Atomic Hydrogen Targets Research Articles

Ionization Cross Sections of Hydrogen Molecule by Electron and Positron Impact.

We present ionization cross sections of hydrogen molecules by electron and positron impact for impact energies between 20 and 1000 eV. A three-body Classical Trajectory Monte Carlo approximation is applied to mimic the collision system. In this approach, the H2 molecule is modeled by a hydrogen-type atom with one active electron bound to a central core of effective charge with an effective binding energy. Although this model is crude for describing a hydrogen molecule, we found that the total cross sections for positron impact agree reasonably well with the experimental data. For the electron impact, our calculated cross sections are in good agreement with the experimental data in impact energies between 80 eV and 400 eV but are smaller at higher impact energies and larger at lower impact energies. Our calculated cross sections are compared with the scaled cross sections obtained experimentally for an atomic hydrogen target. We also present single differential cross sections as a function of the energy and angle of the ejected electron and scattered projectiles for a 250 eV impact. These are shown to agree well with available data. Impact parameter distributions are also compared for several impact energies.

Projectile transverse momentum controls emission in electron vortex ionization collisions

The realization of electron vortex (EV) beams in the past decade has led to numerous proposed applications in fields from electron microscopy to control and manipulation of individual molecules. Yet despite the many unique characteristics and promising advantages of EV beams, such as transverse momentum and quantized orbital angular momentum, there remains a limited understanding of their fundamental interactions with matter at the atomic scale. Collisions between EV projectiles and atomic targets can provide some insight into these interactions and we present here fully differential cross sections (FDCS) for ionization of excited state atomic hydrogen targets using EV projectiles. We show that the projectile’s transverse momentum causes the ionized electron angular distributions to be altered compared to non-vortex projectiles and that the ionized electron’s ejection angle can be controlled by adjustment of the vortex opening angle, a feature unique to vortex projectiles. Additionally, an inherent uncertainty in the projectile’s momentum transfer leads to a broadening of the classical binary peak, making signatures of the target electron density more readily observable. FDCS for aligned 2p targets exhibit structures can be used to determine the alignment.

Сечения образования ионов He-=SUP=-+-=/SUP=- в различных электронных состояниях при столкновениях ионов He-=SUP=-2+-=/SUP=- с атомами водорода

Using collision spectroscopy based on precision measurements of the kinetic energy of projectile ions that capture an electron, we measured the state selective electron capture cross sections of formation of He^+(n) ions at collision 3^He^{2 +} ions with an energy of E = 1.4-10 keV/a.m.u. with hydrogen atoms. The atomic hydrogen target with a degree of dissociation 78% at a temperature of tungsten dissociation cell 2180K has been made.

Near-Threshold Cross Sections for Electron and Positron Impact Ionization of Atomic Hydrogen.

Using classical arguments Wannier [Phys. Rev. 90, 817 (1953)PHRVAO0031-899X10.1103/PhysRev.90.817] proposed an electron-impact ionization cross section for neutral atoms to behave as E^{1.127}, where E is the excess energy above threshold. Using similar arguments Klar [J. Phys. B 14, 4165 (1981)JPAMA40022-370010.1088/0022-3700/14/21/027] obtained E^{2.65} to be the corresponding threshold law for positron impact. Recently, Babij etal. [Phys. Rev. Lett. 120, 113401 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.113401] measured near-threshold positron-impact breakup behavior to be similar to that expected for electrons. Using the convergent close-coupling method for the atomic hydrogen target, we examine cross sections at near-threshold energies for electron and positron impact. Contrary to the experiment, the calculated cross sections are found to behave differently for the two projectiles and consistently with the aforementioned threshold laws, despite the entirely quantum nature of these problems. For electron impact, the threshold behavior holds while the total electron spin asymmetry remains constant, whereas for positron scattering the threshold law holds for breakup while the positronium-formation component of the ionization cross section remains constant.

