorption (XESD) via secondary electrons resulting from valence photoelectrons; indirect PSID is a dominant process in the fastest components.

Introducing the use of the Factor Analysis method to analyze electron emission spectra we identify different mechanisms for electron emission from clean Al surfaces under bombardment by Ar+ and Li+ ions. The analysis of the energy electron emission distributions and their dependence on incoming ion energy allows separation of the direct contribution of Auger neutralization from that of plasmon de-excitation and cascade electrons due to kinetic emission. Similarly, the analysis shows that, under our experimental conditions, plasmons are excited by fast Al-2p Auger electrons produced in symmetric Al-Al encounters in the atomic collision cascades initiated by the projectiles. © 2005 Elsevier B.V. All rights reserved. Fil: Bajales Luna, Noelia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Montoro, Silvia Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Goldberg, Edith Catalina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Baragiola, Raul Antonio. University of Virginia; Estados Unidos Fil: Ferron, Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina

Energy distribution curves (EDC) of secondary electrons from W(1 1 0) were measured for spin-up ( I + ) and spin-down ( I − ) polarization of the incident beam. The intensity asymmetry, defined as A = ( I + − I − )/( I + + I − ), for a certain experimental geometry was measured as large as 7%. This asymmetry was explained in terms of a “spin-filter” effect. The model assumes that the secondary electrons excited by spin-polarized primary electrons are spin-polarized inside the solid. On the way to the vacuum they undergo spin–orbit interaction on the surface barrier that provides the spin-dependent and energy selective transmission through the surface.

The concept, design and performance of the novel dual emission electron spectromicroscope (DEEM) are presented. The DEEM consists of four fully electrostatic lens systems comprising two independent and parallel projective columns, objective lens system and deflector lens system. Switching on the deflector lens potentials causes a parallel shift of the electron beam from the axis of the of the first projective lens column to the axis of the second projective column. This original concept allows for the quasi-simultaneous observation of the complementary diffraction (angular distribution) and imaging planes at the two independent imaging units. The dispersive properties of the deflector lens system can also be used to select the energy range of the electrons emitted from the sample for analysis. Consequently, energetically and laterally resolved measurements are possible and a plurality of image contrast mechanisms are available.

Secondary ions, produced by neutral 1.0 MeV argon beam impact on Al2O3 and LiF thin films, are analyzed by a XY-TOF detection system. Angular distributions and initial velocities are simultaneously determined for ionized species originating from the bulk or from the adsorbate layer. The employed XY-TOF spectrometer was designed to measure accurately radial and axial velocity measurements of positive and negative secondary ions with mass

Secondary ion intensities, substrate composition, work function and valence band spectra during the initial stages of sputter depth profiling Si and SiO 2 with 1 keV Cs + , along with ion induced Auger emissions resulting from 1 keV Xe + have been studied. The results reveal that the increasing O − , Si − , and Si 2 − intensities and decreasing Si + and Cs + intensities noted with increasing Cs + dose from Si substrates, stems primarily from a Cs induced reduction of the work function. Similar intensity variations were noted from SiO 2 substrates. These appear to be due to sub-band states introduced during sputtering. Such trends, along with the matrix dependent transient effects observed in the secondary ion emissions from both Si and SiO 2 , are consistent with resonance charge transfer as being the primary process responsible for secondary ion formation/survival. Minor deviations from the expected trends suggest the presence of Auger de-excitation (Si + and Cs + ) and variable poly-atomic formation (Si 2 − ) probabilities.

