Atomic Collisions Research Articles

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.

Measurement of the Low-Energy Rb+–Rb Total Collision Rate in an Ion-Neutral Hybrid Trap**Supported by the National Natural Science Foundation of China under Grant Nos 11604334, 61227807 and 61575108, and the Tsinghua Initiative Scientific Research Program under Grant No 2013THZ02-3, and the Beijing Natural Science Foundation under Grant No 1164016.

An ion–neutral hybrid trap is built to study low-energy ion–atom collisions. The ion–neutral hybrid trap is combined with two spatially concentric traps: a linear Paul trap for the ionic species and a magneto-optical trap (MOT) for the neutral species. The total ion–atom collision rate coefficient between87Rb atoms and optically dark 87Rb+ ions is measured by monitoring the reduction of the steady-state MOT atoms by sequentially introducing various mechanisms, namely photoionization and ion–atom collisions. In contrast to other experiments, a concise experimental procedure is devised to obtain the loss rates of the MOT atoms due to photoionization and ion–atom collisions in one experimental cycle, and then the collision rate of cold 87Rb atoms with 87Rb+ ions is deduced to m3/s with K measured by the time of flight of the ion signal. The measurements show good agreement with the collision rate derived from the Langevin model.

Six decades of atomic collisions in solids

In response to an invitation by the organizers of the 27th international conference on atomic collisions in solids, a brief survey is presented, starting from the roots of the field in the 1950s and 1960s, of some major discoveries, longstanding problems, surprising findings and memorable controversies in topics covered by the conference. Considering the breadth of the field, the selection of topics is necessarily subjective, but with the emphasis on channeling, stopping and sputtering, three topical areas are discussed which have been active from the early 1960s until now.

Entanglement of two hybrid optomechanical cavities composed of BEC atoms under Bell detection

In this paper, firstly, we consider bipartite entanglement between each part of an optomechanical cavity composed of one dimensional Bose-Einstein condensate (BEC). we investigate atomic collision on the behavior of the BEC in the week photon-atom coupling constant, and use Bogoliubov approximation for the BEC. Secondly under above condition, we propose a scheme for entanglement swapping protocol wich involves tripartite systems. In our investigation, we consider a scenario where BECs, moving mirrors, and optical cavity modes are given in a Gaussian state with a covariance matrix (CM). By applying the Bell measurment to the output optical field modes, we show how the remote entanglement between two BECs, two moving mirrors, and BEC-mirror modes in different optomechanical cavity can be generated.

Dispersive interaction of a Bose-Einstein condensate with a movable mirror of an optomechanical cavity in the presence of laser phase noise

We theoretically investigate the dispersive interaction of a Bose-Einstein condensate (BEC) trapped inside an optomechanical cavity with a moving end mirror in the presence of the laser phase noise (LPN) as well as the atomic collisions. We assume that the effective frequency of the optical mode is much greater than those of the mechanical and the Bogoliubov modes of the movable mirror and the BEC . In the adiabatic approximation where the damping rate of the cavity is faster than those of the other modes, the system behaves as an effective two-mode model in which the atomic and mechanical modes are coupled to each other through the mediation of the optical field by an effective coupling parameter. We show that in the effective two-mode model, the LPN appears as a classical stochastic pump term which drives the amplitude quadratures of the mechanical and the Bogoliubov modes. It is also shown that a strong stationary mirror-atom entanglement can be established just in the dispersive and Doppler regimes where the two modes come into resonance with each other and the effect of the LPN gets very low.

Phase Transitions in Diffusion of Light.

It has been a long time belief that, with increasing the scattering strength of multiple scattering media, the transport of light gradually slows down and, eventually, comes to a halt corresponding to a localized state. Here we present experimental evidence that different stages emerge in this evolution, which cannot be described by classical diffusion with conventional scaling arguments. A microscopic model captures the relevant aspects of electromagnetic wave propagation and explains the competing mechanisms that prevent the three-dimensional wave localization. We demonstrate that strong evanescent-field couplings hinder the localization of wave resonances and, therefore, impede the slowing down of diffusion. The emerging out of equilibrium steady-state process resembles the diffusion of classical particles in spatially correlated random potentials and the thermalization of matter waves due to atomic collisions.

