Collisions Of Hydrogen Atoms Research Articles

Mechanistic Insights into Nonadiabatic Interband Transitions on a Semiconductor Surface Induced by Hydrogen Atom Collisions.

To understand the recently observed enigmatic nonadiabatic energy transfer for hyperthermal H atom scattering from a semiconductor surface, Ge(111)c(2 × 8), we present a mixed quantum-classical nonadiabatic molecular dynamics model based on the time-dependent evolution of Kohn-Sham orbitals and a classical path approximation. Our results suggest that facile nonadiabatic electronic transitions from the valence band to the conduction band occur selectively at the rest atom site, where surface states are doubly occupied, but not at the adatom site, where empty surface states are localized. This drastic site specificity can be attributed to the changes of the local band structure upon energetic H collisions at different surface sites, leading to transient near degeneracies and significant couplings between occupied and unoccupied orbitals at the rest atom but not at the adatom. These insights shed valuable light on the collision-induced nonadiabatic dynamics at semiconductor surfaces.

Energy transfer during hydrogen atom collisions with surfaces

Hydrogen adsorption is an important cornerstone of many natural and technological processes, yet its atomistic details are not fully understood. Here, we review recent advances to understand the interactions between atomic hydrogen and substrates that lead to adsorption and discuss open questions.

Hydrogen atom collisions with a semiconductor efficiently promote electrons to the conduction band

The Born–Oppenheimer approximation is the keystone of modern computational chemistry and there is wide interest in understanding under what conditions it remains valid. Hydrogen atom scattering from insulator, semi-metal and metal surfaces has helped provide such information. The approximation is adequate for insulators and for metals it fails, but not severely. Here we present hydrogen atom scattering from a semiconductor surface: Ge(111)c(2 × 8). Experiments show bimodal energy-loss distributions revealing two channels. Molecular dynamics trajectories within the Born–Oppenheimer approximation reproduce one channel quantitatively. The second channel transfers much more energy and is absent in simulations. It grows with hydrogen atom incidence energy and exhibits an energy-loss onset equal to the Ge surface bandgap. This leads us to conclude that hydrogen atom collisions at the surface of a semiconductor are capable of promoting electrons from the valence to the conduction band with high efficiency. Our current understanding fails to explain these observations.

Nl-Selective Classical Charge-Exchange Cross Sections in Be4+ and Ground State Hydrogen Atom Collisions

Charge-exchange cross sections in Be4+ + H(1s) collisions are calculated using the three-body classical trajectory Monte Carlo method (CTMC) and the quasi-classical trajectory Monte Carlo method of Kirschbaum and Wilets (QCTMC) for impact energies between 10 keV/amu and 300 keV/amu. We present charge-exchange cross sections in the projectile n = 2 and nl = 2s, 2p states. Our results are compared with the previous quantum-mechanical approaches. We found that the QCTMC model is a powerful classical model to describe the state-selective charge-exchange cross sections at lower impact energies and the QCTMC results are in good agreement with previous observations.

Distinctive Features of Charge Exchange Involving the Second Flavor of Hydrogen Atoms—The Candidates for Dark Matter

The second flavor of hydrogen atoms (SFHA) refers to the kind of hydrogen atoms that have only the states of the zero orbital angular momentum (the S-states), both in the discrete and continuous spectra. They were first discovered theoretically in one of my earlier papers, where a proof of their existence was also provided by analyzing atomic experiments concerning the high-energy tail of the linear momentum distribution in the ground state of hydrogen atoms. From a theoretical point of view, the discovery was based on the standard Dirac equation for hydrogen atoms without changing the existing physical laws. Recently, the existence of the SFHA was seemingly also confirmed by two types of astrophysical observations: the allowance for the SFHA explained the puzzling results concerning both the anomalous absorption of the redshifted 21 cm spectral line from the early Universe, and the observations by the Dark Energy Survey (DES) team where it was found that the distribution of dark matter in the Universe is noticeably smoother than predictions employing Einstein’s relativity. In the present review, we exhibit results from two recent papers where attention was brought to a visible difference in the cross-sections of the resonant charge exchange for collisions of the SFHA with incoming protons, compared to collisions of the usual hydrogen atoms with incoming protons. It was shown that, after taking into account the SFHA, there is a better agreement with the corresponding experimental cross-section. Coupled with the previous evidence of the existence of the SFHA, deduced from the analysis of the other kind of atomic experiments, and evidenced by two different kinds of astrophysical observations, this strengthens the standing of the SFHA as the most probable candidate for all or a part of dark matter.

