Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.     

Unusual temperature dependence of the positron lifetime in a polymer of intrinsic microporosity

The performance of polymeric membranes for gas separation is mainly determined by the free volume. Polymers of intrinsic microporosity are interesting due to the high abundance of accessible free volume. We performed measurements of the temperature dependence of the positron lifetime, generally accepted for investigation of free volume, in two polymers of intrinsic microporosity (PIM‐1 and PIM‐7) in the range from 143 to 523 K. The mean value of the free volume calculated from the ortho‐positronium lifetime is in the range of typical values for high free volume polymers. However, the temperature dependence of the local free volume is non‐monotonous in contrast to the macroscopic thermal expansion. The explanation is linked to the spirocenters in the polymer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Positron diffusion and restoration of local symmetry breaking induced by positrons in liquid metals

We present the effective Lagrangian with spontaneously broken density (the hedgehog-like fluctuation) and the massive internal gauge fields, and propose one explanation for the increase of the positron diffusion length in the liquid metal phase with temperature. From the view point of the restoration of the spontaneously broken density around the positron, the positron diffusion length data in the liquid phase of Bi metal are analyzed.     

On the annihilation rate of positronium in dense rare gases

The dependence of the positronium decay rate on the gas density is considered. It is known that the positronium can be localized in a dense gas with the formation of a cavity “bubble” around the positronium. It results in a significant delay of the annihilation. The paper containes the quantitative theory of this phenomenon. A comparison is made with the experimental data for He⁴, He³, Ne.     

Spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces

Recently, the spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces was observed. In the present work, the emission of these ions in ultra-high vacuum has been studied and the long-term stability of the emission efficiency has been investigated.     

Positron mobility in polyethylene in the 60–400 K temperature range

We have determined the positron mobility (₊) in polyethylene samples (67.2% crystalline, glass transition temperatureT _g=151 K) in the 64–400 K temperature range by Doppler shift measurements. A method based on the simulataneous observation of two lines from¹³³Ba and¹³⁷Cs radioactive sources together with the positron annihilation line, was employed to measure the Doppler shift of the 511 keV line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and theS parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation. ₊ at 64 K is 31.7±0.8 cm² V^–1 s^–1 then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (₊=26.9±0.8 cm² V^–s^–). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K. From 250 K to 377 K, ₊ increases and reaches the value of 38.4±1.0 cm² V^–1s^–1. The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).     

Positronium–oxygen interactions in polytrimethylsilylpropine membranes

Received: 25 April 1997/Revised version: 1 August 1997     

Contribution of valence s+p type electrons to the high momentum components of the electron-positron momentum density in gold

It is generally believed that the high momentum components (HMCs) of the angular correlation of annihilation radiation or Doppler broadening spectra map the positron annihilation with predominately core electrons, containing only a small fraction coming from localised d+f-type valence electrons. In present work, we study how far the contribution of valence electrons to the HMCs of the electron-positron (e-p) momentum density can be neglected. Calculations are performed for gold within both the independent particles model (IPM) and including the e-p correlation effects non-locally, using the weighted density approximation (WDA). In particular, the HMCs due to valence s+p-type electrons are compared with their core and d+f-type valence counterparts.     

Positron annihilation in vacancies at grain boundaries in metals

The behavior of vacancies in selected coherent grain boundaries (GBs) in Fe and Ni is studied by means of molecular dynamics simulations. Corresponding positron lifetimes are calculated using the atomic superposition method. There is a difference between the vacancy behavior in Fe and Ni in dependence on temperature. In Ni, vacancies at GBs appear to diminish substantially their free volume (and lifetime) with the increasing temperature, which can be attributed to ‘vacancy delocalization’. Contrary, GB vacancies remain stable up to apparently higher temperatures in Fe.     

Two-dimensional melting of physisorbed submonolayers of argon and nitrogen

We have studied two-dimensional argon and nitrogen physisorbed on grafoil by conducting positron lifetime and Doppler broadening measurements as functions of adsorbate coverage and temperature. Positron lifetime and Doppler broadening parameters show turnarounds near one-half monolayer coverages at 77 K. The s parameter of the Doppler broadening spectra measured for one-half monolayer coverages of argon and nitrogen increases with temperature across the melting phase transition. We discuss the observed coverage and temperature effects following models based on Kosterlitz-Thouless-Nelson-Halperin-Young theory of two dimensional melting and positron localization in surface defects.     

Positron mobility in anthracene

We have measured the positron mobility in a sample of scintillation grade anthracene at two temperatures. We obtain at 300 K: =(26.0±0.9±2.6) cm²V^–1s^–1 and at 77 K: =(33.4±1.1±3.3) cm²V^–1s^–1, where the first error estimate is statistical and the second is systematic. We have also made preliminary measurements on a highly purified sample that yields =(130±3±20) cm² V^–1 s^–1 at 300 K. The data are consistent with the hypothesis that the positron is scattered from both impurities and acoustic phonons in the first sample, and predominantly from photons in the second. It appears that positrons in pure anthracene crystals are delocalized and have a mean free path of about 85 Å at room temperature.     

