Atomic Collisions Research Articles

Continuum distorted-wave method in the two-electron charge exchange theory

Introduction. In the case of collisions with multi-charged ions of atoms, more complex than the hydrogen atom, other than single-electron charge exchange and ionization, other processes may occur. These are, first of all, the processes of two-electron capture, capture with simultaneous ionization or ion excitation and others. We shall stop on the problems of the theoretical description of the processes of two-electron capture in the region of intermediate and high collision velocities of ions with atoms. Purpose . Research the interactions of multi-charged ions with atoms in the region of intermediate and high collision velocities using theoretical methods. Methods. To describe the double charge exchange processes in high-energy ion-atom collisions a four-part formalism of continuum distorted-wave method (CDW-method), based on the integral Dodd-Graider's equations, were developed. Results. We obtained a matrix element that describes the two-step (Thomas) mechanism of electron capture through a continuous spectrum from a target atom into bound states relative to the fast -particle. For the reaction amplitude, in the approximation of the mechanism of simultaneous capture of two electrons, a general expression that takes into account Coulomb effects in the initial and final states is found. Conclusion. The application of the general theory is illustrated by the example of a two-electron charge exchange reaction in collisions of helium atoms with -particles. The results of calculations of the cross sections are in good agreement with the experimental and theoretical data. An attractive side of the proposed formalism is the consistent consideration of the asymptotics of wave functions in both channels of the reaction, which takes into account the long-range nature of the Coulomb interactions.

Spectroscopic study of a diffusion-bonded sapphire cell for hot metal vapors.

Characteristics of a diffusion-bonded sapphire cell for optical experiments with hot metal vapors were investigated. The sapphire cell consisted of sapphire-crystal plates and a borosilicate-glass tube, which were bonded to each other by diffusion bonding without any binders or glues. The glass tube was attached to a vacuum manifold using the standard method applied in glass processing, filled with a small amount of Rb metal by chasing with a torch, and then sealed. The cell was baked at high temperatures, and optical experiments were then performed using rubidium atoms at room temperature. The sapphire cell was found to be vacuum tight, at least up to 350°C, and the sapphire walls remained clear over all temperatures. From the optical experiments, the generation of a background gas was indicated after baking at 200°C. The background gas pressure was low enough to avoid pressure broadening of absorption lines but high enough to cause velocity-changing collisions of Rb atoms. The generated gas pressure decreased at higher temperatures, probably due to chemical reactions.

Free evolution of an incoherent mixture of states: a quantum mechanical approach to the van Cittert–Zernike theorem

We study the time evolution of an incoherent mixture of quantum states and demonstrate, in very simple terms, a quantum mechanical equivalent of van Cittert–Zernike theorem, which can be easily explained to Quantum Physics students with a basic knowledge of the density matrix theory. Finally, we exemplify this result by applying it to the quantitative analysis of the coherence of a beam of particles in atomic collisions.

Phase collapse and revival of a 1-mode Bose–Einstein condensate induced by an off-resonant optical probe field and superselection rules

Phase collapse and revival for Bose–Einstein condensates are nonlinear phenomena appearing due to atomic collisions. While it has been observed in a general setting involving many modes, for one-mode condensates its occurrence is forbidden by the particle number superselection rule (SSR), which arises because there is no phase reference available. We consider a single mode atomic Bose–Einstein condensate interacting with an off-resonant optical probe field. We show that the condensate phase revival time is dependent on the atom–light interaction, allowing optical control on the atomic collapse and revival dynamics. Incoherent effects over the condensate phase are included by considering a continuous photo-detection over the probe field. We consider conditioned and unconditioned photo-counting events and verify that no extra control upon the condensate is achieved by the probe photo-detection, while further inference of the atomic system statistics is allowed leading to a useful test of the SSR on particle number and its imposition on the kind of physical condensate state.

Vortex rings in the ionization of atoms by positron impact

We uncovered the morphology of quantum vortex structures in positron-atom ionization collisions. By performing an exhaustive calculation of the position of vortices in the final momentum space of the problem with only one kinematic restriction, we could elucidate that quantum vortices form a closed structure, which is one of the two possible scenarios for their emergence in three dimensions. This is the first time that these structures are fully calculated for positron atom collisions.

