Hydrogen Collisions Research Articles

Elastic scattering of positrons from hydrogen atoms with exponential cosine-screened Coulomb potentials

We have investigated the elastic scattering of positrons from the ground state of hydrogen atoms interacting with exponential cosine-screened Coulomb potential (ECSCP). A simple expression for partial-wave scattering amplitude, derived within the framework of the second-order distorted wave Born approximation, has been used to calculate the scattering parameters. A detailed study has been made on differential and total cross sections in the energy range 20–300 eV. To the best of our knowledge, this study on the differential and total cross sections for elastic positron–hydrogen collisions in ECSCP is reported for the first time.

The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

Chemical abundances as measured from stellar spectral lines are often subject to uncertainties due to lack of accurate data for inelastic collisions, which is needed for non-local thermodynamic equilibrium (non-LTE) modelling. For cool stars, understanding of collision processes with electrons and hydrogen atoms is required to achieve high precision measurements. In this thesis, I have investigated the role of these collisions on the non-LTE formation of Li and Mg spectral lines in late-type stars.In the case of Li, electron impact excitation processes were calculated using the R-matrix with pseudo states method and the results found to agree well with recent calculations using the convergent close-coupling technique. These modern data were employed in non-LTE calculations by updating an existing model atom, which already included modern data for hydrogen collision processes. Our results were compared with calculations using older semi-empirical approximation calculations and only small differences were found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence of uncertainties in the electron collision data on non-LTE calculations is negligible. Indeed, together with the collision data for the charge transfer process Li + H ↔ Li+ + H- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.In the case of Mg, electron impact excitation processes were again calculated with the Rmatrix with pseudo states method, and used together with recent hydrogen collision calculations to build and test a model atom, without free parameters, for non-LTE modelling. Both electron and hydrogen collision processes, including charge transfer and excitation, are found to be important thermalising agents in various cases. The modelled spectra agree well with observed spectra from benchmark stars in the optical and infrared. The modelling predicts non-LTE abundance corrections ∆A(Mg)NLTE–LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Results of calculations in a large grid of 1D model atmospheres are presented, and the implications for studies of Mg discussed. The propagation of uncertainties in the inelastic collision data to those in stellar abundances is investigated, and found to lead to small uncertainties, once again typically less than 0.01 dex (2%), although for few stellar models in specific lines (e.g., metal-poor suns, in the 7691 A line) uncertainties can be as large as 0.03 dex (7%).

Cold three-body collisions in hydrogen–hydrogen–alkali-metal atomic systems

We have studied hydrogen-hydrogen-alkali three-body systems in the adiabatic hyperspherical representation. For the spin-stretched case, there exists a single $X$H molecular state when $X$ is one of the bosonic alkali atoms: $^{7}\mathrm{Li}$, $^{23}\mathrm{Na}$, $^{39}\mathrm{K}$, $^{87}\mathrm{Rb}$, or $^{133}\mathrm{Cs}$. As a result, the only recombination process is the one that leads to formation of $X$H molecules, $\text{H}+\text{H}+X\ensuremath{\rightarrow}X\text{H}+\text{H}$, and such molecules will be stable against vibrational relaxation. We have calculated the collision rates for recombination and collision-induced dissociation as well as the elastic cross sections for $\text{H}+X\text{H}$ collisions up to a temperature of 0.5 K, including the partial wave contributions from ${J}^{\ensuremath{\Pi}}={0}^{+}$ to ${5}^{\ensuremath{-}}$. We have also found that there is just one three-body bound state for such systems for ${J}^{\ensuremath{\Pi}}={0}^{+}$ and no bound states for higher angular momenta.