Analysis of positron diffusion data

In this paper we present and discuss experimental methods to determine the positron diffusion coefficient from slow positron beam measurements. We also evaluate the use of the annihilation line Doppler-broadening technique in positron diffusion measurements, as compared on the more commonly used method of positronium fraction. The effects of incomplete positron thermalization and uncertainties of the positron implantation profile at low-positron incident energies to the measured data are discussed. We apply the presented methods to the model case of A1(110) system in the temperature range from 20 to 500 K. This data shows that Doppler-broadening and positronium fraction measurements give consistent results for the positron diffusion coefficient in A1(110), where D₊(300 K) = 1.7(2) cm²/s with the temperature dependence D₊ T ^–0.62(3).     

High-momentum analysis in Doppler spectroscopy

Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence.By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. ⁶⁸Ge, instead of the widely used positron emitter ²²Na. The ⁶⁸Ge-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low.When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 × 10⁻³m₀c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors.     

Diffusion of positrons to surfaces

Low-energy positrons implanted into a crystal can diffuse to the surface where they may be detected, for instance, by observing the formation of positronium. We describe the positron motion by a one-dimensional diffusion equation, and discuss the measurement of vacancy activation energies, positron diffusion constants, and positron stopping profiles.     

Performance of the Beijing pulsed variable-energy positron beam

In order to study the depth-dependent characteristics of open-volume defects in thin surface layers, the variable-energy positron lifetime spectroscopy (VEPLS) has been enabled by pulsing a continuous positron beam. The buncher is a quarter-wave coaxial resonator and the RF-signal is fed in by a coupling loop with a frequency of 149.89 MHz and the reflection factor of 0.05 measured by a Network Analyzer. Three synchronic signals with their phases and amplitudes adjusted independently are supplied for start signal of the positron lifetime measurement and the power signal by an electronic system. The stop signal is derived from a detector, a BaF₂ scintillator coupled to a photomultiplier-tube (Hamamatsu). The time resolution of 295 ps (FWHM) was achieved for a Kapton film and a Ti sample at positron energies in the range between 1 keV and 30 keV.     

Determination of the positron diffusion length in Kapton by analysing the positronium emission

Doppler profile spectroscopy and Compton-to-peak ratio analysis have been used to study the positronium (Ps) emission from the Kapton surface as a function of the positron implantation energy E.Two different positions for the sample have been performed in the experiment.In the first case the sample and the Ge-detector are perpendicular to the positron beam. With this geometry the emission of para-positronium (p-Ps) is detected as a narrow central peak.In the second case, by rotating the sample 45° with respect to the beam axis, the emission of p-Ps is detected as a blue-shifted fly away peak. The implantation of the positrons is described by the Makhov profile, where we used the modified median implantation for polymers as given by Algers et al. [J. Algers, P. Sperr, W. Egger, G. Kögel, F.H.J. Maurer, Phys. Rev. B 67 (2003) 125404].Thermalised positrons can diffuse to the surface and may pick up an electron to be emitted as Ps. We found a thermal and or epithermal positron diffusion length L₊ = 5.43 ± 0.71 nm and L₊ = 5.51 ± 0.28 nm correspondingly for both cases, which is much more than the one found by Brusa et al. [R.S. Brusa, A. Dupasquier, E. Galvanetto, A. Zecca, Appl. Phys. A 54 (1992) 233]. The respective efficiency for the emission of Ps by picking up an electron from the surface is found to be f_pu = 0.247 ± 0.012 and f_pu = 0.156 ± 0.003.     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.     

Comment on “Amorphous hydrogenated silicon studied by positron lifetime spectroscopy”

H.-E. Schaefer, R. Wurschum, R. Schwarz, D. Slobodin, and S. Wagner [Appl. Phys. A40, 145 (1986)] have recently assigned positron lifetimes to various vacancy sizes. In this comment we will show that their discussions are ill founded.     

Emission of positronium in a nanometric PMMA film

Positron beam experiments have been performed for the first time on a self-supporting polymethyl metacrylate (PMMA) film of 310 nm-thick made by spin coating. The positronium (Ps) emission from the PMMA surface is studied as a function of the positron implantation energy by using Doppler profile spectroscopy and Compton-to-peak ratio analysis. When the sample and the Ge-detector are perpendicular to the positron beam, the emission of para-positronium (p-Ps) is detected as a narrow central peak. By rotating the sample 45° with respect to the beam, the emission of p-Ps is detected as a blue-shifted fly-away peak. The bulk Ps fraction, the efficiency for the emission of Ps by picking up an electron from the surface, and the diffusion lengths of positrons (thermal and or epithermal), p-Ps and ortho-positronium (o-Ps) are obtained.     

Study of silica aerogel grain surfaces by using a positron age–momentum correlation technique

The effect of heat treatment on silica aerogel has been studied by a positron age–momentum correlation technique and infrared measurement. A difference was observed between the momentum distributions of the electrons on the first layers of the silica aerogel grain surfaces and the electrons in the grains in an as-supplied sample, but not in the sample heat treated at 800 °C. A large change in the S parameter for the momentum distribution of the electrons on the first layer occurs around 200 °C. This change correlates well with that of the infrared spectra, which show oxidation of the methoxyl groups at temperatures above around 200 °C. This correlation reveals that those groups are mainly located on the first layer of the silica grains. Received: 13 November 2000 / Accepted: 23 July 2001 / Published online: 30 October 2001