Collisions Of Hydrogen Atoms Research Articles

Nonadiabatic transitions upon atomic collisions

The standard interpretation of the adiabatic approach and the t matrix method for the calculation of the probabilities of nonadiabatic transitions and inelastic cross sections in atomic collisions is analyzed, and a model problem of collisions of hydrogen atoms with neutrons is considered. It is shown that the application of the standard approach results in a physically inconsistent result—considerable values of the excitation cross sections of hydrogen atoms by neutrons. The application of the t matrix method improves the result by two to three orders of magnitude even for a restricted basis set.

Nonadiabatic coupling in cold collisions of spin-polarized metastable hydrogen atoms

Previous calculations of low-temperature cross sections for collisions between spin-polarized metastable hydrogen atoms are improved to include nonadiabatic radial and angular coupling at large interatomic separations. The electrostatic dipole-quadrupole interaction produces nonadiabatic radial coupling between (2s,2p) and (2p,2p) states, while the Coriolis interaction produces nonadiabatic angular coupling. Both of these long-range contributions are handled in a space-fixed atomic gauge that is particularly convenient for a spin-polarized system. The improved theoretical results are compared with an existing experiment.

Negative-ion formation and mutual neutralization in atom–atom and ion–ion collisions below keV energies

Dynamics and cross-section values for ion-pair formation and its reverse process, mutual neutralization in atom–atom and ion–ion collisions are reviewed for hydrogen–hydrogen atom collisions and hydrogen–alkali atom collisions. For ion-pair formation process, the initial state shows a weak polarization potential, while the final ion-pair state arises from a strong Coulomb potential. Hence, the final state shows a series of avoided crossings with many intermediate states which correspond to electronic excited states of one of pairs. Therefore, the ladder climbing mechanism is the driving dynamics for ion-pair formation. This fact, that is, many sates are involved in dynamics, makes theoretical treatments more complex and cumbersome for an accurate description. We overview the current experimental and theoretical status of these processes.

Change of atomic spins in slow collisions

In the present work, slow atomic collisions with changes of atomic spins are examined. In this case, the electron spin exchange in the process of collisions causes the polarization characteristics of atoms to change. Formalism has been constructed based on the adiabatic approximation and polarization operator technique for a description of changes of arbitrary spins of colliding atoms. Singlet-triplet transitions, analogous to those of inelastic exchange electron scattering on two-electron atoms, are described for collisions of hydrogen and alkali metal (H, K, Na, Cs, and Rb) atoms with two-electron atoms or ions (He, Li +, and Hg). The processes of the type H(2S)+He(1S)→H(2S)+He(3S) and \(P\left( {^4 S_{3/2} } \right) + He\left( {^3 S_1 } \right) \to P\left( {^2 S_{1/2} } \right) + He\left( {^3 S_1 } \right)\) are also considered.

The role of projectile double scattering in positron–atom collisions

We present the doubly differential cross sections of the ionization for positron and hydrogen atom collisions at 50 eV impact energy. We treat the collision problem in the framework of the full and reduced three-body classical trajectory Monte Carlo (CTMC) model. The role of projectile double scattering in light projectile and atom collisions are discussed.

Energy losses of swift protons to hydrogen atoms

Full and reduced three-body classical trajectory Monte Carlo (CTMC) theory are used to calculate energy losses of the projectile in proton–hydrogen atom collisions as a function of impact energy. The total stopping power is presented along with partial contributions of the energy deposition among the channels of excitation, ionization and capture. Contributions of the projectile–electron and projectile–target nucleus interactions to the total stopping power are also investigated. Our results are compared with other theoretical calculations and experimental data. The present calculations verify that high order effects should be included for a proper description of electronic stopping power.

Vibrationally resolved collisions in cold hydrogen plasma

Slow collisions (0.5–100eV) of hydrogen atoms and ions with vibrationally excited hydrogen molecules and molecular ions are studied using fully quantal (below 10eV) and semi-classical approaches. Considered transitions include charge transfer, excitation, dissociation, three-body diatomic association, as well as energy and angular spectra of dissociation fragments. Mutual relations of various elastic and inelastic processes, the relevant physical mechanisms governing vibrational dynamics in inelastic channels and comparison of the fully quantal and semi-classical results are also reported.

Electron transfer in collisions of keV hydrogen atoms and ions with methane

Absolute differential cross sections are reported for electron capture and loss by 1–5 keV H atoms incident on CH4 for laboratory scattering angles up to 1.62°, and for charge transfer of 0.5–5 keV H+ with CH4 for scattering angles up to 2.09°. Electron-loss collisions are seen to result in comparatively large scattering angles and a very clear similarity exists between the present differential cross sections and those reported for other molecular targets. The present charge-transfer differential cross sections are consistent with that of Gao et al (1990 Phys. Rev. A 41 5929–33) but not with the calculations of Kimura et al (1995 Phys. Rev. A 52 1196–205). Prior experimental studies of electron-loss and charge-transfer are generally in good accord with the integral values reported here as are the calculations of Kusakabe et al (2000 Phys. Rev. A 62 062715).

