Atomic Collisions Research Articles

Atomic spin-exchange collisions in magnetic fields

Spin-exchange collisions among alkali-metal atoms affect the coherence time and gyromagnetic ratio of atoms by redistributing the ground-state population. We theoretically study the interactions between magnetic fields and atoms in the presence of spin-exchange collisions. Using the modified Bloch equations derived from the density-matrix equation in a low spin-polarization limit, we obtain explicit formulas describing the role of spin-exchange collisions in the interactions between atoms and magnetic fields, especially the nonresonant radio-frequency fields, which are widely used to manipulate atoms. The results derived from these equations agree well with density-matrix simulations and previous experimental results, and provide a simple and precise way to quantitatively analyze the effect of spin-exchange collisions on atoms. Our paper is helpful to gain physical insight into the spin-exchange collisions and optimize the performance of atom-based precision measurement instruments.

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.

A quantum heat engine driven by atomic collisions

Quantum heat engines are subjected to quantum fluctuations related to their discrete energy spectra. Such fluctuations question the reliable operation of thermal machines in the quantum regime. Here, we realize an endoreversible quantum Otto cycle in the large quasi-spin states of Cesium impurities immersed in an ultracold Rubidium bath. Endoreversible machines are internally reversible and irreversible losses only occur via thermal contact. We employ quantum control to regulate the direction of heat transfer that occurs via inelastic spin-exchange collisions. We further use full-counting statistics of individual atoms to monitor quantized heat exchange between engine and bath at the level of single quanta, and additionally evaluate average and variance of the power output. We optimize the performance as well as the stability of the quantum heat engine, achieving high efficiency, large power output and small power output fluctuations.

Full set of relaxation constants for 174Yb level (6s6p) 3P1 in collisions with rare gases

For the first time, a complete set of decay parameters, due to collisions of 174Yb atoms with noble gas atoms and with ytterbium atoms, was experimentally found for the 174Yb (6s6p) 3P1 level. The method of stimulated photon echo with special angles between the polarization vectors of light pulses forming an echo at the 174Yb (6s2) 1S0—(6s6p) 3P1 transition (type ) is used. For each buffer gas of He, Ne, Ar, Kr, Xe, and Yb, the decay rate constants of population, orientation and alignment for the collision destruction of the 3P1 level of 174Yb atoms are determined. All the decay rates demonstrated their linear increase with a buffer gas pressure, and the corresponding relaxation constants are determined. Each of the indicated relaxation constants, as well as the difference between the alignment and orientation relaxation constants, increases with the increasing buffer gas mass.

Dipole and generalized oscillator strengths-dependent electronic properties of helium atoms immersed in a plasma

Atoms subjected to extreme environmental conditions are of fundamental importance due to the modification of their electronic properties. In this work, we study the helium atom when immersed in a plasma environment. In order to describe the plasma medium, we use two models when solving the Schrodinger equation in a restricted Hartree–Fock approach: the Debye–Huckel screened (DHS) potential and a more general exponential-cosine screened Coulomb (ECSC) potential. The plasma length parameter, $$\lambda $$ , in both model potentials characterizes the plasma screening effects which cause an increase or decrease in the electronic properties of the helium atom. We report results for the total electronic ground state energy, orbital energy, dipole oscillator strengths, generalized oscillator strengths (GOS), mean excitation energy, electrostatic dipole polarizability, and electronic stopping cross section. We find that the ECSC plasma model produces a less bound system than the DHS plasma model at the same value of the screening length, $$\lambda $$ . However, the ECSC model potential has a stronger dipole transition from the 1s to the 2p and 3p states than the DHS model potential. Also, the ECSC potential predicts a higher static dipole polarizability than the DHS model potential, with a consequent lower mean excitation energy. We also find a larger GOS for the ECSC than for the DHS for the same momentum transfer at the same value of the screening length, $$\lambda $$ . Consequently, the ECSC mode potential produces a larger electronic stopping cross section than the DHS for the same $$\lambda $$ and projectile velocity. In the limit of $$\lambda \rightarrow \infty $$ , we have excellent agreement with the free helium properties. These quantitative values for the electronic properties would be useful for the investigations of the atomic structure and collisions of helium atoms immersed in plasmas.