Dissociative Excitation of the Odd Sextet Levels of the Cobalt Atom by Collisions of Electrons with Cobalt Dichloride Molecules

Inelastic collisions of slow electrons with cobalt dichloride molecules, leading to the formation of excited cobalt atoms in odd sextet states, are experimentally studied. At an incident electron energy of 100 eV, thirty six dissociative excitation cross sections are measured for levels belonging to the z6D°, z6F°, and z6G° terms. In the electron energy range of 0–100 eV, ten optical excitation functions are recorded. The full cross sections for the dissociative excitation of the cobalt atom levels and the contribution of cascade transitions to their population are determined. The cross sections for electron–molecule and electron–atom collisions are compared.

Charge Exchange of Proton Sodium Atom Collision

The coupled static approximation is modified for the first time to make it applicable to multi-channels problem of the collision of the proton by alkali atom. The possibility of producing more hydrogen during the proton-alkali atom collision is investigated. The formation of hydrogen H(1s) and excited hydrogen (in 2s- and 2p-states) of the scattering of the proton by sodium atom is treated for the first time to test the convergence of our method. The modified method is used to calculate the total cross-sections of seven partial waves ( , where is the total angular momentum) at incident energies between 50 and 1000 keV. Our p-Na results and those determined by previous authors are in reasonable agreement.

Scattering length calculations via Faddeev approach

The results of calculation of the scattering length for the collision of a 4He atom with a 4He7Li dimer are presented. These investigations are based on the hard-core version of the Faddeev differential equations.

Deterministic loading of an individual atom: Towards scalable implementation of multi-qubit**Project supported by the National Major Scientific Research Program of China (Grant No. 2012CB921601), the National Natural Science Foundation of China (Grant Nos. 61205215, 11274213, and 61475091), and the National Key Research and Development Program of China (Grant No. 2017YFA0304502).

We report the realization of a deterministic single-atom preparation by the method of all-optical feedback. Using a fast-real-time feedback, the light-induced atom desorption effect and blue detuned light-induced atom collision process can increase a success probability of single-atom preparation up to more than 99%. We investigate the dynamics of loading single atom trapped in a trap with a size of hundreds of micrometers into a pair of microscopic tweezers. The detailed experimental results show that the feedback loading is spatially insensitive, which implies that it is possible to use the feedback protocol to simultaneously implement the loading of large number of qubits arrays.

Characteristics of Polarisation in the Ramsauer–Townsend Minima in Strongly Coupled Semiclassic Plasmas

Abstract The influence of quantum shielding on the Ramsauer–Townsend phenomena for the total electron–atom polarisation collision cross-section is investigated in partially ionised strongly coupled semiclassic plasmas. The result shows that the quantum shielding effect changes the position of the Ramsauer energy in partially ionised strongly coupled plasmas. It is also found that the quantum shielding effect enhances the total electron–atom collision cross-section when the collision energy is greater than the Ramsauer energy; however, it suppresses the collision cross-section when the collision energy is smaller than the Ramsauer energy. In addition, it is shown that the plasma screening effect significantly changes the position of the Ramsauer energy and the influence of plasma screening on the magnitude of the collision cross-section is more significant near the Ramsauer energy domain. The variations of the Ramsauer energy and the collision cross-section due to the quantum shielding effect are also discussed.

Ultracold collisions of mixed atoms in optical dipole trap loaded from a dark magneto-optical trap

We study the cold collisions of mixed atoms in an optical dipole trap (ODT), which are loaded from a dark magneto-optical trap (MOT). A comprehensive, phenomenological rate equation is presented to derive the ultracold homonuclear and heteronuclear collision rates in loading and holding procedures. Our results show that the cold atoms in the dark MOT can provide a much better stable, initial atomic sample than MOT. The dependence of the heteronuclear collision rate on the trap depth is attributed to the hyperfine-changing collision by the ODT laser with a broad linewidth. The processes of deriving the collision rate are also universal for other kinds of atoms or even molecules.

Spectroscopic measurement of the softness of ultracold atomic collisions

The softness of elastic atomic collisions, defined as the average number of collisions each atom undergoes until its energy decorrelates significantly, can have a considerable effect on the decay dynamics of atomic coherence. In this paper we combine two spectroscopic methods to measure these dynamics and obtain the collisional softness of ultra-cold atoms in an optical trap: Ramsey spectroscopy to measure the energy decorrelation rate and echo spectroscopy to measure the collision rate. We obtain a value of 2.5 (3) for the collisional softness, in good agreement with previously reported numerical molecular dynamics simulations. This fundamental quantity was used to determine the $s$-wave scattering lengths of different atoms but has not been directly measured. We further show that the decay dynamics of the revival amplitudes in the echo experiment has a transition in its functional decay. The transition time is related to the softness of the collisions and provides yet another way to approximate it. These conclusions are supported by Monte Carlo simulations of the full echo dynamics. The methods presented here can allow measurements of a generalized softness parameter for other two-level quantum systems with discrete spectral fluctuations.