Secondary-electron emission (SEE) from LiF films deposited on Si(0 0 1) surface was studied using time-of-flight two-electron coincidence spectroscopy. A set of energy-distribution curves (EDCs) of secondary electrons excited from a LiF film by electrons with energies in the range of 20–50 eV exhibits emission features at about 7 and 11 eV. The energy positions of these maxima do not depend on the incident energy. To reveal the origin of these features, each of the EDCs was spanned in the second dimension E2 using two-electron coincidence spectroscopy. Two-dimensional mapping of the energy sharing between correlated electrons shows that above 25 eV incident energy, one electron of the pair is preferentially emitted with E1 ¼ 7:2 � 0:3 eV energy and the second one with energy E2 ¼ð Ep � 23:3 Þ� 0:5 eV, where Ep is the incident electron energy. At about 30 eV incident energy, a second favoured emission energy of 10.9 ± 0.3 eV is observed. The unique capability of (e,2e) spectroscopy established the links between electron energy loss process and emission features in the EDCs. It is suggested that the mechanism of SEE from LiF film includes the excitation of two collective excitations with subsequent decay via electron ejection. It was shown that the mechanism of secondary emission from LiF film depends on the film thickness and its structure. � 2003 Elsevier B.V. All rights reserved.

The absolute secondary electron and anion emission probabilities for low energy ( − , N 2 + , Ar + , Ne + and He + impacting an oxygen-adsorbed Mg(0 0 0 1) single crystal surface have been measured as a function of oxygen exposure and of incident ion impact energy. Secondary anion emission was observed throughout the range of adsorbate coverage studied, from that of a “clean” metal (defined by a demonstrated adsorbate-induced work function shift) to that rendering an insulative metal oxide, and was determined to comprise mostly O − . Both electron and anion emission were observed to be largely unaltered during the metal–metal oxide transition. Secondary anion and electron kinetic energy spectra were also measured; no significant dependence on oxygen exposure was observed. The relevance of secondary emission mechanisms that are driven by the ionization potential of the incident ion ranges from limited to none in our experiment in the case of incident O − ; we suggest that decay of a collisionally-excited MgO surface state may account, in part, for secondary emission related to the presence of adsorbed oxygen.

X-ray absorption spectra can be collected in multiple ways, each exhibiting a different probing depth. The total electron yield signal contains contributions from primary, Auger and secondary electrons. We present data on the total electron yield probing depth at core level energies ranging from 77 to 929 eV. By coating materials with chromium overlayers, we find that the maximum probing depth increases with core level energy from 15 to 141 A. We demonstrate that the Auger electron contribution to total electron yield intensity is negligible, therefore X-ray absorption spectra acquired in X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM) are equivalent to spectra acquired by total electron yield. We find that the signal intensity decreases exponentially with coating thickness, and that total electron yield probing depth and Auger electron range (calculated in the continuously slowing down approximation) are similar at low energies, but diverge for kinetic energies above 400 eV.

Desorption of nitrogen adsorbates from amorphous carbon (a-C) surfaces due to swift ion ( Z =6–73, q =6–54, energy 6–13 MeV/u) and electron impact (5 keV) was studied by secondary electron emission (SEE) measurements. The fluence dependence of ion induced electron emission obeys an exponential decay law as the coverage rate is in the sub-mono-layer range. The desorption yield σ varies approximately as σ ∼(d E /d x ) 3/2 or σ ∼ q 3 as a function of the electronic energy loss d E /d x and the projectile charge q , respectively. We discuss the possible desorption mechanisms connected to electronic excitation. Furthermore, we present an ESCA investigation on the mechanism for the influence of nitrogen adsorbates on SEE yields. The mechanism for the SEE enhancement effect is connected to the surface termination: nitrogen adsorbates are believed to provide surface states which generate a downwards energy band bending, which on relative scale pushes up the Fermi level towards the conduction band minimum, reducing the work function.

