Positron annihilation induced Auger electron spectroscopy

Recently, Weiss et al. have demonstrated that it is possible to excite Auger transitions by annihilating core electrons using a low energy (less than 30eV) beam of positrons. This mechanism makes possible a new electron spectroscopy, Positron annihilation induced Auger Electron Spectroscopy (PAES). The probability of exciting an Auger transition is proportional to the overlap of the positron wavefunction with atomic core levels. Since the Auger electron energy provides a signature of the atomic species making the transition, PAES makes it possible to determine the overlap of the positron wavefunction with a particular element. PAES may therefore provide a means of detecting positron-atom complexes. Measurements of PAES intensities from clean and adsorbate covered Cu surfaces are presented which indicate that approx. 5 percent of positrons injected into CU at 25eV produce core annihilations that result in Auger transitions

Positron Induced Electron Emission from Graphite

In this paper, we present and analyze measurements of the positron induced electron spectra (PIES) from highly oriented pyrolytic graphite (HOPG). The spectra were obtained using a time of flight spectrometer attached to a variable energy positron beam. In the first measurements presented, the system was configured to obtain high resolution data from the annihilation induced KVV Auger transition of carbon. In the second set of data presented, PIES spectra were obtained for 3 different positron beam energies (1.25 eV, 3.5 eV and 4.5 eV). The resulting time of flight (ToF)-PIES exhibit contributions arising from either positron annihilation induced Auger processes (PAES), Auger mediated positron sticking (AMPS), or secondary electron emission. Our analysis indicates that for incident positron energies 3.5 eV and less, the ToF-PIES can be accounted for considering only two mechanisms: positron annihilation induced Auger processes or positron sticking.Comment: Presented at 18th International Conference on Positron Annihilation, August 19-24, 2018 | Orlando, USA. The following article has been accepted by AIP Conference Proceedings. After it is published, it will be found at this https://aip.scitation.org/journal/ap

Positronium formation in graphene and graphite

Positronium (Ps) formation on the surface of clean polycrystalline copper (Cu), highly oriented pyrolytic graphite (HOPG) and multi layer graphene (MLG) grown on a polycrystalline copper substrate has been investigated as a function of incident positron kinetic energy (1.5eV to 1keV). Measurments on Cu indicate that as the kinetic energy of the incident positrons increases from 1.5eV to 900eV, the fraction of positrons that form Ps ($f_{Ps}$) decreases from ~0.5 to ~0.3. However, in HOPG and MLG, instead of a monotonic decrease of $f_{Ps}$ with positron kinetic energy, a sharp peak is observed at ~ 5eV and above ~200eV,remains nearly constant in HOPG and MLG. We propose that in HOPG and MLG, at low incident positron energies the Ps formation is dominated either by a surface Plasmon assisted electron pick up process or by an energy dependent back scattering process. Both these processes can explain the peak observed and the present data can help to augment the understanding of Ps formation from layered materials.Comment: Presented at 18th International Conference on Positron Annihilation, August 19-24, 2018 | Orlando, USA. The following article has been accepted by AIP Conference Proceedings. After it is published, it will be found at https://aip.scitation.org/journal/ap

Coincident Measurement of the Energy Spectra of Doppler-Shifted Annihilation Gamma Quanta and Positron-Induced Secondary Electrons

Preliminary results are presented from a new positron beam system currently under development at the University of Texas at Arlington for the coincident energy measurement of Doppler-shifted annihilation quanta and positron-induced Auger electrons. We report data based on an analysis of the pulses resulting from the detection of positron induced secondary electrons by a micro-channel plate detector in coincidence with the pulses resulting from the detection of associated annihilation gamma rays in a NaI(Tl) gamma detector.Comment: Presented at 18th International Conference on Positron Annihilation, August 19-24, 2018 | Orlando, USA. The following article has been accepted by AIP Conference Proceedings. After it is published, it will be found at https://aip.scitation.org/journal/ap

Application of Doppler Broadened Gamma Spectroscopy to Study the Surface of Graphene

We present Doppler broadened gamma spectra, obtained using the newly developed advanced positron beam at the University of Texas at Arlington, from a sample consisting of 6 to 8 layers of graphene (MLG) on polycrystalline Cu. The kinetic energy of the positron beam was varied form 2 eV to 20 keV allowing for a depth resolved measurement. The ratio curves formed by dividing the measured Doppler broadened gamma spectra obtained at low positron kinetic energies (~2eV) to the gamma spectra obtained at 20 keV were compared to ratio curves found by dividing the calculated spectra of bulk graphite to bulk Cu. The ratio curves obtained from the measured results show qualitative agreement with those obtained from the calculated spectra. In particular, both sets of curves indicate a much reduced intensity at high momentum. The agreement between the measured and calculated curves is consistent with the hypothesis that the 2eV spectra correspond to the Doppler broadened spectra from the thin overlayer of Graphene (which we anticipate should be similar to the spectra obtained from bulk graphite) and that the spectra taken at 20 keV corresponds to bulk Cu due to the fact that most of the positrons implanted at this energy annihilate in the Cu substrate. The results taken at 2 eV provide evidence that it is possible to obtain chemically sensitive information from the top atomic layers of surfaces (both internal and external) from an analysis of the high momentum tail of the Doppler broadened gamma spectra obtained from the annihilation of positrons at the surface.Comment: Presented at 18th International Conference on Positron Annihilation, August 19-24, 2018 | Orlando, USA. The following article has been accepted by AIP Conference Proceedings. After it is published, it will be found at https://aip.scitation.org/journal/ap

Monte Carlo analysis of the contributions of long-lived positronium to the spectra of positron-impact-induced secondary electrons measured using an annihilation-gamma-triggered time-of-flight spectrometer

Magnetic bottle Time-of-Flight (ToF) spectrometers can measure the energy spectra of all electrons emitted into a 2$\pi$ sr solid angle simultaneously, greatly reducing data collection time. When the detection of the annihilation gamma ($\gamma$) and the detection of the electron (e) are used as timing signals for ToF spectrometers, the e-$\gamma$ time difference spectra (e-$\gamma$ TDS) are reflective of the positron-induced electron energy distributions provided the times between the impact of the positrons and the emission of the annihilation gammas are short compared to the flight times of the electrons. This is typically the case since positrons have short lifetime in solids ($\sim$ 100 - 500 ps) compared to the flight times of the secondary electrons ($10^2$ ns to $10^3$ ns). However, if the positron leaves the surface as a positronium atom (a bound electron-positron state), the annihilation gamma photons can be appreciably delayed due to the longer ortho-positronium (o-Ps) lifetime. This can result in an e-$\gamma$ TDS having an exponential tail with a decay constant related to the o-Ps lifetime. Here, we present an analysis of the e-$\gamma$ TDS using a Monte Carlo model which estimates the spectral contributions resulting from o-Ps annihilations. By removing the contributions from the delayed gamma signal, the energy spectrum of Positron Impact-Induced Secondary electrons (PIISE) can be isolated. Furthermore, our analysis allows an estimation of the intensity of the exponential tail in the e-$\gamma$ TDS providing a method to measure the fraction of positrons that form Ps at solid surfaces without relying on assumed 100% Ps emitting surfaces for calibration