Well-defined atomic hydrogen target driven by electromagnetic shock wave for stopping power measurement

AbstractThe precise estimation of stopping power is crucial to predict the beam energy loss in the target for heavy-ion fusion and heavy-ion-driven high-energy density physics experiments. The electromagnetic shock wave has been proposed to generate a well-defined atomic hydrogen target for the stopping power measurement with dissociation effects. We measured the angular distribution profile of the discharge plasma and the plasma velocity in the electromagnetic shock tube by high-speed framing cameras. To improve the uniformity of the discharge plasma and the velocity, an external magnetic field was applied in the electromagnetic shock tube. The plasma velocity was up to approximately 40 km/s for an initial hydrogen gas pressure of 100 Pa and the velocity decreased with the initial pressure and the propagation length. The framing cameras showed that angular distributions of the discharge plasmas were not uniform and the initial angular distributions were important for the development of plasma profiles. The interaction of the plasma with the external magnetic field was estimated using the ratio of the plasma dynamic pressure to the magnetic pressure. The estimations offer more magnetic fields to improve the discharge uniformity due to the interaction.

Estimating positronium formation for plasma applications

The basic physics of ${e}^{+},{e}^{\ensuremath{-}}$ creation and annihilation is overviewed. It is shown that for atomic hydrogen targets electron capture by a free positron to form positronium is vastly more probable than in-flight annihilation. Cross sections are presented using the classical trajectory Monte Carlo (CTMC) approach for the charge-exchange process:${e}^{+}+{A}^{q+}\ensuremath{\rightarrow}Ps+{A}^{(q+1)+},$where ${A}^{q+}$ is some target ion of charge $q$. Charge-exchange cross sections for hydrogenic ion targets are presented. It is found that while the CTMC gives adequate results for positronium formation for ${e}^{+}$-hydrogen and ${e}^{+}$-cesium collisions, its high-energy behavior for hydrogenic ions is not in agreement with quantum mechanical predictions. Since we are interested in situations where many multicharged ions will be present we have looked for an alternative approach. Scaling rules are proposed and used to estimate the charge-exchange cross sections for both neutral atoms and multicharged ions.

Classical-trajectory calculations of the electronic stopping cross-section for low-energy H and H+ projectiles by H2-molecules

A model that enables the classical-trajectory simulation of the interaction between an atomic particle and a target containing one or more electrons is devised. It makes use of the so-called Gaussian kernel approximation and ad-hoc potentials. In this way, the most relevant quantum properties of the electron can be preserved and, at the same time, still using classical mechanics to solve the response of the electronic system to the presence of a moving, heavy charge. As a first step to assessing the proposed model we calculate the electronic stopping cross-section for 1–20 keV Protons and Hydrogen impinging upon atomic and molecular Hydrogen targets. The results show a fairly good agreement between experiments and previous theoretical calculations over the entire bombarding energy studied in this paper.

Low-energy charge transfer betweenC5+and atomic hydrogen

Charge transfer with carbon ions has been identified as important in both magnetic fusion plasma devices and, more recently, in solar wind interactions with comets, planets, or neutrals in the heliosphere. A merged-beams technique is used to measure the absolute total charge-transfer cross section for ${\mathrm{C}}^{5+}$ and atomic H over four orders of magnitude in collision energy, from 12 000 to 0.64 eV/u. The present measurements are compared with previous measurements using an atomic hydrogen target and benchmark available classical trajectory Monte Carlo and molecular-orbital close-coupling calculations. An observed increasing cross section below 10 eV/u is attributed to trajectory effects due to the ion-induced dipole attraction between reactants.

THE BLAST EXPERIMENT: POLARIZED ELECTRON SCATTERING FROM HYDROGEN AND DEUTERIUM

At the MIT-Bates Linear Accelerator Center, the nucleon form factors have been measured by scattering polarized electrons from vector-polarized hydrogen and deuterium. The experiment used the longitudinally polarized electron beam stored in the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic hydrogen and deuterium target provided by an atomic beam source. The measurements were carried out with the symmetric Bates Large Acceptance Spectrometer Toroid (BLAST). Results are presented for the proton form factor ratio, [Formula: see text], and for the charge form factor of the neutron, [Formula: see text]. Both results are more precise than previous data in the corresponding Q2 ranges.