Multichannel excitation cross sections of sodium atoms at slow collisions with hydrogen atoms

Excitation cross sections at slow collisions of hydrogen and sodium atoms are calculated based on two sets of quantum-chemical data. The results of calculations permit one to conclude that, upon the excitation of the sodium atom from the ground state in the region of near-threshold energies, the cross sections are highly sensitive to matrix nonadiabaticity elements. In addition, the matrix nonadiabaticity element was varied for the transition 3s → 3p of the sodium atom at fixed collision energy near the reaction threshold. It was found that the variation leads to a significant change in the excitation cross section 3s → 3p, and the range of the energetic dependence of this cross section was determined.

Molecular dynamics simulation of energy exchange during hydrogen collision with graphite sheet containing a vacancy

Molecular dynamics simulation is applied to investigation of energy exchanges during hydrogen collision with graphite sheet containing a vacancy. The effects of the monovancancy defect on the energy exchanges are discussed in detail. This paper analyzes the energy loss of the incident hydrogen atom, the energy range for the adsorption process, and the energy transfer process for target atom, in the course of a hydrogen atom bombarding the carbon atom at the edge of monovacancy defect in the graphite sheet. The simulation results show that the adsorption process proceeds more easily when the graphite sheet contains a vacancy than when the graphite sheet has perfect crystalline structure. In certain areas of the graphite sheet, adsorption of an incident hydrogen atom can occur in two energy ranges. The sp2 structure as well as overhang configuration occurs when a hydrogen atom is adsorbed. This adsorption process does not reduce the C—C bond energy. It is found that the carbon atom at the edge of monovacancy defect can adsorb an incident hydrogen atom more easily but can not diffuse the gained energy as efficiently as in a perfect graphite sheet. These results are helpful for understanding the chemical erosion of carbon based materials and the ensuing tritium retention in fusion devices.

Absolute differential and total cross sections for neutral fragments from dissociative collisions of triatomic hydrogen like ions on He

Neutral fragment products from dissociative collisions of triatomic hydrogen like ions incident on He atoms were studied. Absolute differential and total cross sections are reported here in the energy range of 1.00–5.00 keV and scattering angles between −5.0° and 5.0°. The differential cross sections show decreasing behaviour with a slight structure around 2.0°. The total cross sections for all triatomic molecular ions studied in this work are found to be comparable for the same velocity (E/M). The measured cross sections are between 0.7 × 10−17 cm2 and 0.9 × 10−16 cm2. The present results for the neutral total cross section correlate very well with previously measured total ions cross section for H+3, D+3 and HD+2 on He.

Single electron loss in the collisions of lowly charged ions and atomic hydrogen

In this paper, the projectile electron loss cross sections of He +, Li +, Li 2+ and C 2+ colliding with atomic hydrogen are studied in the frame work of the extended over-barrier model at intermediate velocities (25–600 keV/u). The electron loss cross sections are calculated in terms of the interaction between the screened target nucleus and the active projectile electron and of the interaction between the projectile electron and the target electron. Compared with the measurements, this model satisfactorily reproduces the experimentally obtained energy dependence of the single electron loss cross sections over the energy range investigated here.

Molecular dynamics simulation of energy exchanges during hydrogen collision with graphite sheets

Experiments show that the energy of particles incident on divertor plates in fusion devices seldom exceeds 100 eV. Trim code and its variants are not suitable to predict the sputtering yield of carbon-based divertor plates for this energy range and, therefore, a dynamic model, taking into account the C–H bond formation and breaking, and the structure of carbon, is needed. In this paper, the molecular dynamics method is employed to investigate collision processes between incident hydrogen atoms and a graphene sheet. The simulation results demonstrate that the collision processes cannot be adequately described by a simple binary approximation. The energy transfer from the projectile to the graphite sheet exhibits a very complicated behavior when the kinetic energy of the incident hydrogen atom is below 30 eV, strongly depending on the impact position. When its kinetic energy is lower than 0.35 eV, the incident hydrogen is always reflected back from the single, perfect graphite sheet; when its kinetic energy is higher than 0.35 eV, then whether the incident particle penetrates the graphite sheet, is reflected back or is adsorbed depends on the impact position. In certain areas of the graphite sheet, either adsorption or reflection of an incident hydrogen atom can occur in two different energy ranges.