Friction and Structural Responses of Ion Implanted Polyoxymethylene Copolymer (POM-C)

Polyoxymethylene copolymer (POM-C) round block was implanted with 120 KeV ions of He to doses of 5 x 1016 and 1 x 1016 ions cm-2. It was also implanted with 120 KeV ions of Ar + He and He + Ne to dose of 1 x 1016 ions cm-2, respectively. The friction coefficient behavior of both implanted and unimplanted POM-C blocks was investigated using a ball on disk tribometer mechanism. The friction coefficient of He ion implanted POM-C block at a dose of 5 x 1016 ions cm-2 is lowest compared to all unimplanted and others ions doses implanted POM-C blocks. It also shows the moderate surface texturing (atomic rearrangement), lower surface micro-hardness and average surface roughness compared to both unimplanted and other ions doses implanted POM-C blocks due to well adjusted carbonization, cross-linking and ions-target atoms collisions, which is ascertained from SEM-EDS, Raman spectroscopic and surface profiling observations. The other ions doses implanted POM-C blocks demonstrate the higher friction coefficient and surface roughness with polymer surface deformation (crazing, cracking, pitting and gas evolution, bond breaking) due to severe chain scission, surface dose delivered atomic displacements and chemical structural degradation. It is concluded that the variation in friction coefficient behavior of POM-C block resulted from its structural response for ion beam implantation on the top surface.

Head-on Collisions of Xe Atoms Against Superfluid $$^4$$ 4 He Nanodroplets

We study the head-on collision of a heliophilic xenon atom with a superfluid \(^4\)He droplet made of 1000 atoms. At variance with the findings for a heliophobic cesium atom of a similar atomic weight, it is found that the xenon atom has to hit the droplet with a large kinetic energy in order to get across it without being captured. When it is not captured, the xenon impurity does not emerge as a bare atom; instead, due to its heliophilic character it carries away some helium atoms.

Product lambda-doublet ratios as an imprint of chemical reaction mechanism.

In the last decade, the development of theoretical methods has allowed chemists to reproduce and explain almost all of the experimental data associated with elementary atom plus diatom collisions. However, there are still a few examples where theory cannot account yet for experimental results. This is the case for the preferential population of one of the Λ-doublet states produced by chemical reactions. In particular, recent measurements of the OD(2Π) product of the O(3P)+D2 reaction have shown a clear preference for the Π(A′) Λ-doublet states, in apparent contradiction with ab initio calculations, which predict a larger reactivity on the A′′ potential energy surface. Here we present a method to calculate the Λ-doublet ratio when concurrent potential energy surfaces participate in the reaction. It accounts for the experimental Λ-doublet populations via explicit consideration of the stereodynamics of the process. Furthermore, our results demonstrate that the propensity of the Π(A′) state is a consequence of the different mechanisms of the reaction on the two concurrent potential energy surfaces

Determination of the state production ratios , and from fast () mixed-state He-like ion beams in collisions with H2 targets

New results are presented on the ratio concerning the production cross sections of Li-like quartet and doublet P states formed in energetic ion–atom collisions by single electron transfer to the metastable component of the He-like ion beam. Spin statistics predict a value of Rm = 2 independent of the collision system in disagreement with most reported measurements of . A new experimental approach is presented for the evaluation of Rm having some practical advantages over earlier approaches. It also allows for the determination of the separate contributions of ground- and metastable-state beam components to the measured spectra. Applying our technique to zero-degree Auger projectile spectra from 4.5 MeV (Benis et al 2002 Phys. Rev. A 65 064701) and 25.3 MeV (Zamkov et al 2002 Phys. Rev. A 65 062706) mixed state He-like ion collisions with H2 targets, we report new values of for boron and for fluorine. In addition, the ratios of and populations from either the metastable and/or ground state beam component, also relevant to this analysis, are evaluated and compared to previously reported results for carbon collisions on helium (Strohschein et al 2008 Phys. Rev. A 77 022706) including a critical comparison to theory.

