Atomic Hydrogen Targets Research Articles

Laser-assisted elastic electron-atom collisions

The authors analyse the 'elastic' scattering, accompanied by the transfer of l photons, of fast electrons by atoms in the presence of a strong laser field. They focus their attention on the region of small momentum transfers, where the 'dressing' effect due to the dipole distortion of the target by the laser field is shown to produce important modifications of the differential cross section. Detailed calculations are presented for both atomic hydrogen and helium targets at an incident electron energy of 100 eV and for various geometrical configurations.

Low-energy total-electron-capture cross sections for fully stripped and H-like projectiles incident on H and H2.

Total-electron-capture cross sections are reported for fully stripped and H-like ions of C, N, O, F, and Ne colliding with atomic and molecular hydrogen targets in the energy range 0.18--8.5 keV/amu. At energies above 3 keV/amu the cross sections are approximately energy independent with magnitudes in the range (4--7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$, and scale smoothly with projectile charge. Below 3 keV/amu the H-target cross sections for projectiles with even charge are smaller than those for projectiles of odd charge. For the ${\mathrm{H}}_{2}$-target data, the opposite trend prevails. For those systems where calculations exist, molecular orbital and atomic orbital close-coupling calculations are within 20% of the experimental results except at the lowest energies investigated.

An experimental apparatus for translational energy spectroscopy for low energy X q+ on atomic hydrogen

The design and performance of a system for performing translational energy spectroscopy on atomic hydrogen targets is presented. Low energy highly-charged ions are produced in a recoil ion source through collisions between target gases and a heavy, fast pump beam from the EN tandem accelerator. These ions are directed through a double-focusing magnetic spectrometer which selects the desired projectile charge state. An atomic hydrogen target is produced in a thermal dissociation oven. The energy and final charge states of the product ions are determined by a double-focusing, hemispherical electrostatic analyzer. The current work concentrates on the change in energy of the projectile ions accompanying electron capture for 250 eV per charge argon projectiles.

Electron-capture cross sections for low-energy highly charged neon and argon ions from molecular and atomic hydrogen.

Electron-capture cross sections for low-velocity (${10}^{6}$--${10}^{7}$ cm/s) highly charged ${\mathrm{Ne}}^{\mathrm{q}+}$ (2\ensuremath{\le}q\ensuremath{\le}7) and ${\mathrm{Ar}}^{\mathrm{q}+}$ (2\ensuremath{\le}q\ensuremath{\le}10) projectiles incident on molecular- and atomic-hydrogen targets have been measured. A recoil-ion source that used the collisions of fast heavy ions (1 MeV/amu) with target gas atoms was utilized to produce slow highly charged ions. Atomic hydrogen was produced by dissociating hydrogen molecules in a high-temperature oven. Measurements and analysis of the data for molecular- and atomic-hydrogen targets are discussed in detail. The measured absolute cross sections are compared with published data and predictions of theoretical models.

Intrashell mixing following electron capture from atomic hydrogen targets by slow ions. I — Fully stripped projectiles

The influence of the Stark effect on the l distribution of excited states produced by electron capture from atomic hydrogen into slow fully stripped ions is studied. The Stark effect is strong enough, in these monoelectronic systems, to preclude any relation between the final l distribution and the primary process of electron capture. The latter determines the m distribution through «geometrical» effects, which supports the utility of measuring the polarization of radiation emitted by decay of the excited states formed in the collision Etude theorique du role de l'effet Stark sur la distribution en l des etats excites formes par capture d'electrons dans l'hydrogene atomique par des ions lents nus (C 6+ , O 8+ , Ne 10+ ): dans ces systemes monoelectroniques, l'effet Stark est suffisamment fort pour que la distribution en l ne garde aucune trace du processus primaire de capture electronique, qui se manifeste cependant par des effets «geometriques» observables dans la distribution en m

A high-density effusive target of atomic hydrogen

The design and construction of a DC discharge (Wood's tube), used as source for an atomic hydrogen target in collision experiments with multiply charged ions, is described and the characteristic parameters (nozzle diameter, wall temperature, pumping speed, discharge tube preparation and discharge pressure) are investigated. The ratio obtained for the atomic and molecular hydrogen target thickness was of the order of four in the collision region. The target thickness of the atomic hydrogen could be varied between 5*1016 m-2 and approximately 3*1017 m-2. The effectiveness of the arrangement is demonstrated by charge transfer measurements of multiply charged Krq+ ions in atomic hydrogen.