Hydrogen and helium charge exchange collisions at kiloelectronvolt energies: an electron–nuclear dynamics review

The electron transfer processes that occur during the collision of ions with atomic and molecular systems are of fundamental importance in the understanding of neutralization, fragmentation and bond rearrangement. In this work we present a review of the actual state in charge exchange processes in the few electronvolts up to several kiloelectronvolts within the electron–nuclear dynamics approach. We present the effects of single and double electron capture on stopping power, molecular fragmentation, differential and total cross sections. For the case of α particles, we present the effects of the target mass (target isotopic effect) when colliding with H, D and T atoms, together with the question of control of charge transfer by means of a short, intense laser pulse.

Oxygen lines in solar granulation

Aims: we seek to provide additional tests of the line formation of theoretical 3D solar photosphere models. In particular, we set out to test the spatially-resolved line formation at several viewing angles, from the solar disk-centre to the limb and focusing on atomic oxygen lines. The purpose of these tests is to provide additional information on whether the 3D model is suitable to derive the solar oxygen abundance. We also aim to empirically constrain the NLTE recipes for neutral hydrogen collisions, using the spatially-resolved observations of the OI 777 nm lines. Methods: using the Swedish 1-m Solar Telescope we obtained high-spatial-resolution observations of five atomic oxygen lines (along with lines for other species) for five positions on the solar disk. These observations have a high spatial and spectral resolution, and a continuum intensity contrast up to 9% at 615 nm. The theoretical line profiles were computed using the 3D model, with a full 3D NLTE treatment for oxygen and LTE for the other lines. Results: at disk-centre we find an excellent agreement between predicted and observed line shifts, strengths, FWHM and asymmetries. At other viewing angles the agreement is also good, but the smaller continuum intensity contrast makes a quantitative comparison harder. We use the disk-centre observations we constrain S_H, the scaling factor for the efficiency of collisions with neutral hydrogen. We find that S_H=1 provides the best match to the observations. Conclusions: overall there is a very good agreement between predicted and observed line properties over the solar granulation. This further reinforces the view that the 3D model is realistic and a reliable tool to derive the solar oxygen abundance.

Hyperfine transitions in ultracold hydrogen-antihydrogen collisions

We consider the hyperfine transitions in ultracold collisions of hydrogen (H) and antihydrogen $(\overline{\text{H}})$ atoms. The cross sections for transitions between various spin states are calculated. We show that hyperfine transitions in $\overline{\text{H}}\ensuremath{-}\text{H}$ collisions are basically driven by the strong force between proton and antiproton.

Energy relaxation in collisions of hydrogen and deuterium with oxygen atoms

Collision energy transfer processes between hydrogen, deuterium, and oxygen atoms in the upper atmospheres of the terrestrial planets are studied. A new set of interaction potentials has been constructed using an accurate ab initio method. Full orientation‐dependent scattering cross sections have been obtained quantum mechanically and have been incorporated into the construction of the linear Boltzmann kinetic equation describing the energy relaxation process. The isotope and temperature dependence of the energy relaxation parameters have been analyzed. Distributions of the secondary energetic recoil atoms have been computed and the fractions of hot atoms capable of escaping from the atmospheres of the terrestrial planets have been determined. For applications to atmospheric physics and astrophysics, we have computed effective hard sphere cross sections for O + H and O + D collisions that closely reproduce the energy relaxation kinetics obtained from the linear Boltzmann equation. These effective cross sections, which are functions of the laboratory frame collisional energy and the temperature of the bath gas, may be used in simulations of the thermalization of hot O, H, and D atoms and their escape from planets.

Spin exchange in hydrogen–antihydrogen collisions

We consider spin exchange in ultracold collisions of hydrogen (H) and antihydrogen atoms. The cross sections for transitions between various hyperfine states are calculated. We show that the process of spin exchange in –H collisions is basically driven by the strong force between a proton and an antiproton.

Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity

We present a detailed study of the non-Local Thermodynamic Equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in MARCS model atmospheres representative of late-type stars with spectral types F to K. We carried out the calculations using the statistical equilibrium code MULTI, including estimates of the impact on elemental abundance analysis. The atomic model employed includes, in particular, recent quantum-mechanical electron collision data. We confirm that the O I triplet lines form under non-LTE conditions in late-type stars, suffering negative abundance corrections with respect to LTE. At low metallicity, large line opacity stems from triplet-quintet intersystem electron collisions, a form of coupling previously not considered or seriously underestimated. The non-LTE effects become generally severe for models (both giants and dwarfs) with higher T_eff. Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance corrections tend to rapidly become more severe towards lower metallicity. When neglecting H collisions, they amount to as much as ~ 0.9 dex and ~ 1.2 dex, respectively at [Fe/H]=-3 and [Fe/H]=-3.5. Even when such collisions are included, the LTE abundance remains a serious overestimate, correspondingly by ~ 0.5 dex and ~ 0.9 dex at such low metallicities. Although the poorly known inelastic hydrogen collisions thus remain an important uncertainty, the large metallicity-dependent non-LTE effects seem to point to a resulting "low" (compared to LTE) [O/Fe] in metal-poor halo stars.

Collisional effects in the far red wing of Lyman-$\sf \alpha$

Context. An accurate determination of the line broadening of the Lyman series of atomic hydrogen has been shown to be fundamental to interpretating UV and FUV spectra of DA white dwarfs. Quasi-molecular lines have been detected in the red wing of Lyman-α, Lyman-β, and Lyman-γ. They arise from radiative collisions of excited atomic hydrogen with unexcited neutral hydrogen atoms or protons. Aims. The aim of this paper is twofold. First, we examine the range of validity of the one-perturber approximation widely used to calculate the line wing. Second, we study the relative contributions of the two main transitions contributing to the far wing of the Lyman-α line profile according to the effective temperature and perturber density. Methods. In cool white dwarfs, the perturber density is so high that the one-perturber approximation breaks down and the collisional effects must be treated by using the autocorrelation formalism in order to take into account simultaneous collisions with more than one perturbing atom. Results. We show that, at the low temperatures of cool white dwarfs, the contribution of the singlet X 1 Σ + → B 1 Σ + transition cannot be neglected in the calculation of Lyman-α profile perturbed by neutral hydrogen. A comparison with experimental laboratory spectra shows that the effects of multiple H-perturbers appear in the far wing. Conclusions. A reliable determination of the line profiles for the physical conditions of cool white dwarfs requires a unified theory that takes account of both the singlet and triplet transitions contributing to Lyman-α using accurate interaction potentials and radiative dipole transition moments. Multiple perturber effects must be considered and the autocorrelation formalism permits calculations for the densities of the atmospheres of cool white dwarfs.

NLTE abundances of Mn in a sample of metal-poor stars

Aims. Following our solar work, we perform NLTE calculations of the Mn abundance for fourteen stars with [Fe/H] from 0 to -2.5, mainly to show how NLTE affects Mn abundances in cool stars of different metallicities. Methods. The spectrum synthesis and Mn abundances are based on statistical equilibrium calculations using various estimates fo r the influence of hydrogen collisions. Results. The NLTE abundances of Mn in all studied stars are systematically higher than the LTE abundances. At low metallicities, the NLTE abundance corrections may run up to 0.5 - 0.7 dex. Instead of a strong depletion of Mn relative to Fe in metal-poor stars as found by the other authors, we only find slightly subsolar values of [Mn /Fe] throughout the range of metallicities analyzed here. Conclusions. The [Mn/Fe] trend in metal-poor stars is inconsistent with the predi ctions of galactic chemical evolution models, where Mn is less produced than Fe.

