Electron Pickup Research Articles

Interfacial tuning in FeP/ZnIn2S4 Ohm heterojunction: Enhanced photocatalytic hydrogen production via Zn-P charge bridging

Interfacial regulation is key to photocatalytic performance, yet modulating interfacial charge transfer in heterostructures remains challenging. Herein, a novel nanoflower-like FeP/ZnIn2S4 Ohm heterostructure is first designed, with Zn atoms in ZnIn2S4 (ZIS) acting as potential anchoring sites around P atoms, forming liganded Zn-P bonds. Combining 1D FeP nanowires and 2D ZIS nanosheets enhances the mobility of photogenerated electrons. The synergistic chain-type “electron pickup” mechanism of the Ohm heterojunction coupled with the Zn-P bond speeds up electron transport at the interface. The Ohm heterojunction initiates an internal electric field, creating a driving force to further transfer photogenerated electrons through the Zn-P rapid electron transport channel to FeP, which acts as a reservoir for active sites to release H2. The optimized FeP/ZIS demonstrates a remarkable H2 evolution rate at 4.36 mmol h−1 g−1, 3.6 times that of pristine ZIS. This work provides novel insights into optimizing photocarrier dynamics via interfacial microenvironment modulation.

Development of the antenna adjustable power coupler for 325 MHz superconducting cavities

In this paper, we present the multi-physics design and high-power tests of a fundamental power coupler for superconducting cavities. The fabricated power coupler was a coaxial capacitive-type coupler based on a conventional 318-inch Electronic Industries Alliance 50-Ω coaxial transmission line with a titanium nitride (TiN)-coated single ceramic window. The coupler had three diagnostic ports for vacuum, arc, and electron pick-ups to monitor radio frequency (RF) breakdowns. To reduce the thermal load required to reach the cryogenic temperature, the thermal intercepts were located at 4.5 K and 40 K for liquid helium use and at 4.5 K and 77 K for liquid helium and nitrogen use. A transition box for adjusting the antenna penetration depth and matching impedance was designed. A high-power test was performed with a 20 kW and 325 MHz solid-state power amplifier. The power coupler was connected to a rectangular test cavity with high vacuum and various measuring equipment, such as an arc detector, a power meter, and an electron pick-up probe. The interlock system under vacuum and arc instrumentations prevented the RF window from breaking the power coupler window during the high-power test. We conducted high-power tests for more than 12 h at 12 kW in a 325 MHz continuous wave mode to verify the performance of the designed power coupler.

Electron flux and pressure dynamic in the LHC vacuum pilot sector as a function of beam parameters and beam pipe properties

The Large Hadron Collider (LHC) is affected by the electron cloud (EC) phenomenon that can provoke beam instabilities, detrimental heat loads and pressure increases in the vacuum system. An innovative dedicated system called vacuum pilot sector (VPS) provides a continuous monitoring of the electron flux and of the pressure signals thanks to electron pickup and vacuum gauges. The VPS system is installed in a room temperature, field-free part of the LHC storage ring. Several technical surfaces, such as ex situ nonevaporable getter (NEG), amorphous carbon coating and copper, are simultaneously tested. The main outcomes of this study show that the EC signals have: (1) a linear dependence upon the number of bunches and upon the bunch population in the multipacting regime, (2) a multipacting threshold at a given bunch population, (3) a reduction under beam conditioning, (4) a strong dependence on the filling pattern and beam energy. The comparison between different surfaces shows that amorphous carbon coating reduces drastically the EC buildup, thanks to its low secondary electron yield (SEY) and photoelectron yield (PY), while copper and ex situ NEG coated surfaces suffer of EC multipacting, even after several months of operation. The multipacting rate coefficients are higher for copper than for ex situ NEG, as predicted from the SEY estimation. Other detailed experimental observations are discussed in this paper.