Nonlinear propagation of fast and slow magnetosonic waves in collisional plasmas

AbstractLow‐frequency fast and slow magnetosonic waves propagating in electron ion plasmas with damping effects through ions and neutral atoms collisions are investigated. Linear wave analysis is performed to obtain dispersion relation. The reductive perturbation method is applied and it is shown that fast and slow modes of nonlinear magnetosonic wave are governed by damped Korteweg‐de Vries (DKdV) equation in the presence of ion neutral collisions in plasmas. The analytical solution of DKdV soliton is presented under the assumption of weak collisional effects and numerical solutions of DKdV equation are also obtained using two‐level finite difference scheme with the help of Runge–Kutta method at different plasma parameters. The damping of nonlinear fast and slow magnetosonic wave structures at different times are discussed in the context of space plasma situations where ions and neutral atoms collisions exist.

An asymptomatic theory of charge-exchange process in ion-ion collisions

An theory of charge transfer at intermediate collision velocities is elaborated. Within the framework of the two-wave-function model, we analyze the intermediate stages of the population of the Rydberg states $$\nu _A=(n_A=8,l_A=0-2,m_A=0)$$ of highly charged ions escaping the target ion after atomic collision. The specificity of this approach is reflected in the time-symmetric description of the electron capture process by using two functions. The focus of the work is on determining the transition probabilities and corresponding rates whose analysis can be used to estimate the dominant partial neutralization distances at fixed initial and final states of the ion-ion system under consideration.

Measurement of the principal quantum number distribution in a beam of antihydrogen atoms

The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration plans to measure the ground-state hyperfine splitting of antihydrogen in a beam at the CERN Antiproton Decelerator with initial relative precision of 10^{-6} or better, to test the fundamental CPT (combination of charge conjugation, parity transformation and time reversal) symmetry between matter and antimatter. This challenging goal requires a polarised antihydrogen beam with a sufficient number of antihydrogen atoms in the ground state. The first measurement of the quantum state distribution of antihydrogen atoms in a low magnetic field environment of a few mT is described. Furthermore, the data-driven machine learning analysis to identify antihydrogen events is discussed.Graphic

Modeling of supersonic crowdion clusters in FCC lattice: Effect of the interatomic potential

Among a wide variety of point defects, crowdions can be distinguished by their high energy of formation and relatively low migration barriers, which makes them an important agent of mass transfer in lattices subjected to severe plastic deformation, irradiation, etc. It was previously shown that complexes and clusters of crowdions are even more mobile than single interstitials, which opened new mechanisms for the transfer of energy and mass in materials under intense external impacts. One of the most popular and convenient methods for analyzing crowdions is molecular dynamics, where the results can strongly depend on the interatomic potential used in the study. In this work, we compare the characteristics of a crowdion in an fcc lattice obtained using two different interatomic potentials — the pairwise Morse potential and the many-body potential for Al developed by the embedded atom method. It was found that the use of the many-body potential significantly affects the dynamics of crowdion propagation, including the features of atomic collisions, the evolution of energy localization and the propagation path.

Evaluation of the Charge-Changing Cross Sections for Ion—Atom Collisions

Evaluation of the Charge-Changing Cross Sections for Ion—Atom Collisions

Molecular dynamics simulation of stress induced by energetic particle bombardment in Mo thin films

Molecular dynamics (MD) simulations are performed to analyze the evolution of defects and stress caused by low-energy (25-400 eV) Ar bombardment of polycrystalline Mo thin films. Simulations were performed under different conditions to explore the role of grain boundaries (GBs), Ar-atom's kinetic energies, and incident directions on defect generation. The results show that the GBs enhance the production of interstitial defects, producing compressive stress at much larger depths than the implantation range. This is attributed to sequences of atomic collisions that knock atoms into GBs instead of a diffusional process. Decreasing the grain size or increasing the kinetic energy of the incoming particles increases the number of interstitials in the film, which increases the compressive stress. The incident angle has little influence on the number of interstitials in the film, but the sputtering yield depends on the polar angle. The stress distribution can be modeled by a superposition of the different defect distributions with the appropriate relaxation volumes.

