High Momentum Electron Research Articles

Femtosecond Single Cycle Pulses Enhanced the Efficiency of High Order Harmonic Generation.

High-order harmonic generation is a nonlinear process that converts the gained energy during light-matter interaction into high-frequency radiation, thus resulting in the generation of coherent attosecond pulses in the XUV and soft x-ray regions. Here, we propose a control scheme for enhancing the efficiency of HHG process induced by an intense near-infrared (NIR) multi-cycle laser pulse. The scheme is based on introducing an infrared (IR) single-cycle pulse and exploiting its characteristic feature that manifests by a non-zero displacement effect to generate high-photon energy. The proposed scenario is numerically implemented on the basis of the time-dependent Schrödinger equation. In particular, we show that the combined pulses allow one to produce high-energy plateaus and that the harmonic cutoff is extended by a factor of 3 compared to the case with the NIR pulse alone. The emerged high-energy plateaus is understood as a result of a vast momentum transfer from the single-cycle field to the ionized electrons while travelling in the NIR field, thus leading to high-momentum electron recollisions. We also identify the role of the IR single-cycle field for controlling the directionality of the emitted electrons via the IR-field induced electron displacement effect. We further show that the emerged plateaus can be controlled by varying the relative carrier-envelope phase between the two pulses as well as the wavelengths. Our findings pave the way for an efficient control of light-matter interaction with the use of assisting femtosecond single-cycle fields.

Pothole-rich Ultrathin WO3 Nanosheets that Trigger N≡N Bond Activation of Nitrogen for Direct Nitrate Photosynthesis.

Nitrate is a raw ingredient for the production of fertilizer, gunpowder, and explosives. Developing an alternative approach to activate the N≡N bond of naturally abundant nitrogen to form nitrate under ambient conditions will be of importance. Herein, pothole-rich WO3 was used to catalyse the activation of N≡N covalent triple bonds for the direct nitrate synthesis at room temperature. The pothole-rich structure endues the WO3 nanosheet more dangling bonds and more easily excited high momentum electrons, which overcome the two major bottlenecks in N≡N bond activation, that is, poor binding of N2 to catalytic materials and the high energy involved in this reaction. The average rate of nitrate production is as high as 1.92 mg g-1 h-1 under ambient conditions, without any sacrificial agent or precious-metal co-catalysts. More generally, the concepts will initiate a new pathway for triggering inert catalytic reactions.

Pothole‐rich Ultrathin WO3 Nanosheets that Trigger N≡N Bond Activation of Nitrogen for Direct Nitrate Photosynthesis

AbstractNitrate is a raw ingredient for the production of fertilizer, gunpowder, and explosives. Developing an alternative approach to activate the N≡N bond of naturally abundant nitrogen to form nitrate under ambient conditions will be of importance. Herein, pothole‐rich WO3 was used to catalyse the activation of N≡N covalent triple bonds for the direct nitrate synthesis at room temperature. The pothole‐rich structure endues the WO3 nanosheet more dangling bonds and more easily excited high momentum electrons, which overcome the two major bottlenecks in N≡N bond activation, that is, poor binding of N2 to catalytic materials and the high energy involved in this reaction. The average rate of nitrate production is as high as 1.92 mg g−1 h−1 under ambient conditions, without any sacrificial agent or precious‐metal co‐catalysts. More generally, the concepts will initiate a new pathway for triggering inert catalytic reactions.

Investigation by slow positron beam of defects in CLAM steel induced by helium and hydrogen implantation

Hydrogen and helium ion beams delivering different doses are used in the ion implantation, at room temperature, of China Low Activation Martensitic (CLAM) steel and the induced defects studied by Doppler broadening of gamma-rays generated in positron annihilation. Defect profiles are analysed in terms of conventional S and W parameters, measures of relative contributions of low and high-momentum electrons in the annihilation peak, as functions of incident positron energies E up to 30keV. The behaviours of the S–E, W–E and S–W plots under different implantation doses indicate clearly that the induced defect size has obvious variation with depth, taking values that interpolate between surface and bulk values, and depend mainly on helium ion fluences. The S–W plot indicates that two types of defects have formed after ion implantation.

Recollision Excitation, Electron Correlation, and the Production of High-Momentum Electrons in Double Ionization

The production of high-momentum electrons in double ionization of helium by near-infrared lasers is investigated using three-dimensional classical ensembles. The nucleus' role is examined by systematically adjusting the nuclear potential. The primary source of the high-energy electrons is found to be backscattering off the nucleus at recollision. Recollision excitation with backscattering of the unbound electron is found to be especially important. It is shown that recollision excitation with ionization before the next field maximum can lead to a correlated electron pair.

Depth-dependent positronium formation in γ-irradiated polymers after 30-month aging

Depth-dependent positronium (Ps) formation and annihilation study of Co-60 γ-irradiated polyethylene and polytetrafluorethylene were performed. After 30-month aging, no free radicals effect on Ps formation was detected for the studied polymers. Efficient trapping of back-diffusion positrons was observed in heavily irradiated samples, where Ps formation probability near polymer surface and in the bulk turns to be equivalent to each other. Results shown that positron annihilation Doppler broadening in polytetrafluorethylene is prevailingly influenced by high momentum electrons of fluorine atoms instead of Ps intensity.