Protonium formation in antiproton–hydrogen-atom collisions

Expressions for momentum distributions of electrons in antiproton--hydrogen-atom collisions are derived in the framework of the advanced adiabatic approach for the time- independent Schr\"odinger equation. Protonium formation cross sections for states with different $n$ and $l$ spherical quantum numbers are also obtained. Total ionization and protonium formation cross sections are compared with other calculations in the interval of impact energies from $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to $10\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. We show that diabatic states promoted into the continuum can be rigorously defined within the advanced adiabatic framework.

Formation of theH−ion in the positronium–hydrogen-atom collision

The cross sections (both differential and total) for ${\mathrm{H}}^{\ensuremath{-}}$ formation in the process $\mathrm{Ps}(1s)+\mathrm{H}(1s)\ensuremath{\rightarrow}{e}^{+}+{\mathrm{H}}^{\ensuremath{-}}$ in the energy range $10--500\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ have been studied. The calculations have been performed in the framework of the Coulomb-modified eikonal approximation and take account of the long-range Coulomb attraction between the outgoing ${e}^{+}$ and ${\mathrm{H}}^{\ensuremath{-}}$ ion in the final channel.

Hyperfine Level–changing Collisions of Hydrogen Atoms and Tomography of the Dark Age Universe

We present accurate rates for spin exchange processes in collisions of hydrogen atoms for temperatures in the range 1 K < T < 300 K. A fully quantal, multichannel theory is employed in the calculations. We discuss the respective roles of spin exchange and long-range magnetic interactions in collisional redistribution of sublevel populations. We use the rates to calculate the spin temperature of hydrogen in the universe at 20 < z < 200 and explore its sensitivity to the collision data. We stress the important role of atomic collisions in enabling 21 cm tomography, recently proposed by Loeb & Zaldarriaga, of the dark age universe.

Excessive Balmer line broadening in a plane cathode abnormal glow discharge in hydrogen

Results of a Doppler spectroscopy study of the hydrogen Balmer alpha line in an abnormal glow discharge operated in pure hydrogen are reported. Measurements of line shapes are performed side-on to the discharge axis in a low electric field region of negative glow. The excessive Balmer alpha broadening is detected and its presence and linewidth is related to the collisions of fast hydrogen atoms with molecular hydrogen. The collision model enabled also an estimation of effective cross section data from the Balmer alpha axial intensity decay curves. Large excessive Balmer alpha line broadening in pure hydrogen and its dependence upon the direction of observation with respect to the electric field is in contradiction to the resonance transfer model, proposed byMills et al. in several publications [see, e.g., IEEE Trans. Plasma Sci. 31, 338 (2003)].

Total, Partial, and Differential Ionization Cross Sections in Proton–Hydrogen Atom Collisions in the Energy Region of 0.1–10 keV/u

Single-differential, partial, and total ionization cross sections for the proton–hydrogen atom collision system in the energy region of 0.1–10 keV/u are determined by using the molecular-orbital close-coupling method within a semiclassical formalism. The present cross sections are in an excellent agreement with the recent experiments of Shah et al. [J. Phys. B. 31, L757 (1998)], but decrease more rapidly than the cross sections measured by Pieksma et al. [Phys. Rev. Lett. 73, 46 (1994)] with decreasing energy. The numerical data for all calculated cross sections are included in this paper. A critical evaluation of the existing data for the ionization process in the keV energy range is performed both for the experiment and theory. The recommended data are obtained from a converged close-coupling expansion, which in total includes 362 bound and continuum channels with their wave functions augmented by the electron translation factor in order to insure the correct scattering boundary condition.

Origin of H− in collisions of hydrogen atoms with an adsorbate-covered Cu(100) surface

Results are presented from an experimental study of electron capture and loss during collisions of low-energy hydrogen atoms and ions with a residual gas covered metal surface. Ground-state hydrogen atoms (H1s) and the atomic ions (H−,H+) were scattered from a Cu(100) surface with energies ranging from 25to200eV. Energy loss analysis indicates that the dominant process leading to H− formation proceeds directly through the adsorbed vacuum species, with the metallic crystal acting primarily as a physical scattering platform. In addition, ion beam experiments reveal effects due to the charge state of the incident particle that are inconsistent with the results expected for clean metal surfaces. Measurements indicate that the neutralization of incident ions as expected for clean metals is suppressed, a feature that is attributed to the band gap and dielectric strength of the adsorbed layer.