Magnetic-resonance-frequency shifts in spin-exchange collisions of Rb and Cs alkali-metal atoms

On the basis of data on spin-exchange complex cross sections in the Rb–Rb and Cs–Rb systems, the magnetic-resonance-frequency shifts of rubidium atoms upon collisions in a mixture of Cs–Rb alkalimetal atoms are calculated. The results obtained are compared with the experimental data and the previous theoretical data.

Simple Atomic Quantum Memory Suitable for Semiconductor Quantum Dot Single Photons.

Quantum memories matched to single photon sources will form an important cornerstone of future quantum network technology. We demonstrate such a memory in warm Rb vapor with on-demand storage and retrieval, based on electromagnetically induced transparency. With an acceptance bandwidth of δf=0.66 GHz, the memory is suitable for single photons emitted by semiconductor quantum dots. In this regime, vapor cell memories offer an excellent compromise between storage efficiency, storage time, noise level, and experimental complexity, and atomic collisions have negligible influence on the optical coherences. Operation of the memory is demonstrated using attenuated laser pulses on the single photon level. For a 50ns storage time, we measure η_{e2e}^{50 ns}=3.4(3)% end-to-end efficiency of the fiber-coupled memory, with a total intrinsic efficiency η_{int}=17(3)%. Straightforward technological improvements can boost the end-to-end-efficiency to η_{e2e}≈35%; beyond that, increasing the optical depth and exploiting the Zeeman substructure of the atoms will allow such a memory to approach near unity efficiency. In the present memory, the unconditional read-out noise level of 9×10^{-3} photons is dominated by atomic fluorescence, and for input pulses containing on average μ_{1}=0.27(4) photons, the signal to noise level would be unity.

Modeling the response of mesospheric sodium to pulsed-laser excitation.

A simulation modeling excitation of the sodium D2 line by nanosecond time scale pulsed lasers is described. By numerically integrating transition rates in the sodium hyperfine structure, the return flux per sodium atom is predicted as a function of laser power. The simulation should be useful for studies of mesospheric sodium and adaptive optics. Applications include the estimation of sodium column density from lidar return flux, and of laser guide star brightness for different pulsed laser formats. The simulation assumes that the pulse repetition frequency is sufficiently low (smaller than a few kilohertz) that atomic collisions restore local thermodynamic equilibrium between pulses. It is also assumed that the pulse length is short compared to the Larmor precession time scale. The numerical results are well-approximated by a simple analytic model for a three-level atom. The number of emitted photons is found to be primarily dependent on the product of the length of the laser pulse and the energy density.

On the treatment of ℓ-changing proton–hydrogen Rydberg atom collisions

Energy-conserving, angular momentum changing collisions between protons and highly excited Rydberg hydrogen atoms are important for precise understanding of atomic recombination at the photon decoupling era and the elemental abundance after primordial nucleosynthesis. Early approaches to ℓ-changing collisions used perturbation theory only for dipole-allowed (Δℓ = ±1) transitions. An exact non-perturbative quantum mechanical treatment is possible, but it comes at a computational cost for highly excited Rydberg states. In this paper, we show how to obtain a semiclassical limit that is accurate and simple, and develop further physical insights afforded by the non-perturbative quantum mechanical treatment.

A simple recipe for rapid all-optical formation of spinor Bose–Einstein condensates

We present a simple recipe for the rapid formation of spinor Bose–Einstein condensates of 87Rb atoms in a three-beam crossed optical dipole trap (ODT). Two laser Gaussian beams were crossed at a small angle to form an effective single potential in the intersection region with an independent and widely tunable trap volume and aspect ratio, for both efficient atom loading and number density enhancement. A third Gaussian beam was then introduced to create a potential dimple at the trap center to further increase the atom number density and collision rate, which enabled rapid evaporation to a Bose condensation. In this configuration, atoms were loaded into the three-beam crossed trap from a vapor cell magneto-optical trap, and condensates of atoms were created after 3–5 s of rapid evaporation. The demonstration of efficient and robust BEC production in this ODT suggests the applicability of this trapping geometry to lasers with any far off-resonant wavelengths. To prepare the spinor condensates with a wide tunability of spin population, control of the magnetic fields was first employed during evaporation to polarize the spin states, and the tuning mechanism is discussed. Spinor condensates with an arbitrary spin population were then created via multiple microwave pulses driving the ground hyperfine transitions.

