Energy spectra of electrons and positrons emitted from surfaces as a result of low energy positron bombardment

Weiss, A. H.; Jung, Eun-Sung; Kim, J.H.; Nangia, A.; Venkataraman, R.; Starnes, S.; Bräuer, G.

doi:10.1016/s0169-4332(96)01075-6

Cited by 11 publications

(7 citation statements)

References 21 publications

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“…The temperature dependence of the Auger intensity recorded with PAES was studied on clean Cu (100). In contrast to collisional induced AES, which is inde- [196]). pendent on temperature, a strong decrease of the PAES intensity was observed between 300 o C and 700 o C. Simultaneously, an increase of Ps annihilation was measured by detecting the 3γ/2γ ratio.…”

Section: Adsorbed Layers On Cumentioning

confidence: 99%

“…In first PAES studies using a cylindrical mirror analyzer with an energy resolution of ∆E E =2.5% the double peak structure of the CuM 2,3 VV transition could be observed [195]. In addition, by appropriate analyzer biases Weiss et al succeeded in clearly separating the energy distribution of "true secondaries" from that of "redistributed primary" particles by bombarding Ge (100) with 370 eV positrons and electrons [196]. The application of a positron beam allows to record the spectrum of "true secondaries", i.e.…”

Section: Core-annihilation Probability and Line Shape Analysismentioning

confidence: 99%

“…Panel (d) shows the spectrum of re-emitted positrons as a result of bombardment with 370 eV positrons. For comparison, in panel (b) the electron induced electron spectrum is plotted with a linear combination of the positron induced secondary electron spectrum and the backscattered positron spectrum (figure from[196]).…”

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confidence: 99%

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Positrons in surface physics

Hugenschmidt¹

2016

Surface Science Reports

126

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Within the last decade powerful methods have been developed to study surfaces using bright low-energy positron beams. These novel analysis tools exploit the unique properties of positron interaction with surfaces, which comprise the absence of exchange interaction, repulsive crystal potential and positron trapping in delocalized surface states at low energies. By applying reflection highenergy positron diffraction (RHEPD) one can benefit from the phenomenon of total reflection below a critical angle that is not present in electron surface diffraction. Therefore, RHEPD allows the determination of the atom positions of (reconstructed) surfaces with outstanding accuracy. The main advantages of positron annihilation induced Auger-electron spectroscopy (PAES) are the missing secondary electron background in the energy region of Auger-transitions and its topmost layer sensitivity for elemental analysis. In order to enable the investigation of the electron polarization at surfaces low-energy spin-polarized positrons are used to probe the outermost electrons of the surface. Furthermore, in fundamental research the preparation of well defined surfaces tailored for the production of bound leptonic systems plays an outstanding role. In this report, it is envisaged to cover both, the fundamental aspects of positron surface interaction and the present status of surface studies using modern positron beam techniques.

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Section: Adsorbed Layers On Cumentioning

confidence: 99%

Section: Core-annihilation Probability and Line Shape Analysismentioning

confidence: 99%

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Positrons in surface physics

Hugenschmidt¹

2016

Surface Science Reports

126

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“…The kinetic energy of the positrons was set to E 0 (e + ) = 15 eV. The intensity peak of the secondary electron distribution is typically positioned at E peak = f (E 0 (e + ) -eU sample -Φ -) -eU sample , with f = 1-10% [4]. Five different sample potentials were chosen: -400 V, -301 V, -200 V, -98 V and -51 V. Figure 2 shows the time-of-flight spectra for secondary electrons for all sample potentials.…”

Section: Variation Of the Analyzed Energy Rangementioning

confidence: 99%

A novel time‐of‐flight spectrometer for PAES

Legl

Hugenschmidt

2007

Phys. Status Solidi (c)

