Low-energy Atomic Collisions Research Articles

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.

Status of CRYRING@ESR and preparations for first experiments

SynopsisCRYRING was moved from Stockholm to Darmstadt, modernized and integrated into the GSI/FAIR beamline topology behind ESR. As CRYRING@ESR, it will receive and store heavy, highly charged ions from all species the present accelerator chain is capable of producing. An extensive research program on low-energy atomic collisions, spectroscopy and nuclear reactions was proposed. The facility is gradually completing commissioning, ion beams from the local injector branch have already been stored and prototype experiments performed. We present the machine status and highlight some planned experiments.

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.

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.

Correlated Dynamics in a Synthetic Lattice of Momentum States.

We study the influence of atomic interactions on quantum simulations in momentum-space lattices (MSLs), where driven transitions between discrete momentum states mimic transport between sites of a synthetic lattice. Low-energy atomic collisions, which are short ranged in real space, relate to nearly infinite-ranged interactions in momentum space. However, the added exchange energy between atoms in distinguishable momentum states leads to an effectively attractive, finite-ranged interaction between atoms in momentum space. In this Letter, we observe the onset of self-trapping driven by such interactions in a momentum-space double well, paving the way for more complex many-body studies in tailored MSLs. We consider the types of phenomena that may result from these interactions, including the formation of chiral solitons in zigzag flux lattices.

Low-energy inelastic atomic collisions of magnesium and hydrogen

The available quantum calculations of inelastic cross sections were analyzed for processes in collisions Mg + H and Mg+ + H–. The cross sections with large values were obtained with high accuracy. These processes are of particular interest for astrophysics.

The dependence of scattering length on van der Waals interaction and reduced mass of the system in two-atomic collision at cold energies

The static-exchange model (SEM) and the modified static-exchange model (MSEM) recently introduced by Ray [1] is applied to study the elastic collision between two hydrogen-like atoms when both are in ground states considering the system as a four-body Coulomb problem in the center of mass frame, in which all the Coulomb interaction terms in direct and exchange channels are treated exactly. The SEM includes the non-adiabatic short-range effect due to electron-exchange. The MSEM added in it, the long-range effect due to induced dynamic dipole polarizabilities between the atoms e.g. the Van der Waals interaction. Applying the SEM code in different H-like two-atomic systems, a reduced mass dependence on scattering length is observed. Again applying the MSEM code on H(1s)-H(1s) elastic scattering and varying the minimum values of interatomic distance, the dependence of scattering length on the effective interatomic potential consistent with the existing physics are observed. Both these basic findings in low and cold energy atomic collision physics are quite useful and are being reported for the first time.

Excitation and charge transfer in low-energy hydrogen-atom collisions with neutral atoms: Theory, comparisons, and application to Ca

A theoretical method for the estimation of cross sections and rates for excitation and charge transfer processes in low-energy hydrogen atom collisions with neutral atoms, based on an asymptotic two-electron model of ionic-covalent interactions in the neutral atom-hydrogen atom system, is presented. The calculation of potentials and non-adiabatic radial couplings using the method is demonstrated. The potentials are used together with the multi-channel Landau-Zener model to calculate cross sections and rate coefficients. The main feature of the method is that it employs asymptotically exact atomic wavefunctions, which can be determined from known atomic parameters. The method is applied to Li+H, Na+H, and Mg+H collisions, and the results compare well with existing detailed full-quantum calculations. The method is applied to the astrophysically important problem of Ca+H collisions, and rate coefficients are calculated for temperatures in the range 1000-20000 K.