An estimate of the dust pickup current at Enceladus

We demonstrate that the acceleration of submicron dust originating at Enceladus by a reduced co-rotating E-field is capable of creating a dust pickup current perpendicular to the magnetic field with values ranging from 3 to 15kA (depending upon the effective grain charge). Such a current represents a new contribution to the total pickup current in the region. As such, we suggest that dust pickup currents, along with ion and electron pickup currents, are all active within the plume.

Simulations of pickup ion mediated quasi‐perpendicular shocks: Implications for the heliospheric termination shock

The microstructure of the heliospheric termination shock and the accompanied local acceleration processes of both ions and electrons are investigated by utilizing one‐dimensional full particle‐in‐cell simulations for a variety of parameters. The relative pickup ion density is assumed to be 20–30%. The magnetic field and the shock potential profiles exhibit significant differences, since the former mostly reflects the dynamics of solar wind ions, whereas the latter is mainly sustained by the bulk motion of the reflected pickup ions in the extended foot. The discrepancy between the magnetic field profile and the potential profile increases with Alfvén Mach number. Most of the downstream thermal energy is gained by the pickup ions, while some heating of the solar wind ions and electrons occurs through the modified two‐stream instability excited in the extended foot. Self‐reformation can occur when the relative pickup ion density is 20% but is blurred when it becomes as large as 30%. Reformation is also suppressed if the local solar wind ion temperature in the extended foot is high, which can either be due to heating by the modified two‐stream instability or is already determined by the solar wind temperature far upstream. In all runs presented in this study no evidence for shock surfing acceleration of pickup ions could be found. Nonthermal particle acceleration occurs for oblique shocks. Electron (pickup ion) shock drift acceleration is evidenced when the shock angle is below 80° (60°).

DIFFERENCES OF LEPTONIC AND HADRONIC RADIATION PRODUCTION IN FLARING BLAZARS

In order to distinguish the leptonic from the hadronic origin of the non-thermal emission of a flaring blazar, we present a semi-analytical model that describes the temporal development of the emergent particles (i.e., photons and neutrinos) based on their leptonic and hadronic origins, respectively. The approach starts with the transport equation of the injected relativistic particles and takes spatial diffusion and continuous energy losses into account. On the one hand, a relativistic electron pick-up is considered, which leads to synchrotron, as well as external Compton emission; and on the other hand, a relativistic proton pick-up, which results in high-energy photons and neutrinos by inelastic proton-proton collisions. The temporal development of the emergent photon and neutrino intensities of blazar flares in hadronic and leptonic interaction scenarios are calculated, given useful predictions of flare durations and time lags between photons of different wavelength and high-energy neutrinos.

Time lags of flaring AGN

In order to distinguish the leptonic from the hadronic origin of the non-thermal emission of a flaring AGN we present a semi-analytical model that describes the temporal development of the emergent particles (i.e. photons and neutrinos) based on their leptonic and hadronic origin respectively. The approach starts with the transport equation of the injected relativistic particles and takes spatial diffusion and continuous energy losses into account. On the one hand, a relativistic electron pick-up is considered, that leads to synchrotron, as well as external Compton emission and on the other hand, a relativistic proton pick-up, which results in high energy photons and neutrinos by inelastic proton-proton collisions. The paper ends up in the temporal development of the emergent photon and neutrino intensities of AGN flares in hadronic and leptonic interaction scenarios and gives useful predictions of flare durations and time lags between photons of different wavelength and high energy neutrinos.

