Electron Emission Angle Research Articles

Doubly-differential ionization cross sections of positron impact on argon

We report the first measurement of doubly-differential ionization cross sections for positron impact on argon atoms. Energy- and angle-resolved measurements of ejected electrons in time correlation with the produced and detected ions have been performed. Corresponding measurements with incident electrons were made for comparison. With positrons and electrons as primary particles of 100 eV energy and ejected electrons of 15 eV, our measurements were extended over electron-emission angles from 0‡ to 90‡. Lacking theoretical predictions for the doubly-differential ionization of argon, we compare our measured data with the theoretical doubly-differential ionization cross sections, calculated for positron and electron impact on hydrogen by Klar and Berakdar (Freiburg) [1]. The angular dependence of positron and electron cross sections for argon agrees well with the theory for hydrogen. In particular, we found that for small angles of electron ejection the cross section for positron impact ionization exceeds that for electron impact by an order of magnitude in accordance with the predictions of Klar and Berakdar.

Direct experimental measurement of electron impact ionization‐excitation branching ratios: 2. Angular distribution of secondary electrons from N2 at 100 eV

The coincidence ionization experiment used previously to measure ionization‐excitation branching ratios for the X ²Σg+, A ²Πu, and B ²Σu+ states of N2+ at a fixed secondary electron emission angle of 74° has been modified to allow the secondary emission angle to be scanned. Angular distributions of electrons which have excited the three available electronic states of the N2+ ion have been measured at 100 eV. For the small, primary electron scattering angles where most ionization events occur the distribution for the X ²Σg+ state is found to have a peak at 57° with respect to the incident beam direction, while that for the A ²Πu state has a broad peak near 75°. The distribution for the much less intense B ²Σu+ state is intermediate between the two. The partial branching ratios deduced for the 74° fixed angle case have been corrected to reflect the more accurate partial branching ratios obtained by integrating the secondary electron angular results. The corrected values for the X ²Σg+ A ²Πu, and B ²Σu+ states are 0.56±0.05, 0.35±0.03, and 0.09±0.02, respectively. These results show that the A ²Πu state branching ratio is much smaller than deduced from a number of previous studies

Relative secondary-electron yields from clean metals under fast-atom bombardment and angle-integrated UV-photoelectron spectra in a photoelectron spectromicroscope

This article describes the construction and operation of a source of 2–5 keV argon atoms for use in the strong (1–7 T) B-field environment of a photoelectron spectromicroscope. The velocity spread of the atom beam has been determined from the time of flight. Atom-induced secondary-electron yield measurements on selected areas of various metallic elemental samples are described and the results show that yields differ by up to a factor of 4×103 between elements and trend towards lower yields with increasing atomic number. A number of ultraviolet photoelectron spectra of the fast-atom sputter-cleaned sample surfaces are also presented. These spectra are angle-integrated over the full 2π of electron-emission angles.

Search for Fermi shuttle mechanisms in electron emission from atomic collision sequences

In electron spectra induced by slow heavy ion bombardment of solids a high energy tail can be observed, which is suggested to be explained by multiple collision sequences. In order to find those multiple collision effects like the “Fermi shuttle” acceleration mechanism we measured doubly differential electron emission cross sections for H + (33.5–700 keV) impact on different targets (He, Ne, C and Au) as a function of projectile energy and electron emission angle. We observed a surprising target dependence of the electron emission within the range of electron energies close to that of the binary encounter electrons for all observed angles of emission.

Surface Roughness Effect on Secondary Electron Emission from Beryllium under Electron Bombardment

A direct Monte Carlo model is developed to simulate secondary electron emission from beryllium with a flat surface and Gaussian-ripple surfaces. The calculated electron yield and energy distribution of secondary electrons are in reasonable agreement with the experimental data. The emphasis is in this study put on the effect of surface roughness on secondary electron emission. The number of secondary electrons emitted largely depends on the position of bombardment of primary electrons on the ripple surface. The energy distribution of secondary electrons emitted from the ripple surface shifts towards low-energy side in comparison with the distribution for the flat surface. The over-cosine and gourd-shaped angular distributions, depending on the position of bombardment, are calculated for emission angle of electrons from the ripple surface; the distribution for the flat surface agrees quite well with the cosine distribution.

Positron-atom doubly differential ionization cross sections.