Simulation of ion beam sputtering with SDTrimSP, TRIDYN and SRIM

• We present a comparison between experimental sputter data and binary Monte Carlo simulation programs SRIM, TRIDYN and SDTrimSP, which provide a detailed insight into the capabilities of the commonly used programs to simulate ion sputtering in a quantitative way. • We show that SDTrimSP is able to describe different features of ion beam erosion quantitatively and in good agreement with many experimental data. • We reveal very important discrepancies for simulation results obtained with SRIM, in particular the angular distribution of sputtered atoms for light target elements and the sputter yield dependence on ion incidence angle. • We also present a comparison with experimental data which shows the limitations of the commonly used ZBL potential to correctly describe low energy atomic collisions. A quantitative simulation of ion beam sputtering and related collision cascade effects is essential for applications of ion beam irradiation in thin film deposition, surface treatment and sculpting with focused ion beams, ion beam smoothing of surfaces and ion-induced nanopattern formation. The understanding of fundamental ion–solid interaction processes relevant for nanostructure formation, ion-induced mass redistribution, sputter yield amplification, ion beam mixing and dynamic compositional changes requires reliable simulations of ion-solid interaction processes in particular at low ion energies. In this contribution we discuss the possibilities, the key benefits and the limitations of three popular binary collision Monte Carlo simulation programs (SDTrimSP, TRIDYN and SRIM). The focus will be set to the calculation of angle dependent sputter yields, angular distribution of sputtered particles, sputter yields for compound materials, sputter yield amplification effects, as well as the extraction of parameters relevant for modeling ion-induced surface pattern formation from vacancy and recoil atom distributions.

Classical interpretation of probability oscillations in low-energy atomic collisions

We perform close-coupling molecular and classical trajectory Monte Carlo (CTMC) calculations of charge-exchange probabilities in low-energy ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}+\mathrm{H}$ collisions. We consider head-on collisions with a zero impact parameter and study the dependence of the charge-exchange probability on the impact velocity. This probability exhibits oscillations generally understood as signatures of quantum phase effects. CTMC calculations with dimensionality reduced to one fairly reproduce the oscillations and show that these latter stem from the back and forth motion of the electron between the nuclear centers in the internuclear region. In three dimensions, the probability oscillations are generally washed out in the statistical CTMC framework because of the electron transverse degrees of freedom. However three-dimensional Bohmian trajectories ascertain the classical interpretation of the oscillations in a quantum framework.

On inelastic hydrogen atom collisions in stellar atmospheres

The influence of inelastic hydrogen atom collisions on non-LTE spectral line formation has been, and remains to be, a significant source of uncertainty for stellar abundance analyses, due to the difficulty in obtaining accurate data for low-energy atomic collisions either experimentally or theoretically. For lack of a better alternative, the classical “Drawin formula” is often used. Over recent decades, our understanding of these collisions has improved markedly, predominantly through a number of detailed quantum mechanical calculations. In this paper, the Drawin formula is compared with the quantum mechanical calculations both in terms of the underlying physics and the resulting rate coefficients. It is shown that the Drawin formula does not contain the essential physics behind direct excitation by H atom collisions, the important physical mechanism being quantum mechanical in character. Quantitatively, the Drawin formula compares poorly with the results of the available quantum mechanical calculations, usually significantly overestimating the collision rates by amounts that vary markedly between transitions.

Nuclear reactions induced by stau atomic collisions in big-bang nucleosynthesis

Calculations of low energy atomic collision are performed for the new types of big-bang nucleosynthesis (BBN) reactions that are enhanced by a supersymmetric (SUSY) particle stau, a scalar partner of the tau lepton. If a stau has a lifetime > 103 s, it would capture a light element previously synthesized in standard BBN and form a Coulombic bound state. The bound state, an exotic atom, is expected to induce various nuclear reactions as in the muon catalyzed fusion. A negatively charged stau works as a catalyzer. We use a few-body calculational method developed by the authors, and provide precise cross sections and rates of the stau-catalyzed BBN reactions for the BBN network calculation.

Electron translation and asymptotic couplings in low-energy atomic collisions

The calculation of cross sections for inelastic and charge-exchange atomic collisions encounters a principal difficulty. The coordinates used to describe the collision event are usually not appropriate for the description of the collision partners long before and after the collision. As a consequence, the coupling matrix elements that govern inelastic transitions can remain nonzero for large internuclear distance. We present a simple procedure to deal with this situation in the framework of a full quantum-mechanical treatment. It applies to radial as well as rotational coupling mechanisms. We demonstrate the practical applicability by numerical examples.

