Slow positrons and thermal Ps production using thin foils

A one-dimensional diffusion model for positrons implanted in Cu and Al single crystal thin foils was solved. The fraction of thermalized positrons reaching the surface was obtained for various film thicknesses as a function of the incident positron energy in a transmission-mode geometry. The results indicate that for foil thicknesses of the order of the diffusion length (1000 Å) the reemission fraction is roughly half the fraction of a semi-infinite crystal. For thicker foils the annihilation of the positrons in the bulk and smearing effects due to a higher implantation energy effectively reduce the surface reemission. In this paper it is shown that thin foils can also be used to efficiently produce low-energy (from thermal to a few eV) Ps beams, physically separated from the primary positron beam.     

Second bound state of PsH

The existence of a second bound state of PsH that is electronically stable and also stable against positron annihilation by the normal 2gamma and 3gamma processes is demonstrated by explicit calculation. The state can be found in the ;{2,4}S;{o} symmetries with the two electrons in a spin-triplet state. The binding energy against dissociation into the H(2p)+Ps(2p) channel was 7.03 x 10;(-4) hartree. The dominant decay mode of the states will be radiative decay into a configuration that autoionizes or undergoes positron annihilation. The NaPs system of the same symmetry is also electronically stable with a binding energy of 1.514 x 10;(-3) hartree with respect to the Na(3p)+Ps(2p) channel.     

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).     

Positron states between positronium and “free positron” states in condensed matter

Positronium (Ps), the bound state of an electron and a positron, is formed in many insulating solids and liquids. Recently, several cases of a transition from a Ps state to a non-Ps state have been observed. This has been found for liquid CS₂ if the temperature is decreased from about –40 °C to the melting point, –110.8 °C. In some solid aromatic hydrocarbons, such as naphthalene, biphenyl and p-terphenyl, similar transitions occur upon decreasing the temperature. These transitions are unique in physics since Ps is the only bound state of two light particles in normal low-energy physics. Measurements of the positron lifetime, angular correlation of the annihilation radiation, Ps chemical reactions, and Zeeman effect of Ps give information on the transitions.Deceased.     

真空电子偶素原子的产生

用慢正电子束入射固体靶表面,通过测量湮没光子能谱随靶温度和入射慢正电子能量的变化,用“峰法”确定慢正电子产生电子偶素原子的转换率。转换率依赖于靶材料、靶温度和入射慢正电子能量。对材料锗转换率可达80％。 关键词：     

Correlations between electron and positron workfunctions on copper surfaces

We have performed contact potential difference measurements on low-index faces of copper in ultrahigh vacuum using positrons as positive test particles in a retarding field analyzer. For negative positron affinity surfaces bombarded with keV positrons we also measured energy distributions of reemitted slow positrons and found them to sharply peaked in energy about a value which we label ?φ⁺. Both adsorbing sulfur on a Cu(111) sample and raising its temperature cause changes in φ⁺ which are equal and opposite to the contact potential change of the sample, i.e. the electron workfunction change. This result is in complete accordance with φ⁺ being a measure of the negative positron workfunction of the sample and high temperature or adsorbates inducing a change only in the electrostatic surface dipole layer.     

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.     

Influence of surface orientation and defects on early-stage oxidation and ultrathin oxide growth on pure copper

We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300?K and 600?K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600?K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surface defects. (110) shows modest oxidation at 300?K but the highest oxidation is observed at 600?K. By surface disorder and reconstruction, the oxide layer is produced continuously. The (111) surface is sensitive to ambient temperature and surface defects, showing that surface reconstruction is a key element for further oxidation. The charge distribution of oxidized Cu atoms indicates multiple groups of stoichiometric oxides, while the fraction of CuO-like characteristics increases significantly on the (110) and (111) orientations at higher temperature (600?K). The energetics and mechanisms of oxidation on Cu metal substrates at the nanoscale are discussed in detail, and comparisons with available experimental and other theoretical studies are presented wherever possible.     