Thermal accommodation at cold Ar atoms collisions with small Fe clusters at different temperatures

Thermal accommodation coefficients at collisions of solitary argon atoms at room temperature with iron clusters at temperatures 200-2500 K are calculated. Molecular dynamics method is applied. Finnis-Sinclair potential is used to describe interaction between iron atoms in clusters. The incident atom interacts with a cluster through the Buckingham potential. The number of incident atom trajectories is from 10000 to 100000. Initial velocities of incident atoms obey the Maxwell distribution. Thermal accommodation coefficient is calculated for 2 sizes of cluster, 9 and 27 atoms.

Dipole–dipole interactions between atoms in a partly excited resonance gas

In this paper, our experimental studies of dipole-dipole interactions in excited resonance gases are discussed. In weakly excited gas the dipole broadening of optical transitions is proportional to its density N. A gas medium can be identified by its density N as rarefied gas, where dipole broadening of atomic transitions is much less than Doppler width, or as dense gas, where dipole broadening is much more than Doppler width. According to our experiments the dipole broadening in rarefied gas does not depend on the optical excitation. This behavior is in a good agreement with predictions of standard theory for impact atomic collisions. In excited dense gas we observed the strong reduction of the dipole broadening. These experimental results are not described by standard theory for impact collisions. We suggested a simple description of the optical properties for excited dense gas.

Accurate transport properties for O(3P)-H and O(3P)-H2.

Transport properties for collisions of oxygen atoms with hydrogen atoms and hydrogen molecules have been computed by means of time-independent quantum scattering calculations. For the O(3P)-H(2S) interaction, potential energy curves for the four OH electronic states emanating from this asymptote were computed by the internally-contracted multi-reference configuration interaction method, and the R-dependent spin-orbit matrix elements were taken from Parlant and Yarkony [J. Chem. Phys. 110, 363 (1999)]. For the O(3P)-H2 interaction, diabatic potential energy surfaces were derived from internally contracted multi-reference configuration interaction calculations. Transport properties were computed for these two collision pairs and compared with those obtained with the conventional approach that employs isotropic Lennard-Jones (12-6) potentials.

Optical Dipole Trap for Ultracold Atoms Loaded from Dark SPOT

In this work, we load a 1070 nm crossed optical dipole trap (ODT) from a 133Cs dark spontaneous emission optical trap. Obvious improvements in the loading rate and atomic lifetime are demonstrated, comparing with the values of the atoms which are loaded directly from a normal magneto-optical trap. We analyze the atomic number as a function of time in the ODT’s loading and holding processes based on the constant density and the constant volume approximations, respectively. We also demonstrate that the atomic temperature in holding process is related to the atomic collision, which consists of two-body collision and background gas induced collision. Finally the relationship between atomic collision and optical trap depth is measured and analyzed. The atomic loading way we present can be easily expanded to other species to obtain an atomic sample with large number and high density.

High-performance GPU parallel solver for 3D modeling of electron transfer during ion–surface interaction

This study presents parallelized time-dependent Schrödinger equation solver (GPU TDSE Solver) for 3D modeling of resonant electron transfer (RCT) during ion–surface interactions and atomic collisions. The computer modeling of RCT process is based on the numerical one-electron TDSE solution in relatively large spatial domain (about 103–104 nm3). Due to the numerical complexity of direct 3D TDSE solution in such domains, most of RCT calculations use approximations that reduce problem to 2D calculations (e.g. cylindrical symmetry). Last years the TDSE Solver was developed for 3D RCT modeling in large-scale nanosystems (Gainullin and Sonkin, 2015). It was shown to have rather good performance due to the effective parallel implementation of simple numerical scheme on GPUs (explicit finite-difference method in Cartesian coordinates). Note, that usage of FDM in Cartesian coordinates requires ∼100 times greater numerical grid, comparing to the finite-element or finite-volume methods. For the majority of RCT problems the calculations transfer to the cylindrical coordinates decreases the size of numerical grid by an order of magnitude without loss of the calculations precision. The main problem in the transfer to the cylindrical coordinates is that explicit numerical schemes for parabolic equations, including TDSE, are unstable near the axis ρ=0. This study presents hybrid numerical scheme, which eliminates this instability and preserves the effective parallelization on GPUs. The performance of new version of the GPU TDSE Solver, based on the hybrid numerical scheme, was found to be 6 times greater comparing to the previous version. Such performance gain is stipulated by less discrete points, required for the FDM implementation in cylindrical coordinates. Due to reduction of required GPU memory, new version of GPU TDSE Solver can handle spatial domains about 103 nm3 using an ordinary personal computer (equipped with modern GPU, e.g. Tesla k20 or better) or up to 105 nm3 using supercomputers. GPU TDSE Solver was applied to the calculation of the resonant charge transfer in nanosystems. The calculated neutralization probability for Li+ ions impinging on the Ag(100) surface shows a good quantitative agreement with the experimental data.