Doubly differential ionization cross sections for positron impact on argon atoms were determined by energy- and angle-resolved measurements of ejected electrons in time correlation with the produced ions. Corresponding measurements with incident electrons were made for comparison. With primary particles (positrons or electrons) of 100 eV and ejected electrons of 15 eV, measurements were extended over electron-emission angles from 0\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{}. Lacking theoretical predictions for the ionization of argon we compared our data with the doubly differential ionization cross sections of Klar and Berakdar computed for positron and electron impact on hydrogen. The measured angular dependence of positron and electron cross sections for Ar agrees well with the theory for H. In particular, we find that at small angles of electron ejection the cross section for positron impact exceeds that for electron impact by an order of magnitude in accordance with the calculations of Klar and Berakdar (unpublished).

Systematics and scaling of differential ionization cross sections in multicharged ion-atom collisions

Absolute doubly differential cross sections for electron emission are presented for 0.5-MeV/u multicharged ion-atom collisions. The collision systems investigated are ${\mathrm{B}}^{\mathit{q}+}$, ${\mathrm{C}}^{\mathit{q}+}$ (q=2--5) and ${\mathrm{O}}^{\mathit{q}+}$, ${\mathrm{F}}^{\mathit{q}+}$ (q=3--6) projectiles impacting on helium and ${\mathrm{C}}^{\mathit{q}+}$ (q=2--5) ions impacting on neon and argon targets. Laboratory electron emission angles between 10\ifmmode^\circ\else\textdegree\fi{} and 60\ifmmode^\circ\else\textdegree\fi{} were studied. Under the assumption that the cross sections scale with the square of an effective projectile charge, ${\mathit{Z}}_{\mathit{e}\mathit{f}\mathit{f}}$(\ensuremath{\varepsilon},\ensuremath{\theta}), the scaling was investigated as a function of emitted electron velocity and angle. For distant collisions (low-energy electron emission), we find that ${\mathit{Z}}_{\mathit{e}\mathit{f}\mathit{f}}$(\ensuremath{\varepsilon},\ensuremath{\theta})gq for small q. For the highest values of q investigated, ${\mathit{Z}}_{\mathit{e}\mathit{f}\mathit{f}}$(\ensuremath{\varepsilon},\ensuremath{\theta}) was found to be smaller than the net projectile charge q. The effective projectile charges may be subject to a systematic underestimation since they were determined by referencing the partially stripped ion impact data to fully stripped boron, rather than proton, impact cross sections. In the binary-encounter region, the present data confirm previously observed features--namely that the emission increases as q decreases. For the helium target, the qualitative behavior is roughly in accordance with predictions by Schulz and Olson [J. Phys. B 24, 3409 (1991)]. Neon and argon targets also demonstrate these features, but less dramatically than helium.

Electron and positron emission angle distributions in pair production by relativistic heavy ions

Abstract The cross section for direct electron-positron pair production by relativistic heavy ions, differential in the emission angle of one of the particles, is calculated using lowest order QED theory. A general method is given for computing all the terms in the exact lowest order expression, based on previous work. Some numerical results are given for selected incident particle energies and the results are compared with the corresponding calculation using the Weizsacker-Williams method.

Submonolayer adsorbate reference material based on a low alloy steel fracture sample for Auger electron spectroscopy Part 1 Characterisation

AbstractA homogeneous set of Cr–3·5Ni low alloy steel samples has been made to enable laboratories studying grain boundary segregation by Auger electron spectroscopy to interrelate their studies. This steel has P, Sn, and Ni segregated at the grain boundaries. Data are provided of the effect of variation of signal intensities across a grain, on one grain as a function of the electron emission angle, on many grains, etc. for both unidirectional (concentric hemispherical) and multidirectional (cylindrical mirror) spectrometers. Reference spectra recorded from the sample allow the spectrometer transmission function to be calibrated so that data from all instruments may be interrelated. After correction for the transmission function, the Auger electron peak-to-peak height ratios, for a 5 eV Savitzky and Golay differentiation, are found to be P120/Fe703=0·33±0·06 and Sn430/Fe703=0·12±0·02. Calibrations from the literature show these signals can be attributed to 33 and 31% of a monolayer of P and Sn, respective...

Application of Tougaard background subtraction to Langmuir-Blodgett films

The inelastic background subtraction algorithm proposed by Tougaard is applied for the first time to the X-ray photoemission spectra of metal salts of arachidic acid Langmuir-Blodgett films. The background deconvolution procedure has been done using the inelastic scattering cross-section determined from reflection electron energy loss spectroscopy and using a “universal” inelastic scattering cross-section. Both approaches give physically meaningful and nearly identical primary excitation spectra. The range of validity of the deconvolution formulae is investigated by varying the electron emission angle. Possible reasons for the applicability of the Tougaard algorithm to materials containing molecules with dimensions of the order of the inelastic scattering length, as are obtained for the Langmuir-Blodgett films in this study, are discussed.