Depolarization of spin-polarized hydrogen via collisions with chlorine atoms at ultrahigh density

Recently, the production of ultrahigh-density (∼1019cm−3) spin-polarized deuterium (SPD) atoms was demonstrated, from the photodissociation of deuterium iodide, but the upper density limit was not determined. Here, we present studies of spin-polarized hydrogen (SPH) densities up to 1020cm−3, by photodissociating 5 bar of hydrogen chloride with a focused 213 nm, 150 ps laser pulse. We extract the depolarization cross-section of hydrogen and chlorine atom collisions, which is the main depolarization mechanism at this high-density regime, to be σH-Cl=7(2)×10−17cm2. We discuss the conditions under which the ultrahigh SPH and SPD densities can be reached, and the potential applications to ultrafast magnetometry, laser-ion acceleration, and tests of polarized nuclear fusion.

Excitation and charge transfer in low-energy hydrogen atom collisions with neutral manganese and titanium

Data for inelastic processes due to hydrogen atom collisions with manganese and titanium are needed for accurate modeling of the corresponding spectra in late-type stars. In this work excitation and charge transfer in low-energy Mn+H and Ti+H collisions have been studied theoretically using a method based on an asymptotic two-electron linear combination of an atomic orbitals model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multichannel Landau-Zener model to treat the dynamics. Extensive calculations of charge transfer (mutual neutralization, ion-pair production), excitation and de-excitation processes in the two collisional systems are carried out for all transitions between covalent states dissociating to energies below the first ionic limit and the dominating ionic states. Rate coefficients are determined for temperatures in the range 1000–20 000 K in steps of 1000 K. Like for earlier studies of other atomic species, charge transfer processes are found to lead to much larger rate coefficients than excitation processes.

Kinematically complete scattering cross sections in collisions between positron and hydrogen atom

SynopsisA three-body classical trajectory Monte Carlo (CTMC) method is used to describe positron and hydrogen atom collisions. The total and multi-differential ionization and charge exchange cross sections are calculated and compared with experimental data and other theoretical results.

Efficient Computation of Collisional ℓ-mixing Rate Coefficients in Astrophysical Plasmas

Abstract We present analytical expressions for direct evaluation of ℓ-mixing rate coefficients in proton-excited hydrogen atom collisions and describe a software package for efficient numerical evaluation of the collisional rate coefficients. Comparisons between rate coefficients calculated with various levels of approximation are discussed, highlighting their range of validity. These rate coefficients are benchmarked for radio recombination lines for hydrogen, evaluating the corresponding departure coefficients from local thermal equilibrium.

Atom-Rydberg Atom Collisions in Hydrogen Plasmas: Cross Sections and Rate Coefficients

Non-LTE modelling requires accurate atomic data e.g. collisional excitation and ionization cross-sections and rate coefficients. In order to improve the modeling of the solar photosphere, as well as to model atmospheres of other similar and cooler stars where the main constituent is also hydrogen, it is necessary to take into account the influence of all the relevant collisional processes on the excited-atom populations in weakly ionized hydrogen plasmas. In this context we present the data needed for the inclusion of the specific atomic collisional processes in the investigation of the optical and kinetic properties of weakly ionized stellar atmospheres layers. The ionization processes in collisions of excited hydrogen atoms with atoms in ground states were considered for the principal quantum numbers 2 ≤ n ≤ 20 and temperatures 4000 K ≤ T ≤ 20000 K.

Excitation and charge transfer in low-energy hydrogen atom collisions with neutral carbon and nitrogen

Low-energy inelastic collisions with neutral hydrogen atoms are important processes in stellar atmospheres, and a persistent source of uncertainty in non-LTE modelling of stellar spectra. We have calculated and studied excitation and charge transfer of C I and of N I due to such collisions. We used a previously presented method that is based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions for the adiabatic potential energies, combined with the multichannel Landau-Zener model for the collision dynamics. We find that charge transfer processes typically lead to much larger rate coefficients than excitation processes do, consistent with studies of other atomic species. Two-electron processes were considered and lead to non-zero rate coefficients that can potentially impact statistical equilibrium calculations. However, they were included in the model in an approximate way, via an estimate for the two-electron coupling that was presented earlier in the literature: the validity of these data should be checked in a future work.