Probing dissociative electron attachment through heavy-Rydberg ion-pair production in Rydberg atom collisions.

Electron transfer in collisions between low-n, n = 12, Rydberg atoms and targets that attach low-energy electrons can lead to the formation of heavy-Rydberg ion-pair states comprising a weakly-bound positive-negative ion pair that orbit each other at large separations. Measurements of the velocity and angular distribution of ion-pair states produced in collisions with 1,1,1-C2Cl3F3, CBrCl3, BrCN, and Fe(CO)5 are used to show that electron transfer reactions furnish a new technique with which to examine the lifetime and decay energetics of the excited intermediates formed during dissociative electron capture. The results are analyzed with the aid of Monte Carlo simulations based on the free electron model of Rydberg atom collisions. The data further highlight the capabilities of Rydberg atoms as a microscale laboratory in which to probe the dynamics of electron attachment reactions.

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.

The Hulthén Potential Model for Hydrogen Atoms in Debye Plasma

In this paper, the well-known Hulthen potential is used for describing the plasma screening effect on a hydrogen atom embedded in weakly coupled plasma. As the radial Schrodinger equation with Hulthen potential gives analytical solutions, the energy levels and wave functions are obtained in a closed form. Structural properties such as dipole polarizability, transition probability, and oscillator strength are calculated easily using the corresponding analytical formulas. Moreover, the analytic solutions of the Hulthen potential enable us to obtain many other observables reliably. Asymptotic iteration method solutions of the Hulthen potential for different screening parameters are considered to calculate the transition energies, oscillator strength, and transition probability values for a few dipole allowed states of hydrogen atom embedded in weakly coupled plasma. Use of the Hulthen potential to determine the screening effects on hydrogen atom embedded in weakly coupled plasma would be useful for modeling Debye plasma in investigations of the atomic structure and collisions in the plasma physics field.

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.

Surface Activation and Planarization with Gas Cluster Ion Beam for Wafer Bonding

Surface activated bonding (SAB) has been widely used for various semiconductor wafers. In the SAB process, atomic Ar ions bombard wafer surface to form activated surface. In order to form activated surface on materials which are susceptible to irradiation damage, we have been investigating the gas cluster ion beam (GCIB) as an alternative to atomic ion beams. Gas clusters are aggregates of several thousands of gaseous atoms. Therefore, energy per atom of a gas cluster ion can be easily reduced to several eV. Although energy/atom of a gas cluster ion is low, thousands of low-energy atoms bombard a target surface at the same location simultaneously. As a result, the energy density on the target surface became extremely high and non-linear collisions of atoms are induced. They induces unique irradiation effects of GCIB such as high-yield sputtering, surface analysis of organic materials, thin film deposition at low temperature, and surface smoothing, which are beneficial for surface activated bonding. In this paper, preliminary irradiation effects of surface activation and planarization with glancing incidence GCIBs will be reported. From in-situ XPS analysis of O1s peak intensity on Si(100) with native oxide, Ar-GCIB irradiation at 70o removed native oxide efficiently. On the contrary, mixing of native oxide by normal incidence GCIB might occur, which caused residual oxide layer formation. In addition, the surface roughness at oblique incidence showed lowest value (Ra < 0.5 nm) without ripple formations. Cross-sectional TEM observation with oblique incidence GCIB irradiation showed that the sub-surface damage was almost negligible. These results indicated the possibility of use of GCIB as low-energy ions for wafer bonding.