The understanding of the basic mechanisms responsible for electron induced degradation (EID) of thin insulating films is an important issue in the successful development of industrial applications based on the ability of dielectric surfaces to sustain electron irradiation. In this work, we have used a DC current technique to measure the secondary electron emission yield ( δ ) of E-beam evaporated MgO layers deposited on top of a metallic titanium electrode. This conductive film is required to polarize the sample and to supply electrons to the insulator where positive holes have been generated by the departure of secondary electrons. From simple considerations based on the current limitation imposed by the layer resistivity, we have established that two mechanisms may balance out the excess charge across the film: the so-called radiation induced conductivity and a tunneling injection from the metal-dielectric back barrier into the conduction band of MgO. As surface charging is easily detected in our experimental procedures, we show that the load conditions ( E p =100–300 eV; I p =5–18 mA; j p =50–300 mA/cm 2 ) are accurately quantified. Then, using a cross analysis method which accounts for the δ -determination, the surface analysis performed by XPS and the I – U characteristic of the sample placed in a diode configuration in front of a OsRuI-cathode, we show that the mass loss rates of the film due to EID can be quantified. The δ -degradation due to EID is found to be negligible whereas mass loss rates can play an important role according to the load conditions.

The electron emission yield is measured for impact of 3 keV electrons on polycrystalline aluminum, which is exposed to oxygen. At different oxygen gas pressures the yield is measured as a function of time. The oxygen pressure is in the range between 2×10 −9 and 3×10 −6 mbar. It is found that the yield depends not only on oxygen exposure but also on the oxygen pressure. In the begin of oxygen exposure the yield decreases due to chemisorption of oxygen, but then the yield increases due to oxide formation. A model based on rate equations with adjustable parameters is proposed, which describes the oxidation process and electron emission yield. The parameters are fitted to give best agreement with the measured yield curves. The parameters include sticking coefficients and time constants of oxidation. Whenever possible, evaluated parameters were compared with published data and reasonable agreement is found.

The growth modes of Cu overlayers deposited on oxygen-predosed Ni(1 0 0) surfaces in the Cu coverage range 0–8 monolayers (ML) have been studied using secondary ion mass spectrometry (SIMS) and ion-induced secondary electron emission measurements. On a c(2×2)-O/Ni(1 0 0) surface prepared by 40 L O 2 exposure of Ni(1 0 0), Cu grows according to the Frank-van der Merwe (FVDM, or layer-by-layer) mode up to at least 2 ML Cu coverage. In contrast, Cu grows by the Volmer–Weber mode on a NiO(1 0 0) surface prepared by 400 L O 2 exposure of Ni(1 0 0) up to at least 8 ML Cu coverage; three-dimensional growth of Cu clusters commences at about 0.2 ML Cu coverage. The characterisation of Cu overlayer growth modes from the measurement of SIMS intensity variations hinges largely on the behaviour of the Ni 2 + and Cu 2 + cluster ions which mainly originate from the surface layer of the sample. For example, the deposition of 1 ML Cu on c(2×2)-O/Ni(1 0 0) completely quenches Ni 2 + emission, whereas on NiO(1 0 0) this process is not complete even after deposition of 8 ML Cu. This behaviour can be formally described with a simple analytical model. On the basis of comparisons between the secondary ion and secondary electron emission properties of O/Cu/Ni(1 0 0) and Cu/O/Ni(1 0 0) surfaces, it is concluded that the Cu layers deposited on the c(2×2)-O/Ni(1 0 0) substrate are covered by an adsorbed layer of oxygen.

Ag surface plasmon excitation induced by low keV scattering of hydrogen and He ions is reported. Observation of plasmon excitation in this system was possible in the radiative decay–photon emission channel. The intensity of plasmon radiation was observed to increase quadratically with ion energy. Plasmon excitation was observed for both H + and H − incidence. Here it can be due to neutralisation of H + and H − ionised to H + , as well as secondary electron production. In case of He 0 no plasmon excitation was observed. For He + we ascribed plasmon excitation to mainly secondary electron production. For He ++ plasmon excitation was much larger than for He + . It is assigned to neutralisation into He +* as well as secondary electron emission processes.