The Electron Beam Method of Production of Highly Charged Ions and Its Applications

Based on the works performed at the Joint Institute for Nuclear Research, the present-day situation and prospects of development are considered of the electron beam method of production of highly charged ions and its applications in various fields of physics.At present an ionization factor jτi (the product of electron beam density by the time of bombarding a stationary ion target with the electrons) approximately equal to 1.5 × 102 2 cm-2 has been achieved at an electron energy of Ee = 20 keV. This allows one to produce rather highly charged ions including argon nuclei (Ar18+) and He-like ions of krypton (Kr34+) and xenon (Xe52+).As regards the applications of the method, primary consideration is given to positive ion ionization by electron impact, the charge exchange of highly charged ions on atomic and molecular hydrogen targets, the interaction of the ions with the surface of solids and the nuclear properties of the ions.

Isotope Effect in Electron-Capture Differential Cross Sections at Intermediate Energies

The isotope dependence in the angular distribution of electron-capture cross sections for protons and deuterons with equal velocity (E = 40 keV/u) colliding with atomic hydrogen or deuterium targets is predicted theoretically and observed experimentally. A projectile-dependent effect is observed at small scattering angles. No target dependence was detected in the differential cross sections. A scaling relationship is determined which permits the prediction of differential cross sections for hydrogen-isotope projectiles.

Electron Capture by Fast H+, He+, and He++ Ions in Collisions With Atomic and Molecular Hydrogen

Total cross sections for electron capture by H+ (250-600 keV), 4He+ and 4He++ (0.3-2 MeV) incident on atomic and molecular hydrogen have been measured. A directly heated tungsten oven provided an atomic-hydrogen target, and the degree of dissociation of molecular hydrogen and the target-thickness calibration were performed utilizing earlier results reported by Shah and Gilbody.

Excitation and ionisation in collisions of negatively charged projectiles with atoms

Semiclassical coupled-states calculations have been carried out for collisions involving structureless positively and negatively charged projectiles to explore the dependence of the excitation and ionisation cross sections on the sign of the projectile charge. Both asymmetric collisions, where the magnitude of the projectile charge, Zp, is much less than the target nuclear charge, ZT, so that the perturbation of the target by the projectile is weak, and symmetric collisions, where mod ZP mod approximately=ZT, have been considered. The Coulomb deflection of muons is found to have an appreciable effect on the K-shell ionisation cross section. For the symmetric collision case, the authors' considered excitation and ionisation of atomic hydrogen targets both by protons and antiprotons and by electrons. For protons and antiprotons the ratios of the coupled-state cross sections to the corresponding first Born cross sections do not show the same relation to the first Born that is exhibited by the copper ionisation results, and are in fact remarkably the same for ZP=-1 as for ZP=+1. These results demonstrate the projectile charge asymmetry of the collision, that is due to the presence of the charge transfer channel for the positive projectiles, but its absence for the negative projectiles. For e-+H collisions coupled-state corrections to the first Born approximation are found to be appreciable, and overall good agreement with experiment is found.