Triphenylene adsorption on Cu(111) and relevant graphene self-assembly

Investigations on adsorption behavior of triphenylene(TP) and subsequent graphene self-assembly on Cu(111) were carried out mainly by using scanning tunneling microscopy(STM).At monolayer coverage,TP molecules formed a longrange ordered adsorption structure on Cu(111) with an uniform orientation.Graphene self-assembly on the Cu(111) substrate with TP molecules as precursor was achieved by annealing the sample,and a large-scale graphene overlayer was successfully captured after the sample annealing up to 1000 K.Three different Moire patterns generated from relative rotational disorders between the graphene overlayer and the Cu(111) substrate were observed,one with 40 rotation between the graphene overlayer and the Cu(111) substrate with a periodicity of 2.93 nm,another with 70 rotation and 2.15 nm of the size of the Moire supercell,and the third with 100 rotation with a periodicity of 1.35 nm.     

Photoemission of positronium from Si

We have observed that the amount of positronium (Ps) emitted from the surface of p-Si(100) is substantially increased if the sample is irradiated with 532 nm laser light just prior to the implantation of positrons. The energy of the emitted Ps has a constant value of ～0.16 eV and is independent of the Si temperature and the applied laser fluence, while the photoemission yield depends on both of these parameters. These observations are consistent with Ps production via a previously observed excitonlike positron surface state that is populated in response to the production of electron-hole pairs in the Si. Possible applications of Ps photoemission include probing surface electron dynamics on Si, the generation of ultrashort Ps or positron pulses using ps lasers, and efficient production of Ps in cryogenic environments.     

Measurement of the CP asymmetry and branching fraction of B0-->rho0K0

We present a measurement of the branching fraction and time-dependent CP asymmetry of B(0)-->rho0K0. The results are obtained from a data sample of 227x10(6) Upsilon(4S)-->BB decays collected with the BABAR detector at the PEP-II asymmetric-energy B factory at Stanford Linear Accelerator Center. From a time-dependent maximum likelihood fit yielding 111+/-19 signal events, we find B(B(0)-->rho0K0)=(4.9+/-0.8+/-0.9)x10(-6), where the first error is statistical and the second systematic. We report the measurement of the CP parameters S(rho)(0)K(0)S=0.20+/-0.52+/-0.24 and C(rho)(0)K(0)S=0.64+/-0.41+/-0.20.     

Long-range electronic interactions at a high temperature: bromine adatom islands on Cu(111)

Long-range electronic interactions between Br adatom islands, which are formed at approximately 600 K, on Cu(111) are mediated by substrate surface-state electrons at that elevated temperature. Using scanning tunneling microscopy at 4 K, we have quantified nearest neighbor island separations and found favored spacings to be half-multiples of the Fermi wavelength of Cu(111). The strong interaction potential and decay length of the interisland interactions are discussed in terms of the interaction of Br with the substrate surface state.     

New mechanism for positronium formation on a silicon surface

We describe experiments in which positronium (Ps) is emitted from the surface of p-doped Si(100), following positron implantation. The observed emission rate is proportional to a Boltzmann factor exp{-E(A)/kT}, which is dependent on the temperature T of the sample and a characteristic energy E(A)=(0.253±0.004) eV. Surprisingly, however, the Ps emission energy has a constant value of ～0.16 eV, much greater than kT. This observation suggests the spontaneous emission of energetic Ps from a short-lived metastable state that becomes thermally accessible to available surface electrons once the positron is present. A likely candidate for this entity is an electron-positron state analogous to the surface exciton observed on p-Si(100) c(4×2) by Weinelt et al. [Phys. Rev. Lett. 92, 126801 (2004)].     

Positronium formation from silver

Characterization of a positronium (Ps) gas target to be used for the charge conjugate reaction for antihydrogen formation [1], namely proton + Ps-atom → hydrogen + positron, has been performed [2–4]. In the above collision the Ps target utilised is produced efficiently on the transmission side of a hot thin (1900 Å) Ag(100) foil with 12% efficiency at 800 K foil temperature, with a mean velocity of thermally activated Ps of 1.2×10⁵ m/s and with a maximum kinetic energy of prompt emitted Ps of 1.5±0.2 eV.     

Improved slow-positron yield using a single crystal tungsten moderator

A well-annealed W(110) single crystal was used as a fast-to-slow positron moderator. The measured moderator efficiency at room temperature using a⁵⁸Co positron source in the backscattering geometry is =(3.2±0.4)×10^–3, roughly a factor of three better than for the best previously reported Cu(111)+S moderator. We find a stable positron moderation efficiency over a period of several weeks when maintained at pressures around 10^–9 Torr and an energy spreadE = 0.7 eV of the emitted slow positrons. An initial attempt was made to fabricate a hybrid Cu on W(110) moderator, which yielded of about 1.2×10^–3 after annealing.     