Influence of nuclear track potentials in insulators on the emission of target Auger electrons.

The angle, energy, and fluence dependence of electron emission following the interaction of normally incident 100-MeV ${\mathrm{Ne}}^{9+}$ ions with thin polypropylene foils and 170-MeV ${\mathrm{Ne}}^{7+}$ projectiles with Mylar foils has been investigated experimentally. Spectra were taken for electron ejection angles of 0\ifmmode^\circ\else\textdegree\fi{}, 45\ifmmode^\circ\else\textdegree\fi{}, and 120\ifmmode^\circ\else\textdegree\fi{} at fluences in the range of 2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ to 4\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ions/${\mathrm{cm}}^{2}$. A fluence-dependent carbon KLL Auger energy shift of up to 68 eV was observed. Model calculations for the nuclear track potential are consistent with the experimental findings.

High-resolution core-level study of the (100), (110), and (111) surfaces ofCr3Si

High-resolution angle-resolved photoemission investigations of core levels of the (100), (110), and (111) surfaces of ${\mathrm{Cr}}_{3}$Si have been performed using synchrotron radiation. Surface-shifted Si 2p levels were observed on all three surfaces, but no shifted Cr 3p levels could be identified. The core-level shifts were extracted using a curve-fitting procedure, and it was found necessary to use two surface-shifted components for each surface. Shifts of -0.29 and -0.56 eV for the (100) surface, of -0.42 and -0.77 eV for the (110) surface, and of -0.27 and +0.28 eV for the (111) surface were extracted. The reaction rate for these surfaces upon oxygen exposure was investigated, and significant differences were observed. The (100) surface was found to show the highest reaction rate and the (110) surface the lowest. Initial Si oxidation was observed on all three surfaces, while Cr oxidation could be revealed only after higher oxygen doses. Clear diffraction effects were observed in the Si 2p surface-to-bulk intensity ratio both as a function of electron emission angle and photoelectron kinetic energy. These results are presented and discussed.

Two-centre effects in ejected-electron spectra from ionization of helium atoms in collisions with fast, fully stripped ions

A joint experimental and theoretical study of the double-differential cross sections (DDCS) for ejection of electrons from helium by 1.00 and 1.84 MeV amu-1 p+, He2+, C6+ and O8+ is presented. Electron-emission angles from 20' to 160' are surveyed, and the electron energies range from 4 to 2500 eV. Single-differential and total cross sections, obtained by integrating the DDCS over angle and electron energy, are also presented. Large departures from the q2 dependence (where q is the projectile charge) predicted by the first Born approximation are observed in the cross sections. The deviations, which are seen to increase with projectile charge are discussed and compared with calculations, using the first Born approximation and a continuum-distorted-wave eikonal-initial-state model.

Electronic and structural properties of the Cu - Bi 2CaSr 2Cu 2O 8 interface

The formation of the Cu-Bi 2CaSr 2Cu 2O 8 interface has been studied by photoelectron spectroscopy using synchrotron radiation. Photon energies in the range 20–1000 eV were utilized in order to probe both the valence band and to monitor chemical changes upon Cu deposition, as revealed by the core-level shifts. A strong chemical reaction between Bi 2CaSr 2Cu 2O 8 and Cu is manifested by the formation of metallic Bi. From theb density variations as a function of electron emission angle it is shown that the metallic Bi segregates to the top surface layer.

Strong dependence of zero degree binary encounter electron production on the initial and final charge state

Doubly differential cross sections d2 sigma /dEed Omega e and fractional probabilities for binary encounter (BE) electron emission at 0' electron emission angle as a function of initial (qi) and final (qf) projectile charge state have been measured for 0.53 MeV u-1 Fqi on H2. The data confirm the surprising decrease of the partial cross section with increasing charge state of the incoming projectile qi reported by Richard et al. (1990). The authors show by coincident electron projectile detection that the pure ionization channel qi=qf strongly dominates the differential cross section at theta e=0' and projectile capture and loss channels are of negligible importance. In view of a calculation by Olson et al. (1990) they conclude from their data that 0' BE electron emission depends strongly on the shape of the screened projectile potential varying with qi.