OPERATIONAL CHARACTERISTICS OF THE ELECTRON ACCELERATOR BASED ON THE PLASMA-FILLED DIODE

The paper is concerned with the plasma-filled diode performance in the intensive mode regulated by means of external gas puffing. The possibility to smoothly vary the plasma parameters in the discharge gap zone, and thus, to optimize the main diode characteristics (Ucutoff, Icutoff) by the external gas puffing method has been confirmed by experiment. The introduction of additional quantity of neutral gas into the discharge causes the change in the plasma density balance due to elementary processes in physics of electronic and atomic collisions, such as ionization, dissociation, recombination. The deviation of actual voltage/current values from their maximum values can be attributed to the mismatch in the generator-load feed circuit.

High-pressure Ar passivation to enhance the photoluminescence of Si nanocrystals

Photoluminescence (PL) enhancement of Si nanocrystals (Si–NCs) embedded in SiO2 matrix by high-pressure (30 bar) Ar passivation at moderately high temperatures has been observed. For the same pressure, time and temperature of passivation, the PL enhancement of Si–NCs for the Ar passivated sample is even larger than that for the regular H2 passivated one. Electron paramagnetic resonance (EPR) measurement shows that high-pressure Ar passivation causes more reduction in density of dangling bonds which are non-radiative recombination centers, than the regular H2 one. Elemental depth profile analysis shows deep diffusion of Ar atoms into the sample after Ar passivation. The depth profile of PL enhancement after Ar passivation correlates properly with the Ar depth distribution. This effect of inert gas passivation also holds for the case of Kr passivation. The mechanism of such an effect could be that Ar atoms inside the sample alter Si dangling bonds into paired Si ones by a kind of annealing process of thermal atomic collision.

Velocity preserving transfer between highly excited atomic states: black body radiation and collisions

We study the excitation redistribution from cesium 7P1/2 or 7P3/2 to neighboring energy levels by black body radiation (BBR) and inter atomic collisions using pump-probe spectroscopy inside a vapor cell. At low vapor densities we measure redistribution of the initial, velocity-selected, atomic excitation by BBR. This redistribution preserves the selected atomic velocities allowing us to perform high resolution spectroscopy of the 6D → 7F transitions. This transfer mechanism could also be used to perform sub-Doppler spectroscopy of the cesium highly-excited nG levels. At high densities we observe interatomic collisions redistributing the excitation within the cesium 7P fine and hyperfine structure. We show that 7P redistribution involves state-changing collisions that preserve the initial selection of atomic velocities. These redistribution mechanisms can be of importance for experiments probing high lying excited states in dense alkali vapor.

Distributions of Two Atoms Collisions over the Surface of the Condensed Phase

The processes of heat and mass transfer are closely related to the evaporation of a substance from the surface of the condensed phase. The interaction of outgoing molecules from the surface of the condensed phase with condensed phase molecules plays a fundamental role. A simpler case of evaporation is the departure of atoms from the surface of the condensed phase, i.e. the atoms overcome the potential barrier on the surface of the condensed phase. Depending on the evaporation rate, a Knudsen layer appears above the surface of the condensed phase. In this paper, based on the model of rigid spheres, the density distributions of the collision distances and the average values of the collision distances of two atoms emitted simultaneously from the surface of the condensed phase above the surface are analyzed. Distributions of the collision distance depending on the surface temperature, the size of the potential barrier, and the size of the evaporation area are obtained. Computer experiments were performed using the Monte Carlo method. To obtain the results of numerical simulation, a parallel algorithm adapted to calculations on graphics processors with CUDA technology was developed.

Probabilities of Atomic Departures from the Slit System

The theoretical description of experimental data on transport in open systems has been of interest for more than a hundred years. Boltzmann proposed a new view of the transfer of matter. Now it is possible to describe the transfer processes from a microscopic point of view. The solution of the Boltzmann equation and the equations derived from it is a complex problem related to the mathematical problems of solving such equations. On the other hand, the complexity of the solution is related to the geometry of the system in which the transfer process takes place. Fundamental physical calculations are made for systems: flat, slotted, cylindrical, rectangular, etc. In the free molecular mode of gas flow, collisions of molecules occur mainly with the walls of systems. In this connection, there was a direction related to the calculation of the probabilities of atomic transport in the system. In this paper, we propose an approach for determining the probabilities of atomic outcomes from slit systems depending on the relative height of the walls of H systems. Exact formulas are obtained for calculating the probabilities of atomic departures from systems without colliding with walls, the distribution of atomic collisions over the height of the system wall, the probabilities of atoms entering the condensed phase after a single collision with the system walls, and the probabilities of atomic departures from systems after a single collision with walls. The accuracy of the obtained formulas was compared with the data obtained from computer experiments using the Monte Carlo method.