The influence of spectator nuclear motion on the nonresonant formation of muonic hydrogen molecules

A new approach has been developed in order to estimate the influence of the nuclear motion of the hydrogenic molecules on the nonresonant formation of the muonic hydrogen molecules. The rates of such nonresonant reactions, in collisions of the muonic hydrogen atoms with the symmetric and asymmetric hydrogenic molecules, have been calculated for a wide range of kinetic energies up to 100 eV. It has been shown that the internal nuclear motion significantly affects the nonresonant formation processes at specified low collision energies. The energy-dependent formation rates are indispensable in the interpretation of the already-measured data and for the upcoming and planned experiments in low-energy muon physics.

An empirical recipe for inelastic hydrogen-atom collisions in non-LTE calculations

Context. Determination of high-precision abundances of late-type stars has been and always will be an important goal of spectroscopic studies, which requires accurate modeling of their stellar spectra with non-local thermodynamic equilibrium (NLTE) radiative transfer methods. This entails using up-to-date atomic data of the elements under study, which are still subject to large uncertainties.Aims. We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, Na I, Mg I, Al I, Si Iand Ca I) using simple polynomial equations.Methods. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We then compare the results to calculations computed using the published quantum rates for all the corresponding elements. We also compare to results computed using excitation collisional rates via the commonly used Drawin equation for different fudge factors,SMH, applied.Results. We demonstrate that our proposed method is able to reproduce the NLTE abundance corrections performed with the quantum rates for different spectral types and metallicities for representative Na Iand Al Ilines to within ≤0.05 dex and ≤0.03 dex, respectively. For Mg Iand Si Ilines, the method performs better for the cool giants and dwarfs, while larger discrepancies up to 0.2 dex could be obtained for some lines for the subgiants and warm dwarfs. We obtained larger NLTE correction differences between models incorporating Drawin rates relative to the quantum models by up to 0.4 dex. These large discrepancies are potentially due to ignoring either or both CT and ionization collisional processes by hydrogen in our Drawin models.Conclusions. Our general empirical fitting method (EFM) for estimating hydrogen collision rates performs well in its ability to reproduce, within narrow uncertainties, the abundance corrections computed with models incorporating quantum collisional rates. It performs generally best for the cool and warm dwarfs, with slightly larger discrepancies obtained for the giants and subgiants. It could possibly be extended in the future to transitions of the same elements for which quantum calculations do not exist, or, in the absence of published quantum calculations, to other elements as well.

Excitation and charge transfer in low-energy hydrogen atom collisions with neutral iron

Data for inelastic processes due to hydrogen atom collisions with iron are needed for accurate modelling of the iron spectrum in late-type stars. Excitation and charge transfer in low-energy Fe+H collisions is studied theoretically using a previously presented method based on an asymptotic two-electron linear combination of atomic orbitals model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multi-channel Landau–Zener model. An extensive calculation including 166 covalent states and 25 ionic states is presented and rate coefficients are calculated for temperatures in the range 1000–20 000 K. The largest rates are found for charge transfer processes to and from two clusters of states around 6.3 and 6.6 eV excitation, corresponding in both cases to active 4d and 5p electrons undergoing transfer. Excitation and de-excitation processes among these two sets of states are also significant.

Excitation and charge transfer in low-energy hydrogen atom collisions with neutral oxygen

Excitation and charge transfer in low-energy O+H collisions is studied; it is a problem of importance for modelling stellar spectra and obtaining accurate oxygen abundances in late-type stars including the Sun. The collisions have been studied theoretically using a previously presented method based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multichannel Landau-Zener model. The method has been extended to include configurations involving excited states of hydrogen using an estimate for the two-electron transition coupling, but this extension was found to not lead to any remarkably high rates. Rate coefficients are calculated for temperatures in the range 1000–20 000 K, and charge transfer and (de)excitation processes involving the first excited S-states, 4s.5So and 4s.3So, are found to have the highest rates.