Electron capture induced dissociation of peptide ions: Identification of neutral fragments from secondary collisions with cesium vapor

In high-energy collisions between peptide dications and cesium vapor, cations picked up an electron, which led to N–C α bond breakage to give the characteristic c + and z + type fragments together with their complementary neutral fragments. Neutrals were converted to anions in secondary collisions with cesium. Peaks corresponding to z − anions dominate the product ion spectra, whereas c − anions could not with certainty be identified, instead O −, OH −, CN −, CNH −, C 2O −, and OCN − were formed. A fragment ion is also assigned to HCOO − formed from dissociation of z −. We ascribe the outcome of secondary collisions to the fact that a z fragment is a radical that when accepts an electron becomes a stable carbanion whereas electron capture to an even-electron c fragment produces a reactive radical anion that dissociates within the timescale of the experiment (few microseconds). Such an experimental scheme in which anions are measured implies that we are not limited to the study of dications; indeed, selective N–C α bond cleavages of peptide monocations were identified from negative fragment ion spectra that were again dominated by z − type ions. Neutral y fragments from collision-induced dissociation are also easily detected as y − ions after electron pickup in secondary collisions.

Lifetimes of C602− and C702− dianions in a storage ring

C60(2-) and C70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200+/-30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s.

The Stern–Gerlach polarized ion source for the Munich MP-tandem laboratory, a bright source for unpolarized hydrogen and helium ion beams as well

The design of our source of negatively charged, intense and brilliant DC beams of polarized hydrogen and deuterium ions was motivated by the excellent performance of the HERMES atomic beam source and the successful operation of a polarized ion source at TUNL. Deviating from their 30 K atomic beam technology we combine an 80 K atomic beam source (ABS) with subsequent ionization in an electron cyclotron resonance (ECR) plasma. In a separate unit, negative ions are obtained by successive two electron pickup from cesium in a vapour jet target. Our ABS provides a flux of 6.4 × 10 16 atoms / s for hydrogen and of about 5 × 10 16 atoms / s for deuterium into a compression tube of 10 mm diameter and 100 mm length. Polarized negative D - ⇒ / H - ⇒ ion beams of about 9 μ A / 15 μ A with an emittance of 20 π mm rad eV have been observed. After tandem acceleration we have polarized beam intensity of 2 μ A on target of the Q3D spectrograph. The measured vector polarizations of the D ⇒ and H ⇒ ion beams are 72% and 67%, respectively, in accordance with expectations from source operation data. Using an external gas inlet for the ECR region, intense beams of unpolarized 1 H - , 2 D - , 3 He - , and 4 He - ions are obtained.

Charge-exchange cross sections and beam lifetimes for stored and decelerated bare uranium ions

Charge-exchange cross sections and beam lifetimes are studied for decelerated bare uranium ions at the ESR storage ring. By deceleration from the initial energy of 358 MeV/u down to various energies as low as 49 MeV/u, i.e., far below the production energy of bare ionic species, the electron pickup cross sections were obtained for collisions with ${\mathrm{N}}_{2}$, Ar, ${\mathrm{CH}}_{4},$ and Kr gaseous targets. The measured cross sections and beam lifetimes are compared with the theoretical results for radiative and nonradiative electron capture. The present data, along with the theoretical approximations discussed, provide a solid basis for the estimation of beam lifetimes for decelerated bare high-$Z$ ions. Moreover, a normalization procedure is proposed, in which absolute total charge-exchange cross sections are derived by normalizing the simultaneously measured yield of radiative electron capture photons to rigorously calculated relativistic cross sections. This method, along with the unprecedented beam conditions at the ESR storage ring, allows a significant improvement of the accuracy of cross-section data.

Theory of proton neutralization based on the Kleber–Zwiegel model of screening electron pickup

The Kleber–Zwiegel (KZ) model for proton neutralization, in which some electron is picked up from the screening electron cloud around the proton, has been reformulated in terms of quantum field theory of solids. Using the reformulated KZ model, the cases of an Al (high electron density case) and a Cs (low electron density) metal are investigated within the Thomas–Fermi approximation for the screening effects. For high electron density metals, the resonant neutralization is a predominant process and it can be regarded as an electron pickup process from the screening electron cloud in the surface region. In contrast, for low electron density metals, it appears that the Auger neutralization, which is disregarded in the KZ model, cannot be neglected.