Elastic collisions of sulfur and hydrogen in their ground states at low temperatures and spectroscopic parameters of SH(X2∏) radical

This paper constructs the interaction potential of the SH(X2∏) radical by using the coupled-cluster singles-doubles-approximate-triples theory combining the correlation-consistent quintuple basis set augmented with the diffuse functions, aug-cc-pV5Z, in the valence range. Employing the potential, it accurately determines the spectroscopic parameters. The present De, Re, ωe, ωeχe, αe and Be values are of 3.7767eV, 0.13424 nm, 2699.846 cm−1, 47.7055cm−1, 0.2639cm−1 and 9.4414cm−1, respectively, which are in excellent agreement with those obtained from the measurements. A total of 19 vibrational states has been found when J = 0 by solving the radial Schrödinger equation of nuclear motion. The complete vibrational levels, classical turning points, initial rotation and centrifugal distortion constants when J = 0 are reported for the first time, which are in good accord with the experimental results. The total and various partial-wave cross sections are computed for the elastic collisions of sulfur and hydrogen in their ground states at low temperatures when two atoms approach each other along the SH(X2∏) potential energy curve. Over the impact energy range from l.0 × 10−11 to 1.0 × 10−4 a.u., eight shape resonances have been found in the total elastic cross sections. For each shape resonance, the resonant energy is accurately calculated. Careful investigations have pointed out that these resonances result from the l = 0, 1, 2, 3, 4, 6, 7, 8 partial-wave contributions.

Hydrogen collisions in planetary atmospheres, ionospheres, and magnetospheres

Hydrogen is the most abundant element in the universe. Molecular hydrogen is the dominant chemical species in the atmospheres of the giant planets. Because of their low masses, neutral and ionized hydrogen atoms are the dominant species in the high atmospheres of many planets. Finally, protons are the principal heavy component of the solar wind. Here we present a critical evaluation of the current state of understanding of the chemical reaction rates and collision cross sections for several important hydrogen collision processes in planetary atmospheres, ionospheres, and magnetospheres. Accurate ab initio quantum theory will play an important role. The collision processes are grouped as follows: (a) H ++H charge transfer, (b) H ++H 2( v) charge transfer and vibrational relaxation, and (c) H 2 (v, J)+H 2 vibrational, rotational, and ortho–para relaxation. In each case we provide explicit representations as tabulations or compact formulas. Particularly important conclusions are that H ++H 2( v) collisions are more likely to result in vibrational relaxation than charge transfer and H 2 ortho–para conversion is at least an order-of-magnitude faster than previously assumed.

Statistical equilibrium of silicon in the solar atmosphere

The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. High-resolution solar spectra were used to determine solar $\log gf\epsilon_{\rm Si}$ values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metal-poor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. The statistical equilibrium of \ion{Si}{i} turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental $f$-values of Garz corrected for the Becker et al.'s measurement is $7.52 \pm 0.05$. Combined with an extended line sample with selected NIST $f$-values, the solar abundance is $7.52 \pm 0.06$, with a nearly perfect ionisation equilibrium of $\Delta\log\epsilon_\odot(\ion{Si}{ii}/\ion{Si}{i}) = -0.01$.

Young Type Interference Effect on the Forward-Backward Asymmetry Parameter in Electron Emission from H2 Under Fast Ion Impact

We have investigated the double differential distribution of electron emission from molecular hydrogen in collisions with fast bare carbon ions in order to investigate the effect of Young type interference on the forward-backward angular asymmetry. The asymmetry parameter, derived from the cross sections for complementary forward and backward angles, shows an oscillatory behaviour as a function of electron velocity which is absent in atomic target such as He. It is shown that the asymmetry parameter which is based on the DDCS of H2 only, can be used as self normalizing way of the obtaining the Young type interference in an inversion symmetric homo-nuclear diatomic molecule like H2. The measured energy and angular distributions as well as the asymmetry parameter are compared with a molecular CDW-EIS (continuum distorted wave-eikonal initial state) model.