Average Low and High Momenta in Singly-Excited 1snl States of the He Atom

Average low and high > momenta are examined for 28 singly-excited 1snl singlet and triplet states (0 ≤ l decreases with increasing n and decreasing l, while the high momentum > is nearly equal to 16/(3π)≅1.698, independent of n and l. Comparing the low and high > momenta with the average electron momentum of a hydrogenlike atom, we show that an electron with a lower momentum behaves like an nl electron of the hydrogen atom and another electron with a higher momentum behaves like the 1s electron of the helium cation. A decomposition of an uncertainty product finds that the largest contribution to comes from an outer electron and a high momentum electron, where and are the usual average electron radius and momentum, respectively.

Analyzing the high-momentum part of positron annihilation Doppler spectra with a single germanium detector

We present a novel way for recording and mathematically analyzing the Doppler-broadening of the 511 keV annihilation line to obtain the momentum distribution of the annihilating electron–positron pair. A necessary precondition is employing either background free positron beams or e.g. a 68Ge-source, instead of the widely used positron emitter 22Na. The 68Ge-source emits positrons with an end-point energy of about 1.9 MeV, while the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low. Compared to 22Na, this results in a significant lower background in the photo peak. Employing such a 68Ge-source allows an analysis of the high-momentum part of the annihilation radiation. However, this part contains valuable chemical information about the annihilation site of the positron, originating from high momentum electrons. Providing an adequate mathematical analysis in determining the remaining background and its subsequent subtraction, we are able to obtain the contribution of these high electron momenta to the annihilation peak using only a single Ge-detector. After background subtraction, the same information contained in the momentum range up to 35×10 −3 m 0 c can be extracted by analyzing spectra from metals and semiconductors as if measured employing a coincidence setup with two Ge-detectors. With the method presented here it is possible to obtain the whole information of a positron annihilation Doppler spectrum employing just a single Ge-detector.

Positron Annihilation Spectroscopy Study of Interfacial Defects Formed by Anodic Oxidation of Aluminum

Positron annihilation spectroscopy (PAS) measurements were carried out to characterize open-volume defects associated with anodic oxidation of aluminum. The annihilation fractions with low and high momentum electrons (S and W spectral lineshape parameters, respectively) of the annihilation photopeak were determined, as a function of the positron beam energy. A subsurface defect layer, containing nanometer-scale voids in the metal near the metal/oxide film interface, was found after oxide growth, and was shown to contain new voids created by anodizing. Such interfacial voids in the metal are of interest because of their possible role as corrosion initiation sites. The S parameter characterizing the defect-containing layer was obtained by simulation of the S-energy profiles. On samples with two different surface conditions, remained constant at its initial value during anodizing. Because is related to the void volume fraction in the interfacial metallic layer containing the voids, that result suggests that formation of metallic voids, and their subsequent incorporation into the growing oxide layer, occurred repeatedly at specific favored sites. © 2003 The Electrochemical Society. All rights reserved.

Positron annihilation study of defects and Si nanoprecipitation in sputter-deposited silicon oxide films

Positron annihilation spectroscopy was conducted to study defects and Si nanoprecipitation in sputter-deposited silicon oxide films. For as-deposited SiO 0.9 and SiO 1.9 films, Doppler broadening spectra are strongly influenced by the type of paramagnetic defects ( P b or E ′). However, the disappearance of these defects after annealing at 1050 °C in a vacuum cannot account for the corresponding change of the Doppler broadening. In annealed film of SiO 1.9, positronium formation is reduced from defect-free SiO 2 because of the presence of Si precipitates; meanwhile, in annealed film of SiO 0.9 with nanocrystalline Si (nc-Si), positron annihilation with high momentum electrons is enhanced as a result of efficient positron trapping at the nc-Si surface with oxygen.

Measurement of the branching fraction for inclusive semileptonicBmeson decays

A largely model-independent measurement of the inclusive electron momentum spectrum and branching fraction for semileptonic decays of B mesons is presented based on data recorded at the Υ(4S) resonance with the BABAR detector. Backgrounds from secondary charm decays are separated from prompt B decays using charge and angular correlations between the electron from one B meson and a high momentum electron tag from the second B meson. The resulting branching fraction is B(B→Xeν)=[10.87±0.18(stat)±0.30(syst)]%. Based on this measurement we determine the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |Vcb|. Received 14 August 2002DOI:https://doi.org/10.1103/PhysRevD.67.031101©2003 American Physical Society

Defects in silicon-on-insulator wafers and their hydrogen interaction studied by monoenergetic positron beams

Vacancy-type defects in separation-by-implanted oxygen wafers were probed using monoenergetic positron beams. We measured the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positrons. The species of the defects in Si-on-insulator (SOI) layers were identified as oxygen clusters or small oxygen precipitates, and the positrons are considered to be trapped by open spaces adjacent to such defects. The mean size of the open spaces was estimated to be larger than that of a hexavacancy. After annealing in a hydrogen atmosphere at 600 °C, both the lifetime of positrons trapped by the defects and the annihilation probability of positrons with high-momentum electrons decreased. These facts were attributed to the trapping of hydrogen atoms by the open spaces. The defects in bond-and-etchback SOI wafers and their interaction with hydrogen were also studied.