Neighboring Atom Collisions in Solid-State High Harmonic Generation

High harmonic generation (HHG) from solids shows great application prospects in compact short-wavelength light sources and as a tool for imaging the dynamics in crystals with subnanometer spatial and attosecond temporal resolution. However, the underlying collision dynamics behind solid HHG is still intensively debated and no direct mapping relationship between the collision dynamics with band structure has been built. Here, we show that the electron and its associated hole can be elastically scattered by neighboring atoms when their wavelength approaches the atomic size. We reveal that the elastic scattering of electron/hole from neighboring atoms can dramatically influence the electron recombination with its left-behind hole, which turns out to be the fundamental reason for the anisotropic interband HHG observed recently in bulk crystals. Our findings link the electron/hole backward scattering with Van Hove singularities and forward scattering with critical lines in the band structure and thus build a clear mapping between the band structure and the harmonic spectrum. Our work provides a unifying picture for several seemingly unrelated experimental observations and theoretical predictions, including the anisotropic harmonic emission in MgO, the atomic-like recollision mechanism of solid HHG, and the delocalization of HHG in ZnO. This strongly improved understanding will pave the way for controlling the solid-state HHG and visualizing the structure-dependent electron dynamics in solids.

Zeptosecond Dynamics in Atoms: Fact or Fiction?

Photon exchange due to nuclear bremsstrahlung during nuclear collisions can cause Coulomb excitation in the projectile and the target nuclei. The corresponding process originated in nuclear timescales can also be observed in atomic phenomenon experimentally if it delayed by at least with an attosecond or longer timescales. We have found that this happens due to a mechanism involving the Eisenbud-Wigner-Smith time delay process. We have estimated photoionization time delays in atomic collisions utilizing the nonrelativistic version of random phase approximation with exchange and Hartree-Fock methods. We present three representative processes in which we can observe the phenomena in attosecond timescales even though they originate from excitations in the zeptosecond timescales. Thus the work represents an investigation of parallels between two neighboring areas of physics. Furthermore the present work suggests new possibilities for atomic physics research near the Coulomb barrier energy, where the laser is replaced by nuclear bremsstrahlung.

Elliptically polarized laser assisted elastic electron-hydrogen atom collision and differential scattering cross-section

In the present study, we have investigated scattering of an electron by hydrogen atoms in the presence of the elliptical polarized laser field. We have discussed the polarization effect of laser field on hydrogen atom and effect of the resulted polarized potential on differential scattering cross-section is studied. We assume the scattered electrons having kinetic energy (~3000 eV) and laser field of moderate field strength because it is permitted to treat the scattering process in first Born approximation and the scattering electron was described by Volkov wave function. We found that the differential scattering cross-section area increases with the increase of the kinetic energy of the incident electron and there is no effect of changing the value of polarizing angle on the differential cross-section with kinetic energy. We observed that differential scattering cross-section in elliptical polarization in the high energy region depends upon the laser intensity and the incident energy for a linearly polarized field.

On the CN production through a spark-plug discharge in air-CO2 mixture

This work presents a simulation study of the cyanide production through an electric discharge in a vehicular spark plug in dry air containing typical outdoor amounts of CO 2 . For such a task, a previously reported 2D theoretical and numerical framework is used. Simulations are carried out at atmospheric pressure. The starting gas mixture is formed by molecular nitrogen, oxygen, and CO 2 (3996:999:5) ratio. The mathematical formalism includes heat and species diffusion and convection considering sub-model for energy transfer in electronic, atomic, and molecular collisions. The plasmo-chemical kinetics model includes 69 species and 710 collision processes . Principal pathways for the cyanide formation are discussed. The spatial and temporal evolution of species of interest are also reported.