The fine structure of the experimental true secondary electron emission spectra (SEES) along the normal to a clean W(1 0 0) surface is interpreted theoretically. It is shown that the fine structure of the SEES is mainly due to the electronic structure of unoccupied high-level electronic states (above the vacuum level Evac) to which electrons are scattered or from where they are emitted. The predominant role of the bulk energy-band structure effects in the spectra formation is shown. In addition, a comparison to existing theoretical and experimental data is given.

We present a study of the autoionising state production in N, O, Ne and Ar ion collisions with a clean and potassium covered Al(1 1 1) surface in an energy range from 5 to 50 keV. In case of clean Al, we only observe production of 2p4nln′l′, 2s2p5nl, 2s2p6nl autoionising states for Ne+ projectiles; for N+, O+ and Ar+ scattering no sign of production of doubly excited states was found, although ion backscattering is known to occur. In case of K covered surfaces we in all cases observe production of sputtered excited K atoms in 3p54s2 autoionising states. For Ar+ the quasi-symmetric collision with K produces excitation of Ar in 3p44s2 states. For N+ scattering the electron spectra reveal a small structure which we attribute to the formation of metastable N− ( 1 D). K adsorption does not lead to the appearance of any new Ne ∗∗ related structures.

Secondary anion and electron yields resulting from low-energy (50–500 eV) Na + bombardment of an oxygen-adsorbed Si(1 0 0) substrate have been measured as a function of oxygen exposure and of Na + impact energy. Adsorbate coverage ranges from none to over half a monolayer. The dominant sputtered anion was found to be O − with SiO 2 − being a minor constituent. Kinetic energy distributions of the secondary anions and electrons were also measured. The presence of an adsorbate was observed to enhance secondary anion emission to a significant degree whereas secondary electron emission was minor, in sharp contrast to what has been observed for metallic substrates. The mechanism for secondary emission appears to involve electronic excitation of Si x O − ; it is suggested that electron emission is governed by a process similar to Penning ionization, in which a vacancy created by the excitation of Si x O − may be filled by an electron from the valence band. The variation in the work function as oxygen accumulated on the surface was observed to be small.

We report on spin- and energy-resolved secondary electron emission, induced by impact of fast protons and electrons on ultrathin Cr films epitaxially grown on Fe(1 0 0). Based on a simple model, we extract spin- and energy-dependent probing depths from experimental polarization spectra. For grazing impact of protons the probing depth strongly depends on the energy of secondary electrons.

The fine structure (FS) of the experimental secondary electron emission spectra (SEES) along the normal to Ir(1 1 1) surface and the target (total) current spectra (TCS) of Cu(1 1 1) surface are interpreted theoretically. It is shown that the FS of SEES and TCS is mainly due to the electronic structure of finite states to which the electrons come or from which they emit. The predominant role of the effects of bulk energy-band structure in the formation of spectra is shown.

We have studied electron emission from decay of plasmons excited in the interactions of slow single charged Ne ions with Al surfaces as a function of incident energy ranging from 0.7 to 8 keV. Bulk plasmon excitation occurs above a threshold incident energy of ∼1 keV and initially coexists with potential excitation of surface modes. The excitation of bulk plasmons in Al is determined by fast electrons traveling inside the solid.

We have investigated fcc Fe x Co 1− x /Cu(1 0 0) alloy films with linear magnetic dichroism in photoemission. Specifically we studied the emission from the Fe 3p core level. For the first time, we have observed fine structure for linear dichroism in the photoelectron emission of the Fe 3p core level and confirmed our assignments using spin-polarized photoelectron spectroscopy. This conclusively demonstrates the validity of the single electron, density of states model for the 3p manifold of 3d magnetic materials.

Absolute probabilities for generating secondary electrons and negative ions have been measured as a function of adsorbate coverage and impact energy for 50–500 eV Na + incident upon a tungsten surface. Adsorbate coverage ranges from none up to a monolayer or more of oxygen or chlorine. Kinetic energy distributions of the emitted anions and secondary electrons have also been measured, and the identity of the anions has been determined. The presence of an adsorbate is observed to significantly enhance secondary emission of electrons and anions for both adsorbates. The dependence of the emission probability on Na + impact energy differs markedly for the two adsorbates. The results are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both negative ion and electron emission into the vacuum.