Formation of nanoislands in the course of copper deposition on a Cu(111)-(9 × 9)-Ag surface

The process of copper deposition on a structured Cu(111)-(9 × 9)-Ag surface, which represents a (9 × 9) loop dislocation network, is studied by scanning tunneling microscopy. It is found that, when the substrate temperature is 100 K and the copper coverage is 0.1–0.4 of a monolayer, islands of a size no greater than 50 Å are formed at the Ag/Cu(111) interface. The islands remain stable as the sample is heated to room temperature. The shape and boundaries of the nanoislands follow the initial surface superstructure and are determined by the nonuniformity of the interaction of the upper silver layer with the copper substrate. The mechanism of island formation and the origin of their stability are explained in terms of the atom exchange between the adsorbate and substrate.     

Direct observation of a (3 x 3) phase in alpha-Pb/Ge(111) at 10 K

We have investigated the recently reported structural phase transition at low temperature (LT) for alpha-Pb/Ge(111) [from a (3 x 3) symmetry to a disordered phase] using scanning tunneling microscopy (STM). By tracking exactly the same surface regions with atomic resolution while varying the sample temperature from 40 to 140 K, we have observed that substitutional point defects are not mobile, in clear contrast to previous assumptions. Moreover, STM data measured at the lowest temperatures ever reported for this system (10 K) show that while filled-state images display the apparent signature of a glassy phase with no long-range order, in empty-state images honeycomb patterns with (3 x 3) periodicity, and not distinguishable from data measured at much higher temperatures, are clearly resolved. These new observations cast serious doubts on the nature and/or on the existence of a disordered phase at LT.     

Positronium formation in Debye plasma

Positronium (Ps) formation during positron-hydrogen collisions in Debye plasma has been studied in detail by using the second-order distorted-wave approximation with the inclusion of the adiabatic dipole polarization potential. Reliable results have been reported at several incident positron energies in the range from the Ps formation threshold to 500 eV for different values of the Debye screening parameter μ. Resonances for S-, P- and D-wave partial Ps formation cross sections appear to exist for 0.03 ≤ μ ≤ 0.3. Interesting structures in the differential cross sections are displayed in the surface plots. It is reported here for the first time that there is a huge probability of Ps formation in dense plasma corresponding to Debye screening parameter μ ≥ 0.3.     

Al-4%Ag合金中Ag原子偏析的低温正电子湮没参数研究

分别研究了823 K淬火处理和20%形变量的Al-4%Ag低温下Ag析出物对正电子的捕获行为的变化。采用正电子湮没寿命谱(PALS)技术和符合多普勒展宽能谱(CDBS)在温度范围10~293 K内对其进行表征。多普勒展宽能谱结果表明2种样品中均存在Ag析出物。正电子寿命谱的解谱结果中的各组分给出了Ag析出物随测量温度的变化规律。在170 ~273 K之间,正电子湮没行为具有较强的温度依赖性。但对于两个具有不同类型缺陷的样品,在低于170 K时观察到样品中Ag析出物捕获正电子能力出现了差异。随着测量温度的降低,淬火样品中的Ag析出物的正电子寿命和强度基本不变。在低于170 K的测量中,形变样品中的Ag析出物对正电子的捕获能力仍旧存在着较强的温度依赖性,但是变化幅度在逐渐减弱。当测量温度提升到室温(273~293 K),越来越多的正电子从Ag析出物中逃逸,逐渐回到自由状态或被其他深陷阱所捕获,失去了对温度的依赖性。     

Application of positron beams to the study of positronium-forming solids

Doppler broadening of annihilation radiation (DBAR) and positron annihilation lifetime spectroscopy (PALS) have been successfully applied to the study of positronium (Ps)—forming amorphous solids such as polymers and silicon oxide in the bulk. Implementing depth selectivity to DBAR and PALS by combining them with variable-energy positron beams considerably broadens their applicability. Variation of incident positron energy over a wide range enables depth-profiling, whereas tuning of the beam energy enables the studies of surfaces, interfaces and thin films. In this paper, we discuss fundamentals and applications of energy variable DBAR and PALS for Ps—forming polymers and silicon oxide.