Laboratory Detection of the Lyβ 1150 Å Quasi-molecular Satellite Seen in Far Ultraviolet Spectroscopic Explorer Observations of the White Dwarf G226-29

We report a laboratory measurement of the 1150 Å collision-induced quasi-molecular satellite of Lyβ. It was first identified in Far Ultraviolet Spectroscopic Explorer observations of the cool DA white dwarf G226-29. The satellite, predicted by theoretical calculations of the complete profile of the hydrogen Lyα and Lyβ lines, arises from transitions occurring during close collisions of hydrogen atoms. Fits of model atmospheres with these profiles to observed spectra provide accurate constraints on stellar temperature and gravity. The comparison here of the far-ultraviolet absorption spectrum of a laser-produced hydrogen plasma with the theoretical absorption profiles and a model of the plasma confirms the underpinning atomic and molecular theory.

Far Ultraviolet Spectroscopic Explorer Observations of G226-29: First Detection of the H 2 Quasi-molecular Satellite at 1150

We present new far-ultraviolet observations of the pulsating DA white dwarf G226-29 obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This ZZ Ceti star is the brightest one of its class and the coolest white dwarf observed by FUSE. We report the first detection of the broad quasi-molecular collision-induced satellite of Ly? at 1150 ?, an absorption feature that is due to transitions that take place during close collisions of hydrogen atoms. The physical interpretation of this feature is based on recent progress of the line-broadening theory of the far wing of Ly?. This predicted feature had never been observed before, even in laboratory spectra.

Electron capture by protons and electron loss from hydrogen atoms incollisions with hydrocarbon and hydrogen molecules in the 60–120 keV energyrange

Cross sections for electron capture by 60–120 keV protons and electron loss from60–120 keV hydrogen atoms in collisions with molecular hydrogen, methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), propylene(C3H6), andpropane (C3H8)have been measured. We found that the capture and loss cross sections dependlinearly on the number of carbon atoms and the number of hydrogen atoms in thetarget molecules, as predicted by the simple additivity rule. The success of theadditivity rule allowed the extraction of effective electron capture andloss cross sections for atomic hydrogen and atomic carbon targets.

Multichannel study of spin-exchange and hyperfine-induced frequency shift and line broadening in cold collisions of hydrogen atoms

We calculate the cross sections that characterize the frequency shift and broadening of the resonance magnetic hyperfine transition in atomic hydrogen due to collisions with hydrogen atoms in a masing cavity. We consider collision energies that correspond to the temperature range, $10\mathrm{mK}&lt;T&lt;1000\mathrm{K},$ and introduce and apply a fully quantal, multichannel collision theory for the entire temperature range under consideration. Our results for the spin-exchange induced cross sections at room temperature, are in harmony with previous calculated values and experimental measurements. For the hyperfine-induced cross sections, our predicted values are in fair agreement with measurements at cryogenic temperatures with the exception of the hyperfine frequency shift at 0.5 K and 323 K for which we obtain a different sign. A study of possible resonance structures shows that they cannot be invoked to resolve the discrepancy and nor can any plausible modification of the ${X}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ and ${b}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ interaction potentials.

Cold collisions of spin-polarized metastable hydrogen atoms

Cross sections and rate coefficients for collisions of two spin-polarized metastable hydrogen atoms are calculated at low energies and temperatures using complex scaling and Feshbach projection methods for the determination of the molecular potential-energy curves. The inclusion of the Lamb shift and fine-structure splitting of the excited $n=2$ atoms leads to an attractive van der Waals potential for two ${2s}_{1/2}$ atoms with an exceptionally large coefficient. The quenching of the metastable atoms is dominated at temperatures below 20 mK by ionization. The calculated values for the total loss rate coefficient at 87 \ensuremath{\mu}K and 230 \ensuremath{\mu}K are within a factor of 2 or 3 of the error bars of recent measurements of metastable loss rate coefficients.

Theoretical investigation of the electron capture in collisions of O8+ ion with H and He atoms: Comparison between the Landau–Zener and the over-barrier models

Analytical and semi-analytical theories based on the molecular and classical treatments have been applied for electron capture processes in collisions of hydrogen and helium atoms with low energy (E=0.1,…,10 keV/amu) O8+ bare ion impact. The used Landau–Zener model (LZ), which allows to give analytical formulas for the cross-sections in our recent approach and the modified over-barrier model (OBM) [Phys. Rev. A 62 (2000) 042711] are briefly summarized. The obtained values of the LZ and OBM cross-sections are compared with experimental data and the results of other sophisticated numerical calculations. It is found that the LZ model provides much reliable cross-sections for charge transfer than the OBM model.