We studied the Au thickness and the primary electron energy-dependence of polarization of secondary electrons emitted from Au thin film on an Fe(110) substrate. By fitting to the experimental data using the exponential function, we found that the magnetic information depth λ m of the secondary electron polarization increases with primary electron energy E p from λ m = 6 A at E p = 70 eV to λ m = 26 A at E p = 4 keV. The accuracy of the fit degrades with higher E p . We explain this behavior by considering the difference in secondary yields between Au and Fe.

The oxidation of aluminum by electron bombardment of a water-covered surface at 90 K has been investigated by XPS, EELS and high-resolution electron energy-loss spectroscopy. Electron bombardment converts the adsorbed ice layer into a disordered aluminum oxide film with very high efficiency. The cross-section for water conversion is 2.5×10 −16 cm 2 for an electron energy of 100 eV. The threshold energy of the electronic stimulation has been estimated to be 6±1 eV. The excitation probably proceeds via direct electron attachment, leading to dissociative production of an OH radical species, which is very effective for surface oxidation.

Absolute yields of secondary electrons and negative ions resulting from collisions of Na+ with Mo(100) and a polycrystalline molybdenum surface have been measured as a function of the oxygen coverage of the surface for impact energies below 500 eV. The sputtered negative ions have been identified with mass spectroscopy, and O− is found to be the dominant sputtered negative ion for the surfaces at all oxygen coverages and impact energies. Both the electron and O− yields have an impact energy threshold at about 50 eV and exhibit a strong dependence on oxygen coverage. The kinetic energy distributions of the secondary electrons and sputtered O− were determined as functions of the oxygen coverage and impact energy. The distributions for O− are characterized by a narrow low-energy peak (at ∼1–2 eV) followed by a low-level high-energy tail. The secondary electrons have a narrow (FWHM ∼1–2 eV) kinetic energy distribution, centered approximately at 1–2 eV. The shapes of the distributions and their most probable energies are essentially invariant with impact energy, oxygen coverage and the nature of the Mo surface. The emission is explained and analyzed in terms of a simple model which involves a collision-induced electronic excitation of the MoO− surface state. The decay of this excited state leads to the production of both secondary electrons and O− with energy distributions and yields comparable to those observed.

The dependence of the height of the elastic peak and the Auger signal on the incidence angle of the primary electron beam was used to investigate the crystalline structure of the Ag/Cu(001) adsorption system. For the clean Cu(001) face experimental elastic peak profiles were compared with results of single scattering cluster (SSC) calculations, where main intensity maxima are reproduced. Auger scans for Cu M2,3VV and LMM transitions and elastic peak scans exhibit similar maxima for the same Ep=2.0 keV. For 8 ML of Ag the populations of four possible domains of Ag(111) were determined by an R-factor analysis. The growth of one domain was found to be preferred which can be rationalised by the miscut of the sample and the resulting step orientation.

Kinetic energy distributions and absolute yields of negative oxygen ions and secondary electrons resulting from positive ions impacting an aluminum surface have been determined as a function of the oxygen coverage of the surface. The experiments have been carried out with positive sodium ions at collision energies below 500 eV. Both the negative ion and secondary electron yields are observed to be strongly dependent on the oxygen coverage of the aluminum surface. The kinetic energy distributions of the dominant negative ion, O−, peak at approximately 1.0 eV and have a significant high-energy tail. The secondary electron kinetic energy distributions peak between 0.8–1.0 eV, have widths of 1.0–1.5 eV and exhibit no features indicative of discrete spectra. For both the ions and the electrons, the most probable kinetic energy is essentially independent of the impact energy and there is no correlated electron-anion emission. A mechanism is proposed to augment the collision cascade in order to model the sputtering of O− and the emission of electrons. Finally, the secondary processes are investigated with a partial coverage of sodium which serves to reduce the surface work function.