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Positron annihilation induced Auger electron spectroscopy (PAES) uses the annihilation of a positron with an electron of the inner shell of atoms whereas X-rays or high-energy electrons initiate the ionisation in conventional AES. Advantages of PAES are the extremely high surface sensitivity and the higher signal to noise ratio. In order to benefit from PAES one has to use a low-energy positron beam of high intensity, which is available at NEPOMUC at the new Munich research reactor FRM-II. The energy of the Auger electrons is mostly analysed by hemispherical energy analysers. Due to their small solid angle (typically ≥0.1%) and the necessity to scan over certain energy ranges in ≥1eV steps one still needs several hours using an intense positron source like NEPOMUC (at the FRM-II) or even weeks at the lab-beam based on a beta-emitter to obtain an Auger spectrum. For this reason, a novel time-of-flight spectrometer (TOF-PAES) was developed, which efficiently collects the Auger electrons emitted in a solid angle of 2π around the sample, and hence reduces the measurement time to less than one hour. . Positron annihilation induced Auger-electron spectroscopy (PAES) has several advantages over conventional Auger-electron spectroscopy (AES), in which the sample is irradiated with X-rays or keV-electrons to initiate the Auger-process by photo-or impact ionization (XAES or EAES) of core electrons.Due to the low energy of the implanted positrons (several 10 eV), no collision-induced secondary electron background is produced in the higher energy range of released Auger-electrons, whereas commonly measured Auger-spectra show a high background of secondary electrons released by the incoming X-rays or keV-electrons [1]. Moreover, PAES is much more surface sensitive than AES, since most of the implanted positrons are trapped at the surface bound states and annihilate with electrons in the topmost atomic layer [2,3].The main drawback of PAES is the low intensity of low-energy positrons. Another challenge is improving the electron detection in order to compensate for the lack of positron intensity. Augerspectrometers usaually use hemispherical or cylindrical energy analysers. The limited solid angle (few percent) and the energy scans of the analyzers lead to a long duration of measurement for an Augerspectrum: Therefore we developed a novel design for a time-of-flight apparatus for PAES (TOF-PAES) investigations at the positron beam facility at NEPOMUC. With this new Time-of-Flight spectrometer we expect a reduction of the measurement time for an Auger spectrum from hours to less than one hour.

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“…inset of Fig. 5), we need to incorporate in a theoretical description the positron±lattice interaction [49,50] and the positron±electron correlation [51]. In this work we investigate ®rst the electron±positron pair emission that is due to the electron±positron correlation only.…”

Section: Secondary Electron Ejection From Surfaces By Positron Beamsmentioning

confidence: 99%

Mechanisms of electronic excitations by low-energy positrons: From finite to extended electronic systems

Berakdar

2000

Nuclear Instruments and Methods in Physics Research Section B:

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This work deals with the mechanisms of electronic transitions induced by low-energy positrons. For isolated atomic systems we discuss the role of successive binary collisions, the so-called Thomas mechanisms, between the positron, the excited electron and the atomic core. Furthermore, we consider transfer processes in which one of the charged particles is trapped in a low-lying continuum state of one of the reaction participants. In the second part of the paper, we investigate the various pathways for the secondary-electron emission from metallic surfaces following the bombardment by low-energy positrons. From a formal analysis it is shown that the probability rate for the production of an electron± positron pair is connected to the interacting electron±positron two-particle Green operator. The scattering of the interacting positron±electron subsystem from the crystal potential is treated and it is pointed out that a di raction of the electron±positron pair as a whole might take place leading to characteristic di raction pattern. The analysis is substantiated by numerical studies for the secondary electron emission from Cu(0 0 1) and Fe(1 1 0) (BCC) samples. Ó

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Energy spectra of electrons and positrons emitted from surfaces as a result of low energy positron bombardment

Cited by 11 publications

References 21 publications

Positrons in surface physics

Positrons in surface physics

A novel time‐of‐flight spectrometer for PAES

Mechanisms of electronic excitations by low-energy positrons: From finite to extended electronic systems

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