Installation of an on-line Lamb shift spin-filter polarimeter in the Triangle Universities Nuclear Laboratory atomic beam polarized ion sourcea)

An atomic beam polarized ion source, used heavily since 1989 for producing polarized H± and D± beams for experiments between 25 keV and 20 MeV, has been modified to accept a Lamb shift, spin-filter polarimeter. In this source, polarized ground-state H or D atoms enter an electron cyclotron resonance ionizer where they are stripped to produce an outgoing positive polarized ion beam. When negative ions are desired, cesium vapor is introduced into a downstream charge-exchange canal. The polarimeter, based on an atomic physics concept first developed to produce nuclear-spin-polarized beams at Los Alamos, is designed to monitor the polarization of 2S1/2 metastable H or D atoms emerging from the cesium canal. Metastable 2S1/2 atoms created by electron pickup in a collision with cesium are ‘‘filtered’’ by the polarimeter according to magnetic substate, as the magnetic field imposed on the polarimeter cavity is tuned between 53 and 61 mT. Photons produced by subsequent quenching of these filtered atoms to their ground state are monitored downstream by a phototube to reveal the magnetic substate population of the incident positive beam. To install the polarimeter cavity and phototube assembly, the existing polarized ion source was lengthened by 30 cm. Installation is complete, and comparisons with calibrated nuclear polarimeters have shown agreement to better than 0.023. Principles of operation, a description of the hardware, measurements for cross calibration, and impressions gained from its use are all presented.

Installation of an on-line Lamb–shift spin–filter polarimeter in the Triangle Universities Nuclear Laboratory atomic beam polarized ion source (abstract)a)

An atomic beam polarized ion source, used heavily since 1989 for producing polarized H± and D± beams for experiments between 25 keV and 20 MeV, have been modified to accept a Lamb–shift, spin–filter polarimeter. In this source, polarized ground-state H or D atoms enter an electron cyclotron resonance ionizer where they are stripped to produce an outgoing positive polarized ion beam. When negative ions are desired, cesium vapor is introduced into a downstream charge-exchange canal. The polarimeter, based on an atomic physics concept first developed to produce nuclear–spin-polarized beams at Los Alamos, is designed to monitor the polarization of 2S1/2 metastable H or D atoms emerging from this cesium canal. Metastable 2S1/2 atoms created by electron pickup in a collision with cesium are ‘‘filtered’’ by the polarimeter according to a magnetic substate, as the magnetic field imposed on the polarimeter cavity is tuned between 53 and 61 mT. Photons produced by subsequent quenching of these filtered atoms to their ground state are monitored downstream by a phototube to reveal the magnetic substate population of the incident positive beam. To install the polarimeter cavity and phototube assembly, the existing polarized ion source was lengthened by 30 cm. Installation is complete, its calibration has been checked, and initial impressions gained from its use are presented.

Can molecular ZEKE states be generated by Rydberg electron pickup?

We report experiments displaying simultaneous measurement of mass-selected delayed pulsed field ionization spectra of two isotopically different species in a common laser focus. The experiments show that charge transfer from a ZEKE excited species into high Rydberg states of surrounding ions although definitely observable is only a minor contribution to typical ZEKE spectra at normal low laser intensities and is below the level if ZEKE fragment spectra from up-pumped Rydbergs are recorded.

First Measurement of Helium on Mars: Implications for the Problem of Radiogenic Gases on the Terrestrial Planets

108 ± 11 photons of the martian He 584-Å airglow detected by the Extreme Ultraviolet Explorer satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 ± 10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 ± 20 K, result in a He mixing ratio of 1.1 ± 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He+ by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 × 105, 3 × 104, and 7 × 103 cm-2 sec-1, respectively. The lifetime of helium on Mars is 5 × 104 years. The He outgassing rate, coupled with the 40Ar atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances of K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function δ = δ0(t0/t)1/2 with δ0 = 0.1, 0.04, and 0.0125 Byr-1 for Earth, Venus, and Mars, respectively. After a R2 correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassium is depleted by a factor of 2 in Venus and Mars. Mass ratios of helium and argon are close to 5 × 10-9 and 2 × 10-8 g/g in the interiors of all three planets. The implications of these results are discussed.