Low-energy atomic collision with dipole interactions

We apply quantum defect theory to study low energy ground state atomic collisions including aligned dipole interactions such as those induced by an electric field. Our results show that coupled even ($l$) relative orbital angular momentum partial wave channels exhibit shape resonance structures while odd ($l$) channels do not. We analyze and interpret these resonances within the framework of multichannel quantum defect theory (MQDT).

Present Status of Highly Charged Ion Experiments with an ECR Ion Source at TMU

AbstractA highly charged ion facility built around a 14.25 GHz electron cyclotron resonance ion source has been constructed for the experimental investigation of low energy atomic collisions in Tokyo Metropolitan University. Four kinds of different experiments, namely differential scattering of electrons from ions, polarization spectroscopy for charge transfer reactions, Coulomb‐explosion‐imaging of molecular ions, and secondary ion measurements from solid surfaces, are taking place in parallel in this facility. General features of the facility and some recent results are described in this paper.

Low-energy three-body atomic collision within a coordinate-space integro-differential equation approach: Muon-transfer reaction

Low-energy three-body atomic collision within a coordinate-space integro-differential equation approach: Muon-transfer reaction

極低温移動管質量分析装置

The drift tube method, which is known as one of the powerful techniques for low energy atomic collision experiments, is explained with description on the history of this method. A new selected ion drift tube mass spectrometer, which has been developed for ion swarm experiments at very low temperature, is presented. Gas temperature of 2 K in the drift tube as the lowest one for this apparatus is achieved by liquid helium cooling. Details of this set-up are described, and several technical points for the low temperature experiment, which are concerned in the development of this apparatus, are discussed: analysis of the heat flow rates into the drift tube, thermal contraction of metallic parts, temperature measurements of gas and solid surfaces, and gas pressure correction with respect to the thermal transpiration effect because of large temperature difference between the drift tube and the capacitance manometer. Mass spectra with the injection of doubly charged rare gas ions, Ar2+ and Kr2+, into helium gas are shown as examples of experimental results with this apparatus. Charge transfer, ion-molecule and atomic association reactions take place in the drift tube. Structure of large helium cluster ions K2+Hen (n≤40), which have been observed to have magic numbers n=12 and n=32, is discussed.

Approach to electron translation in low-energy atomic collisions

We consider two conceptional problems that arise in the quantum-mechanical calculation of inelastic atomic collisions. The definition of the coupling matrix elements, which govern the inelastic transitions, seems to be ambiguous and the matrix elements can remain nonzero as the internuclear distance goes to infinity. We show that there is no ambiguity in the coupled equations, which describe inelastic collisions, and we demonstrate that the asymptotic couplings are required for a correct description of the asymptotic part of the scattering wave function. We develop on this basis a straightforward and simple procedure to deal with the asymptotic couplings in the framework of a full quantum approach and we apply it to a simple example of H+Na collisions.

Computer simulation of ion sputtering of polyatomic multilayered targets

The novel binary collision approximation Monte Carlo (BCA-MC) computer codes TRIRS and DYTRIRS for simulating ion sputtering of polyatomic nonuniform amorphous targets are presented. TRIRS simulates the collision cascade in a target and related secondary processes, including sputtering, damage generation etc., being more realistic than similar MC-BCA codes in modeling low-energy interatomic collisions. These improvements ensure better simulation of low-energy atomic collision processes in nonuniform targets, like sputtering, and ultra-low energy ion implantation. DYTRIRS is the extension of TRIRS which simulates the dynamics of the ballistic stage of ion-induced modification and sputtering for a target under high-fluence ion irradiation. The efficiency of DYTRIRS is verified by comparing the simulation of sputtering and secondary-ion mass-spectrometry in-depth compositional profiling of molecular-beam epitaxy grown two-dimensional (Al,Ga)AsGaAs (001) heterostructures, including structures with silicon and aluminum marker layers.