Erratum: Excitation and charge transfer in low-energy hydrogen-atom collisions with neutral atoms: Theory, comparisons, and application to Ca [Phys. Rev. A 93 , 042705 (2016)

Received 30 May 2017DOI:https://doi.org/10.1103/PhysRevA.95.069906©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasScattering of atoms, molecules, clusters & ionsAtomic, Molecular & Optical

Excitation of hydrogen atoms in collisions with helium atoms: the role of electron–electron interaction

Cross sections for producing H(nl) excited state atoms in H(1s) + He(1s2) collisions are calculated using the CTMC method, at impact energies ranging from 20 eV to 100 keV. The role of the electron correlation is studied. In the first step, the interactions between each pair of the three electrons are neglected. This leads to disagreement of the calculated total cross section for producing H(2l) atoms with previous experimental and theoretical results. In a second step, the electron–electron interaction is taken into account in a rigorous way, that is, in the form of the pure Coulomb potential. To make sure that the He target is stable before the collision, phenomenological potentials for the electron–helium-nucleus interactions that simulate the Heisenberg principle are included in addition to the Coulomb potential. The excitation cross section calculated in the frame of this model is in remarkable agreement with previous data in the range between 200 eV and 5 keV. At other energies, discrepancies are revealed, but only by a factor of less than 2 at high energies. The present results show the decisive role of the electron–electron interaction during collisions. In addition, they demonstrate the ability of classical mechanics to take into account the effects of the electron correlation.

Electron capture and excitation processes in H+−H collisions in dense quantum plasmas

Electron capture and excitation processes in proton–hydrogen atom collisions taking place in dense quantum plasmas are studied by employing the two-centre atomic orbital close-coupling (TC-AOCC) method. The Debye–Hückel cosine (DHC) potential is used to describe the plasma screening effects on the Coulomb interaction between charged particles. The properties of a hydrogen atom with DHC potential are investigated as a function of the screening strength of the potential. It is found that the decrease in binding energy of nl levels with increasing screening strength is considerably faster than in the case of the Debye–Hückel (DH) screening potential, appropriate for description of charged particle interactions in weakly coupled classical plasmas. This results in a reduction in the number of bound states in the DHC potential with respect to that in the DH potential for the same plasma screening strength, and is reflected in the dynamics of excitation and electron capture processes for the two screened potentials. The TC-AOCC cross sections for total and state-selective electron capture and excitation cross sections with the DHC potential are calculated for a number of representative screening strengths in the 1–300 keV energy range and compared with those for the DH and pure Coulomb potential. The total capture cross sections for a selected number of screening strengths are compared with the available results from classical trajectory Monte Carlo calculations.

Quantum Scattering Calculations of Transport Properties for the H-N2 and H-CH4 Collision Pairs.

Transport properties for collisions of hydrogen atoms with molecular nitrogen and methane were calculated through close-coupling quantum scattering calculations. For these calculations, potential energy surfaces for the interaction of H atoms with these molecules, with their geometries fixed at the respective equilibrium structures, were obtained with a coupled cluster method that included single, double, and (perturbatively) triple excitations [RCCSD(T)]. The computed transport properties for H-N2 were found to be similar in magnitude to those computed in a previous study but significantly different from those obtained through the conventional approach that employs isotropic Lennard-Jones (LJ) (12-6) potentials. The differences in the transport properties for H-CH4 computed in this work and those estimated with isotropic LJ potentials are somewhat smaller.

Alignment of H(2p) in collisions of protons and antiprotons with hydrogen atoms with screened Coulomb interaction

The effects of screened Coulomb interaction on the alignment of H(2p) state produced in collisions of hydrogen atoms with protons and antiprotons are investigated in the energy range 1–200 keV by using the two-center atomic orbital close-coupling (TC-AOCC) method. It is shown that the decrease of the binding energy of hydrogen nl-states and the reduction of the number of bound states with increasing the strength of the screening affect significantly the alignment degree and its energy dependence. In the case of antiproton-H collision the difference between the alignments with screened and unscreened Coulomb potential increases with increasing the strength of the screening in the entire energy range above 2 keV/u, while in the case of proton-H collision it does so only in the energy range 5–25 keV/u.