Electron-capture collisions at keV energies of boron and other multiply charged ions with atoms and molecules. II. Atomic hydrogen

Single electron-capture cross sections have been measured for boron ions ${\mathrm{B}}^{q+}$ with initial charges $q=2, 3, \mathrm{and} 4$ incident on an atomic hydrogen target. The cross sections ${\ensuremath{\sigma}}_{21}$ and ${\ensuremath{\sigma}}_{43}$ are large and show little dependence on the ion energy for the range studied, $6q$ to $23q$ keV. The cross section ${\ensuremath{\sigma}}_{32}$ increases with increasing energy in this same range. Also reported is a survey of single electron-capture cross sections for ions of carbon ($2\ensuremath{\le}q\ensuremath{\le}4$), nitrogen ($2\ensuremath{\le}q\ensuremath{\le}5$), and oxygen ($2\ensuremath{\le}q\ensuremath{\le}5$) at the single energies $8q$ keV also incident on atomic hydrogen.

Observation of Oscillations in the Charge Dependence of Total Electron-Capture Cross Sections

An oscillatory dependence on ionic charge of total electron-capture cross sections has been observed for multiply charged heavy projectiles incident on an atomic hydrogen target. The oscillatory behavior is attributed to interference caused by the long-range Coulomb and a short-range (screened Coulomb) force acting on the captured electron. The oscillations are superimposed on a monotonically increasing variation with ionic charge. This monotonic increase is also found for lighter ions.

Reduced matrix equations for many-particle scattering. I. Electron-atom scattering at low energies

The reduction method developed previously for many-particle scattering is applied here to derive new sets of coupled equations for electron scattering from atomic targets at low energies, fully taking into account the exchange effect and target distortions. The reduced matrix equations have simpler structure than the Faddeev-Watson equations, with the iteration kernels connected only in those channels which are explicitly taken into account. Scatterings by atomic hydrogen, helium, and lithium targets are considered, and the resulting formalism is compared with the earlier work of Faddeev and others. Various approximation procedures are described, and their applicability is tested on the electron-hydrogen scattering system.

Prototype polarized-electron source through electron-hydrogen spin exchange with teflon containment of hydrogen and a longitudinal magnetic trap

A prototype polarized-electron source has been constructed and results are reported. It is based on electron-hydrogen spin-exchange collisions using a longitudinal-geometry electron trap and teflon coatings to contain the state-selected atomic-hydrogen target. This scaled-down prototype is compatible with an electron linac and produces an extracted pulse of about 107 electrons/μs with about 60% polarization and 50–60 Hz repetition rate. Construction of a more intense (1010 or more electrons per pulse) full-scale device based upon this technique is also discussed. The average measured total electron-hydrogen spin-exchange cross section for 5–10 eV electrons is estimated to be in the range (0.5–1.55) × 10−15 cm2.

Design and Operation of an Atomic Hydrogen Target for Precision Scattering Cross Section Measurements

A new design for a hydrogen atom target is described. An H2 dissociation of 96±4% is obtained for pressures near 3×10-5 Torr and temperatures of about 2400 K. The scattering cell proper is uniformly heated by radiation from a separate resistance heater element. Magnetic field effects are eliminated by gating the 700 A heater current out of phase with the incident particle beam. A method for absolute measurement of the dissociation fraction is described. Target impurities other than H2 are estimated to be less than 0.2%.

Measurements of Resonant Electron Capture in CloseH+-on-H Collisions

Differential measurements of the electron capture probability ${P}_{0}$ are made for protons scattered from atomic hydrogen targets. The incident proton energy ranges from 150 keV down to 0.130 keV and the scattering angle is varied between 0.2\ifmmode^\circ\else\textdegree\fi{} and 6.0\ifmmode^\circ\else\textdegree\fi{}. Seven sharply resonant peaks of ${P}_{0}$ are found, their location depending on both energy and angle. For example, when the scattering angle is held at 6\ifmmode^\circ\else\textdegree\fi{} the peaks are found at 21, 3.9, 1.7, 0.90, 0.54, 0.36, and 0.19 keV, and when the energy is fixed at 0.250 keV, peaks are found at 0.7\ifmmode^\circ\else\textdegree\fi{}, 1.4\ifmmode^\circ\else\textdegree\fi{}, 2.6\ifmmode^\circ\else\textdegree\fi{}, and 4.5\ifmmode^\circ\else\textdegree\fi{}. In some cases the equivalent collision with deuterium, ${\mathrm{D}}^{+}$-D, is used to augment the range of the measurements. These data extend those reported by Lockwood and Everhart (1962) who studied this same phenomenon in ${\mathrm{H}}^{+}$-H collisions over a smaller range in energy and at fixed 3\ifmmode^\circ\else\textdegree\fi{} scattering angle. Relative differential-cross-section data are presented also which show a change in slope with the onset of rainbow-angle scattering conditions.