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements

Doppler-broadening measurements of the electron–positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference material. The positron lifetime technique has been used to detect the presence of positron traps in all the samples. The change in the high-momentum part of the annihilation line due to positron trapping is illustrated. The measured data are in a good qualitative agreement with recent theoretical calculation and constitute the most complete measurement series, up to now, to establish a future data-base for positron annihilation spectroscopy.

Measurement of the $\rm B^+$ and $\rm B^0$ lifetimes and search for CP(T) violation using reconstructed secondary vertices

The lifetimes of the $\rm B^+$ and $\rm B^0$ mesons, and their ratio, have been measured in the OPAL experiment using 2.4 million hadronic $\rm Z^0$ decays recorded at LEP. $\rm Z^0 \rightarrow \rm b\overline{b}$ decays were tagged using displaced secondary vertices and high momentum electrons and muons. The lifetimes were then measured using well-reconstructed charged and neutral secondary vertices selected in this tagged data sample. The results are \begin{eqnarray*} \tau_{B^+} & = & 1.643 \pm 0.037\pm 0.025 \rm\,ps \tau_{B^0} & = & 1.523 \pm 0.057 \pm 0.053 \rm\,ps \tau_{B^+}/ \tau_{B^0} & = & 1.079 \pm 0.064 \pm 0.041 \,, \end{eqnarray*} where in each case the first error is statistical and the second systematic. A larger data sample of 3.1 million hadronic $\rm Z^0$ decays has been used to search for CP and CPT violating effects by comparison of inclusive b and $\rm\overline{b}$ hadron decays. No evidence for such effects is seen. The CP violation parameter $\rm Re(\epsilon_B)$ is measured to be \[ \rm Re(\epsilon_B) = 0.001 \pm 0.014 \pm 0.003 \] and the fractional difference between b and $\rm\overline{b}$ hadron lifetimes is measured to be \[ \left(\frac{\Delta\tau}{\tau}\right)_{\rm b} \equiv \frac{\rm\tau (b \mbox{hadron}) - \tau \mbox{(\rm\overline{b} hadron)}} {\rm\tau (average)} = -0.001\pm 0.012\pm 0.008 \,. \]

A measurement of $R_{\rm b}$ using a double tagging method

The fraction of ${\rm Z^0 \to b\overline b}$ events in hadronic ${\rm Z^0}$ decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The ${\rm Z^0 \to b\overline b}$ decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of \[ R_{\rm b} \equiv \frac{\sigma (\rm e^+e^- \rightarrow{\rm b\overline b} )} {\sigma (\rm e^+e^- \rightarrow hadrons)} = 0.2178 \pm 0.0011 \pm 0.0013 \] was obtained, where the first error is statistical and the second systematic. The uncertainty on $R_{\rm c}$ , the fraction of ${\rm Z^0 \to c \overline c}$ events in hadronic ${\rm Z^0}$ decays, is not included in the errors. The dependence on $R_{\rm c}$ is \[ \frac{\Delta R_{\rm b}}{R_{\rm b}} = -0.056 \frac{\Delta R_{\rm c}}{R_{\rm c}} , \] where $\Delta R_{\rm c}$ is the deviation of $R_{\rm c}$ from the value 0.172 predicted by the Standard Model. The result for $R_{\rm b}$ agrees with the value of $0.2155 \pm 0.0003$ predicted by the Standard Model.

Heat-treatment-induced defects in low-resistivity silicon

Czochralski-grown silicon has been investigated with doping levels up to 5×1018 cm−3 of boron or phosphorus by positron annihilation. For the highly boron-doped samples vacancies could be created upon heat treatment in the 700–1000 °C range up to a concentration of 1017 vacancies cm−3. Reducing either the boron concentration or the interstitial oxygen concentration (by prolonged heat treatment at 1200 °C) reduced the amount of created vacancies. No vacancies were detected in highly phosphorus-doped materials. Combined positron-lifetime and Doppler-broadening measurements indicate that the vacancies are essentially of monovacancy nature and have impurities close by which contribute with high-momentum electrons. Isothermal heat treatments at 700, 750, 900, and 950 °C indicate a reaction of the form X+Y⇄Z, where Z contains a vacancy. The forward reaction has an activation enthalpy of 2.7 eV, while the back reaction has the value 3.0 eV. It is suggested that X corresponds to substitutional boron and Y to interstitial oxygen.

Performance of the CLEO d E/d x system

We describe the performance of the pressurized d E/d x system of the CLEO detector. We show resolution and relativistic rise measurements. The system has been used to identify low momentum hadrons, high momentum electrons and measure the pion fraction at high momentum.