Interactions of Electroless Catalysts with Ozonated Polymer Surfaces: Platability and X‐Ray Photoelectron Spectroscopy Studies

The treatment of polystyrene and poly(styrene‐acrylonitrile) with ozone allows the examination of the effect of a mildly oxidized surface on the activity of electroless catalysts. Ozonation, compared to photo‐oxidation, produces a lightly oxidized surface that inhibits, but does not deactivate, electroless catalysts. The treated polymer surfaces were examined by x‐ray photoelectron spectroscopy (XPS) at several electron emission angles. By varying the electron emission angle, both the amount and the depth distribution of ozonation products can be determined in the near surface region. XPS detected less than a monolayer of oxidation on the ozonated surfaces accompanied by the accumulation of about 0.5 monolayer of aromatic hydrocarbon. Even though total catalyst poisoning does not occur, the fact that catalyst inhibition occurs on a marginally altered surface has implications concerning treatments of polymer surfaces before electroless deposition.

Study of sidewall passivation and microscopic silicon roughness phenomena in chlorine-based reactive ion etching of silicon trenches

Sidewall passivation layers produced in the formation of Si trenches by HCl/O2/BCl3 reactive ion etching have been characterized by angle-resolved x-ray photoemission measurements and secondary electron microscopy. Electron-shading effects observed at grazing electron emission angles and electrostatic charging of insulating portions have been used to differentiate photoemission contribution from (i) the trench sidewall film (moderately insulating), (ii) the oxide mask (highly insulating), and (iii) the Si trench bottom (conductive). The sidewall passivation film for this process is found to be a silicon oxide. Only minor amounts of chlorine are incorporated in this film. The thickness of the sidewall passivation layer depends on the width of the Si trench and ranges from ≂0.1 μm for narrow trenches to ≂0.7 μm for wide Si trenches for our conditions. The Si taper formed in this trench etching process is produced by the simultaneous formation of the sidewall film which protects the underlying Si from etching. It is also shown that microscopic silicon pillars (‘‘black silicon’’ or ‘‘grass’’) form by micromasking under process conditions where large areas of unmasked silicon are exposed. The microscopic Si pillars are protected by the same sidewall passivation film as the trench sidewalls. Islands of black silicon are observed only in large open silicon areas and separated from oxide masked areas by a characteristic distance which is free of silicon pillars. This roughness-free distance is of the order of the mask thickness employed. Ion scattering from the sloped mask sidewalls and sputtering of involatile micromasking material can explain this observation. Our results are consistent with a mechanism in which the processes that cause the formation of the beneficial sidewall film also initiate and control the production of undesirable silicon roughness in large unmasked Si regions.

Investigation of delta -electron emission in collisions of highly charged fast Ne projectiles with carbon-foil targets.

The angle and energy dependence of electron emission following the interaction of 70--170-MeV ${\mathrm{Ne}}^{\mathit{q}+}$ (q=7,10) projectiles with 20 and 100 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$ carbon foils has been investigated experimentally. Absolute emission yields were determined for ejection angles ranging from 0\ifmmode^\circ\else\textdegree\fi{} to 180\ifmmode^\circ\else\textdegree\fi{} and electron energies from 10 eV to 6 keV for normal-incidence ion beams. The experimental \ensuremath{\delta}-electron spectra are compared to results from a newly developed theoretical model based on a separation of energy loss and angular scattering for secondary electrons.

Observation of structures in the binary encounter peak in fast uranium-rare gas collisions

For 1.4 MeV/u U 33+ on rare gas atoms, the double differential cross sections Δ 2σ(E e, ϑ e) ΔE eΔω e as a function of the electron emission energy E e at different emission angles ϑ e were measured. At ϑ e≈40° an unexpected structure in the center of the binar encounter peak was observed, which varies strongly with the electron emission angle.

Unexpected angular distribution of the δ-electron emission in 1.4 MeV u-1 U33+-rare-gas collisions

Doubly differential delta -electron emission cross sections Delta 2 sigma (Ee, theta e)/ Delta Ee Delta Omega e have been measured as a function of the electron energy Ee and the electron emission angle theta e for the collision systems 1.4 MeV u-1 U33+ on neon and argon. The angular distributions of the emission cross sections show a maximum at theta e approximately=60', in agreement with parameter-free n-body classical trajectory Monte Carlo (n-CTMC) calculations. The cross sections show an extreme anisotropy between forward and backward emission compared with those of light-ion data. The cross sections decrease exponentially for theta e90'. The n-CTMC calculations indicate that the large projectile Coulomb force highly polarises the target-atom electrons in the two-centre field.