International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC2017)

The 30th International Conference on Photonic, Electronic and Atomic Collisions (XXX ICPEAC) was held at the Cairns Convention Centre, Queensland, Australia on 26 July - 1 August, 2017, organized by the Australian National University (ANU, Canberra), Curtin University (Perth, Western Australia) and Griffith University (Queensland). Held biannually, ICPEAC is one of the largest conferences on the atomic and molecular physics calendar, and covers the recent progress in photonic, electronic, and atomic collisions with matter. With a history dating back to 1958, the ICPEAC has its second coming to the Sunshine State of Queensland, the first of which being the 1991 Australian ICPEAC in Brisbane. With the selection of Cairns as the venue for the ICPEAC XXX the local organizers began their preparations six years ahead in anticipation of the event.The conference brought together 500 participants from 50 countries and attracted 680 contributed papers for presentation in poster sessions. Among the latter, 311 are presented in issues 2-12 of this volume of the Journal of Physics Conference Series. In addition, five plenary lectures by Linda Young, Clifford Surko, Henrik Cederquist, Tilman Pfau and Reinhard Kienberger have been presented along with 62 progress reports and 26 special reports. The latter “hot topics” were selected by the ICPEAC Program Committee from the contributed papers. Detailed write-ups of most of the invited lectures and reports are presented in issue 1 of this volume, constituting a comprehensive and tangible record of the meeting.Preceeding the conference, three distinguished scientists: Barry Schneider (NIST, USA), Chris Greene (Purdue University, USA) and Thomas Pfeifer (Max-Plank Institute, Germany) presented their tutorial lectures specifically designed for students and early career researchers.During the conference, the 2017 IUPAP Young Scientist Prize was awarded to Johannes Feist (Universidad Autónoma de Madrid, Spain). The Sheldon Datz Prize for an Outstanding Young Scientist Attending ICPEAC was awarded to Dermot Green (Queen's University Belfast, UK).The Local Organizing Committee would like to express its gratitude to Tourism and Events Queensland (TEQ), the International Union for Pure and Applied Physics (IUPAP), all the conference sponsors for their financial support. With particular emphasis we thank the group of volunteers from the ANU and Curtin University for their efforts in orchestrating the event.Finally, the Local Organizing Committee would like to thank all the ICPEAC participants for their support and contributions to the conference.The Editors,Alexander Bray, Sean Hodgman, Joshua Machacek, Anatoli Kheifets(Australian National University, Canberra, Australia)

Many particle spectroscopy of atoms, molecules, clusters and surfaces: international conference MPS-2016

The conference on Many Particle Spectroscopy of Atoms, Molecules, Clusters and Surfaces (MPS-2016) brought together near to a hundred scientists in the field of electronic, photonic, atomic and molecular collisions, and spectroscopy from around the world. We deliver an Editorial of a topical issue presenting original research results from some of the participants on both experimental and theoretical studies involving many particle spectroscopy of atoms, molecules, clusters and surfaces.

On the onset of void swelling in pure tungsten under neutron irradiation: An object kinetic Monte Carlo approach

We propose an object kinetic Monte Carlo (OKMC) model for describing the microstructural evolution in pure tungsten under neutron irradiation. We here focus on low doses (under 1 dpa), and we neglect transmutation in first approximation. The emphasis is mainly centred on an adequate description of neutron irradiation, the subsequent introduction of primary defects, and their thermal diffusion properties. Besides grain boundaries and the dislocation network, our model includes the contribution of carbon impurities, which are shown to have a strong influence on the onset of void swelling. Our parametric study analyses the quality of our model in detail, and confronts its predictions with experimental microstructural observations with satisfactory agreement. We highlight the importance for an accurate determination of the dissolved carbon content in the tungsten matrix, and we advocate for an accurate description of atomic collision cascades, in light of the sensitivity of our results with respect to correlated recombination.