Impact-parameter dependence of the energy loss of fast molecular clusters in hydrogen

The electronic energy loss of molecular clusters as a function of impact parameter is far less understood than atomic energy losses. For instance, there are no analytical expressions for the energy loss as a function of impact parameter for cluster ions. In this work, we describe two procedures to evaluate the combined energy loss of molecules: Ab initio calculations within the semiclassical approximation and the coupled-channels method using atomic orbitals; and simplified models for the electronic cluster energy loss as a function of the impact parameter, namely the molecular perturbative convolution approximation (MPCA, an extension of the corresponding atomic model PCA) and the molecular unitary convolution approximation (MUCA, a molecular extension of the previous unitary convolution approximation UCA). In this work, an improved ansatz for MPCA is proposed, extending its validity for very compact clusters. For the simplified models, the physical inputs are the oscillators strengths of the target atoms and the target-electron density. The results from these models applied to an atomic hydrogen target yield remarkable agreement with their corresponding ab initio counterparts for different angles between cluster axis and velocity direction at specific energies of 150 and 300 keV/u.

Nucleon form factors and the BLAST experiment

Measurements of the charge and magnetic form factors of the nucleon present a sensitive test of nucleon models and QCD-inspired theories. A precise knowledge of the neutron form factors at low Q 2 is also essential to reduce the systematic errors of parity-violation experiments. At the MIT-Bates Linear Accelerator Center, the nucleon form factors have been measured by means of scattering of polarized electrons from vector-polarized hydrogen and deuterium. The experiment used the longitudinally polarized stored electron beam of the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic hydrogen and deuterium target provided by an atomic beam source. The measurements have been carried out with the symmetric Bates Large-Acceptance Spectrometer Toroid (BLAST) with enhanced neutron detection capability.

Projectile-electron loss in collisions with hydrogen at relativistic impact energies

We consider electron loss from hydrogen-like projectiles in elastic and inelastic collisions with atomic hydrogen target. The projectiles move at relativistic velocities v and have atomic numbers Z p which are not very high ( Z p ≪ v). In the elastic collision mode, where the target remains in the ground state, the projectile electron is removed by the interaction with both the target electron and the target nucleus. The inelastic collision mode, in which the target is excited or ionized, is governed by the electron–electron interaction and gives more than 50% of the target contribution to the total loss cross section. Within this mode the loss cross section is dominated by collisions in which the target gets ionized. At sufficiently high impact energies both the elastic and inelastic contributions to the loss cross section become independent of the energy.

Møller polarimetry with atomic hydrogen targets

A novel proposal of using polarized atomic hydrogen gas, stored in an ultra-cold magnetic trap, as the target for electron beam polarimetry based on Moller scattering is discussed. Such a target of practically 100% polarized electrons could provide a superb systematic accuracy of about 0.5% for beam polarization measurements. Feasibility studies for the CEBAF electron beam have been performed. PACS: 07.60.Fs Polarimeters – 29.25.Pj Polarized targets – 67.65.+z Spin-polarized hydrogen and helium

Excitation functions of the analyzing power in elastic proton-proton scattering from 0.45 to 2.5 GeV

Excitation functions A_N(p_{lab},Theta_{c.m.}) of the analyzing power in elastic proton-proton scattering have been measured in an internal target experiment at the Cooler Synchrotron COSY with an unpolarized proton beam and a polarized atomic hydrogen target. Data were taken continuously during the acceleration and deceleration for proton kinetic energies T_{lab} (momenta p_{lab}) between 0.45 and 2.5 GeV (1.0 and 3.3 GeV/c) and scattering angles 30 deg < Theta_{c.m.} < 90 deg. The results provide excitation functions and angular distributions of high precision and internal consistency. The data can be used as calibration standard between 0.45 and 2.5 GeV. They have significant impact on phase shift solutions, in particular on the spin triplet phase shifts between 1.0 and 1.8 GeV.