Measurements of Thermal Dissociation of Hydrogen, Using Fast Protons

Experiments involving study of the angular scattering of kiloelectron-volt protons from atomic and molecular hydrogen targets have been used to measure directly the fraction of hydrogen gas dissociated in equilibrium with hot tungsten. The state of the hydrogen gas in a tungsten furnace is determined by analyzing the scattered incident particles which result from single collisions between the incoming protons and the hydrogen gas in the furnace. In particular, the number of H— ions among the scattered particles is a measure of the molecular hydrogen density, since H— ions cannot be created in single collisions between protons and hydrogen atoms. Under the present experimental conditions it was possible to achieve a fraction dissociated of over 87.5% at a furnace temperature of 2380°K and still higher fractions at higher temperatures. These measurements are made at hydrogen pressures of the order of 10—2 Torr. The practical conditions under which an atmosphere of highly dissociated hydrogen can be created are described and it is pointed out how the dissociation fraction and also the density of the hydrogen can be directly measured in the otherwise inaccessible interior of the gas. These measurements using a proton beam probe do not require that the pressure or temperature of the hydrogen be known and, in fact, do not require thermal equilibrium conditions.

Measurements of Electron Capture in CloseH+-on-He andHe+-on-H Collisions

Differential measurements of electron capture probability ${P}_{0}$ are made for close encounters in the reaction ${\mathrm{H}}^{+}$+He\ensuremath{\rightarrow}H+${\mathrm{He}}^{+}$. The energy range of the incident proton is 1.6 to 180.0 keV and the scattering angle is varied from \textonehalf{} to 4\ifmmode^\circ\else\textdegree\fi{}. The impact parameters associated with these collisions extend from 0.015 to about 0.50 \AA{}. There is little angular dependence to the data. When ${P}_{0}$ is plotted versus energy, a damped resonant structure is seen with peaks at 36, 7, and 2.6 keV with amplitudes of 0.52, 0.16, and 0.05, respectively. The phenomena are discussed in terms of the energy-level diagram for He${\mathrm{H}}^{+}$ and with reference to the existing theories for charge transfer in the nonresonant case. Measurements of the inverse reaction, ${\mathrm{He}}^{+}$ ions incident on atomic hydrogen targets, are also presented and discussed.

Resonant Electron Capture in Violent Proton-Hydrogen Atom Collisions

A new collision chamber design has made possible differential measurements of scattering of protons on atomic hydrogen target atoms. The scattering takes place in the interior of a furnace where hydrogen gas is dissociated. Electron capture probability is measured vs proton energy for protons passing nearly through the center of isolated hydrogen atoms such that the incident particle is scattered through an angle of 3\ifmmode^\circ\else\textdegree\fi{}. This shows a resonant structure with maxima at energies of 0.78, 1.57, 3.92, and 20.1 kev. A simple model is presented for looking at these resonances. In the light of this model and previous theoretical work, the location of the high-energy maximum is unexpectedly low, and this suggests that a phase constant is needed to achieve agreement between experiment and theory. The ${\mathrm{H}}^{+}$ on H data presented here are compared with those for the other combinations, ${\mathrm{H}}^{+}$ on ${\mathrm{H}}_{2}$, ${\mathrm{H}}^{+}$ on He, and ${\mathrm{He}}^{+}$ on He, which also show resonant electron capture.