Selective Rydberg-level population of multiply charged ions at solid surfaces: A dynamic theory for low-angular-momentum ionic states.

Selective Rydberg-level population of multiply charged ions (e.g., Z=6, 7, and 8) at solid surfaces is treated in normal emergence geometry. For the intermediate ionic velocity region (between v\ensuremath{\approxeq}1 and 3 a.u.) a molecular-dynamics-type model of the electron pickup process from the solid valence band into low-angular-momentum ionic states (l=0, 1, and 2) is proposed. Specific features of the Rydberg states and ions (large size, high degeneracy with respect to l, high value of Z) are included in the model. The electron transition amplitude is calculated as a mixed electron-density flux through a moving Firsov plane, whose kinematics is determined by a variational requirement. A multichannel character of the process is taken into account in the framework of a statistical treatment of decoupled channels, based on the approximation of small transition probabilities. The population probability ${\mathit{P}}_{\mathit{n}\mathit{l}}$=${\mathit{P}}_{\mathit{n}\mathit{l}}$(v,Z) of the (n,l) state is in sufficiently good agreement with available beam-foil experimental data (S VI, Cl VII, Ar VIII) not only as a function of the principal quantum number n, but also as a function of l and v. An ``anomalous'' peak at n=11 in the population probability of Ar VIII is briefly discussed from the standpoint of the developed formalism. The predicted maxima in the v dependence of ${\mathit{P}}_{\mathit{n}\mathit{l}}$(v,Z) in the intermediate velocity region calls for further more refined experimental studies.

A spin-filter polarimeter for low energy hydrogen and deuterium ion beams

An efficient polarimeter which reveals populations of individual hyperfine states of nuclear-spin-polarized H ± (or D ±) ion beams has been tested. This device is based on unique properties of a three-level interaction in the 2S 1 2 and 2P 1 2 states of hydrogen (or deuterium) atoms, created when the incident, polarized ion beams undergo electron pickup in cesium vapour. Used on a polarized ion source, its efficiency facilitates both rapid optimization and continual monitoring of parameters that affect the beam polarization. With such sources, and perhaps in applications with polarized gas jet targets, the device has potential for an absolute accuracy of better than 2%.

Formation of the negative muonium ion and charge-exchange processes for positive muons passing through thin metal foils.

The negative muonium ion, ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\equiv}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$${\mathrm{e}}^{\mathrm{\ensuremath{-}}}$, has been produced by double electron pickup as a beam of positive muons passes through thin foils of various materials. The ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$ ions were detected by electrostatically accelerating and analyzing the ions in a magnetic time-of-flight spectrometer. Yields of ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$, Mu and ${\ensuremath{\mu}}^{+}$ as well are reported when ${\ensuremath{\mu}}^{+}$ with kinetic energies from 380 to 800 keV impinge on thin Be, Al, and Au foils. A Monte Carlo simulation provides good agreement with the experimental results.

Erratum: First observation of the negative muonium ion produced by electron capture in a beam-foil experiment

The negative muonium ion Mu/sup -/equivalent..mu../sup +/e/sup -/e/sup -/ has been produced by double electron pickup as a beam of positive muons passes through a foil. A clean signal was detected by electrostatically accelerating and analyzing the ions in a magnetic time-of-flight spectrometer. The Mu/sup -/ beam at the focal plane of our spectrometer had an intensity 1.4 x 10/sup -5/ times that of the incident ..mu../sup +/ beam or 2 s/sup -1/, which corresponds to a Mu/sup -//..mu../sup +/ conversion probability of 0.5 x 10/sup -4/ at the foil in agreement with the calculated value. The kinetic energies of Mu/sup -/ at the foil are less than several keV as expected.