Moller polarimetry with atomic hydrogen targets

A novel proposal of using polarized atomic hydrogen gas, stored in an ultracold magnetic trap, as the target for electron-beam polarimetry based on Moller scattering is discussed. Such a target of practically 100% polarized electrons could provide a superb systematic accuracy of about 0.5% for beam polarization measurements. Feasibility studies for the CEBAF electron beam have been performed.

Cold-target recoil-ion momentum spectroscopy studies of capture from atomic and molecular hydrogen byO8+andAr8+

We have used cold-target recoil-ion momentum spectroscopy to study electron capture from atomic and molecular hydrogen targets by slow $(0.3&lt;\mathrm{v}&lt;0.95\phantom{\rule{0.3em}{0ex}}\text{a.u.})$ ${\mathrm{O}}^{8+}$ and ${\text{Ar}}^{8+}$ ions. For the atomic hydrogen target, we have performed coupled-channel calculations using an atomic orbital expansion to compare to the experimental results. The $Q$-value spectra show strong populations of both $n=5$ and $n=6$ states on the projectile, with a tendency for increasing the population of high-angular momentum states for higher projectile velocity. A strong population of $n=6$ is found, a result not previously reported but in good agreement with the present calculations.

Protonium formation in collisions of antiprotons with hydrogen molecular ions

Protonium formation in molecular collisions, , is theoretically investigated. Within the framework of the adiabatic approximation, the four-body problem can be treated as three-body collisions on a single adiabatic (Born–Oppenheimer) potential energy surface (PES), in the same manner as chemical reaction problems. The adiabatic potential energies of the system are calculated for various configurations. A classical trajectory Monte Carlo calculation is carried out for the collisions on the adiabatic PES. It is found that the dissociative dynamics plays a critical role in protonium formation, and consequently the molecular target is much more effective in protonium formation than the atomic-hydrogen target.

Protonium formation in collinear collisions between antiprotons and hydrogen molecular ions: Quantum-classical hybrid method versus adiabatic approximation

A quantum-classical hybrid (or semiclassical) method is applied to protonium formation $\overline{p}+\mathrm{H}_{2}{}^{+}\ensuremath{\rightarrow}\overline{p}p+\mathrm{H}$ and dissociation $\overline{p}+\mathrm{H}_{2}{}^{+}\ensuremath{\rightarrow}\overline{p}+p+\mathrm{H}$ at kinetic energies up to $200\phantom{\rule{0.3em}{0ex}}\text{eV}$. The electronic motion is accurately solved quantum mechanically, while the motion of the heavy particles $\overline{p}$ and $p$ is described by classical mechanics. The $\overline{p}\text{\ensuremath{-}}p\text{\ensuremath{-}}p$ collinear configuration is assumed as a preliminary to three-dimensional calculations, and to assess the validity of the adiabatic approximation. Vibrational excitation to the dissociative continuum is crucial in $\overline{p}p$ formation in contrast to the importance of electron emission for the atomic-hydrogen target. For this reason, $\overline{p}p$ formation occurs efficiently even well beyond the ionization threshold if the target is a molecule.

Electron-impact ionization of atomic hydrogen at incident electron energies of 15.6, 17.6, 25, and 40 eV

Absolute doubly differential cross sections for the electron-impact ionization of atomic hydrogen have been measured from near threshold to intermediate energies. The measurements are calibrated to the well-established, accurate differential cross section for electron-impact excitation of the atomic hydrogen transition H(12S22S+22P). In these experiments background secondary electrons are suppressed by moving the atomic hydrogen target source to and from the collision region. Measurements cover the incident electron energy range of 14.6–40 eV, for scattering angles of 10°–120° and are found to be in very good agreement with the results of the most advanced theoretical models—the convergent close-coupling model and the exterior complex scaling model.

Ionization of hydrogen targets by short laser pulses

We present a distorted-wave formulation of atomic ionization by short laser pulses based on Coulomb–Volkov states. The method is applied to atomic-hydrogen targets, for different interaction times and frequencies. Results are compared with the predictions of an exact numerical treatment, and good agreement is obtained as long as the convergence conditions of the perturbative series hold. The applicability of the method depends on the field intensity but not on the pulse duration, permitting a unified description of various ionization mechanisms.