The dynamics of nitrogen collisions with metals partially covered by alkali atoms is studied both experimentally and theoretically. Our attention focuses on the formation of N−(1D) metastable ions and their interaction with the surface. We present the electron energy spectra induced by slow collisions of N+ ions with partially cesiated Pd(111) surfaces under grazing incidence. These spectra display, as a function of Cs coverage, a sharp feature which is due to the autodetachment of N−(2p4, 1D) to the N(2p3, 4S) ground state. Our calculations, performed with the coupled angular mode (CAM) method on the basis of the resonant electron exchange between the nitrogen atom in states of the 2p3 configuration and the metal surface, consistently explain how negative ions formed close to the surface can survive against electron loss to the metal during the outgoing trajectory and can later decay as free ions. In order to understand the alkali coverage dependence of the N−(1D)-N(4S) peak intensity, the local character of the nitrogen interaction with the surface partially covered by adsorbate atoms has been taken into account.

The technique of secondary-electron imaging is applied to investigate the structure of Ag films vapor-deposited onto a (0001) surface of Ag 2 Al at room temperature. The observations show that Ag grows in the hexagonal close-packed structure up to a thickness of 6–8 atomic layers, while twinned, face-centered cubic domains develop for thicker films.

The emission of secondary electrons excited by 60–1000 eV primary electrons from NaCl, KCl, LiF and CaF2 layers on W(110) ranging in thickness from less than one to several tens of monolayers is studied with a radial field analyzer. The energy distributions from NaCl and KCl layers are very narrow and have very high peaks at very low energies compared to the energy distributions from metals as exemplified by the substrate. LiF and CaF2 have wider distributions than NaCl and KCl. Nevertheless, most electrons have energies below the ionization threshold of the halogen atoms; this has important consequences for electron-stimulated neutral and ion desorption.

Femtosecond time-resolved two-photon photoemission (2PPE) is used to investigate the hot electron dynamics of Cu(111), Ag(110) and Ta(poly). The experimentally derived relaxation rates of photoexcited electrons scale with the available phase space for scattering with electrons below the Fermi level and are compared with predictions from Fermi liquid theory. A simulation employing rate equations to account for hot electron lifetimes, transport effects and the creation of secondary electrons demonstrates that the relaxation dynamics is strongly influenced by secondary electron cascades. With Ag(110) a pronounced temperature dependence of the photoemission yield is observed, indicating the importance of electron-phonon scattering for momentum conservation in the 2PPE process.

The negative H, O and matrix secondary ion emissions resulting from Ar+, Cs+ and O+2 bombardment of Sc, Ti, V, Co, Ni and Cu foils were found to exhibit a wide range of energy distribution shapes and intensities. These energy distributions could be reconstructed using two velocity dependent functions. Characteristic velocity values extracted from these velocity dependent functions were found to fall into groups according to the primary and secondary ions studied. Characteristic velocity values from within a particular group were found to correlate with work function and electron affinity arguments consistent with the Auger and resonance charge transfer processes (the Auger process predominating at lower emission energies ( 40 eV)). All secondary ion groups (H, O and the matrix secondary ion) resulting from a particular primary ion species could then be related on introducing a function relating the covalent radius of the secondary ion-surface atom pair. This, in turn, allowed for an empirical description of the negative secondary ion yield from metal surfaces to be defined.

Results of angle-resolved ion fraction measurements and an electron spectroscopy study of low keV Ne and Ne + collisions with a Si surface are presented. Similar charge fractions for neutral and ionic projectiles are obtained, indicating that ion production occurs as a result of collisions with surface atoms. Production of Ne∗∗ autoionising states is observed and its efficiency as a function of collision energy is determined. The characteristics of autoionising peak line shapes suggest differences in angular distributions of the different Ne∗∗ states. The effect of oxygen adsorption is studied and it is shown that exposure to oxygen results in a strong decrease of autoionising state production. Differences with oxygen adsorption on Mg and Al targets are underlined.

