A novel time‐of‐flight spectrometer for PAES

Abstract

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.

Positrons in surface physics

“…One disadvantage of these devices is the lower energy resolution compared to electrostatic electron energy analyzers, which was reported to be ∆E/E= 3 eV/30 eV [12]. Therefore, an improved TOF-setup, which combines a trochoidal filter and a flight tube mounted in a Faraday cage, with an energy resolution of about 1 eV at high electron energies up to E ≈ 1000 eV was developed [191] and demonstrated to work in an energy range between 50 eV and 400 eV [192]. Figure 39: PAES spectrometer at NEPOMUC: The positron beam is magnetically guided to the Auger chamber, which is connected with a STM and a sample preparation chamber [190].…”

A multi-stop time-of-flight spectrometer for the measurement of positron annihilation-induced electrons in coincidence with the Doppler-shifted annihilation gamma photon

“…The energy resolution can be further improved by applying a negative potential on the field-free tube with respect to the sample. The negative potential decelerates the electron, increases the ToF, 𝑡, and thus improves the energy resolution, as can be seen in Equation 6and as demonstrated by Ohdaira et al and Hugenschmidt et al [30][31][32].…”

“…It is also possible to apply a negative potential to the ToF tube, with respect to the sample, to reflect electrons with insufficient energy to overcome this applied bias. Additionally, the negative bias decelerates the electrons to a lower energy that persists during their traversal time through the ToF tube, resulting in improved resolution in the determination of the energy of the detected electrons [30][31][32]. After exiting the ToF tube, the electrons enter the 𝐸 ̅ × 𝐵 ̅ system, where they are drifted upwards to the entrance of the electron detector according to equation 1.…”