Energy and angular distribution of secondary electrons (SE) emitted from Si(111)-7 × 7, √3 × √3-Ag and 5 × 2-Ag surfaces, excited by a 15 keV primary electron beam, has been measured in an ultrahigh-vacuum (UHV) chamber. SE from 7 × 7 and √3 × √3-Ag surfaces show angular dependence nearly proportional to cos (θ) with θ denoting the emission angle referred to surface normal; while those from 5 × 2-Au decrease more rapidly with θ. The latter can be explained by an additional field between the sample and the detector, resulting from the larger work function of 5 × 2-Au. The measurement of the spectra of SE emitted normally from the surface reveals that, compared with a 7 × 7 surface, a √3 × √3-Ag surface has much larger intensity in the whole energy range measured (0–10 eV), while a 5 × 2-Au surface shifts the whole spectrum by ∼ 0.5 eV with slight reduction in intensity. These can be well accounted for by the effect of band-bending and the difference in work functions. On a surface where 5 × 2-Au and 7 × 7 domains are simultaneously present, it is experimentally verified that the lowest energy SE emitted from the smaller work function (7 × 7) domains cannot reach the detector. The measurements are in qualitative agreement with observations with UHV scanning electron microscope (SEM), reported by the present authors before, showing that √3 × √3-Ag and 5 × 2-Au domains appear brighter in SE images than simultaneously present 7 × 7 domains.

The rearrangement of (7 × 7) domain boundaries on Si(111) surfaces during annealing has been investigated by using a secondary electron imaging technique: scanning electron surface microscopy. The shape of the domain boundaries changes during annealing for several minutes at 8°C below the (7 × 7)−(1 × 1) transition temperature in order to form stabilized domain structures. The stabilized boundaries are in the 〈110〉 directions, reflecting the structures of the (7 × 7) unit. This is the result of minimizing the energy of the boundaries, which consisted of (1 × 1) structures fluctuating with the size of the (7 × 7) units at the temperature.

Second derivative energy spectra of secondary electron emission (SE) near the onset have been measured in the coadsorption of Cl and Cs on Si(100). Two SE peaks are observed when Cl atoms are deposited on Cs-covered Si(100), while only one onset peak for the reverse order of deposition of first Cl and second Cs. Both two peaks are ascribed to the SE onsets due to cascade electrons rather than electrons due to the electron density of state above the vacuum level. The appearance of the two SE onsets is quite sensitive to primary electron beam energies, E p . For E p E p > 200 eV a single onset appears. This fact suggests that the two SE onsets occur when the secondary electrons are preferentially generated in the selvedge. The surface structure with two SE onsets is still stable even after the Cl atoms are removed by electron stimulated desorption. From a deconvolution analysis of the double peak, we propose a surface structure with Cs rich patches.

We study the electron-induced secondary electron emission of insulating targets by a Monte-Carlo simulation method. The most important collision mechanisms in the energy domain considered are investigated. The usual approximations apparently overestimate the elastic collision rate and empirical corrections are proposed. The self-trapping of the charges is explained by the polarization of the surrounding medium. The role of the polaronic effects in the secondary emission as well as the sensibility of the results to the choice of the different parameters are analyzed. The influence of an internal electric field caused by a “frozen” charge distribution is also investigated.

Results of a study of the formation of doubly excited autoionising states in single and doubly charged inert gas ion collisions with Mg and Al surfaces are presented. In case of Ar++ ion scattering with a Mg surface, formation of such states is observed as a result of electron capture as the ion approaches the surface. This is not observed in case of Ne++ scattering on Mg where these excited states result from a violent binary collision with Mg surface atoms. In case of Ne++ scattering on Al both mechanisms are observed. These results are interpreted in terms of a numerical model which considers a series of Auger and resonant decay and capture processes.