Atomic Collisions Research Articles

Ultracold collisions of Cs atoms in excited Zeeman and hyperfine states

We investigate Cs+Cs scattering in excited Zeeman and hyperfine states. We calculate the real and imaginary parts of the s-wave scattering length; the imaginary part directly provides the rate coefficient for 2-body inelastic loss, while the real part allows us to identify regions of magnetic field where 3-body recombination will be slow. We identify field regions where Cs in its $(f,m_f)=(3,+2)$ and $(3,+1)$ states may be stable enough to allow Bose-Einstein condensation, and additional regions for these and the $(3,0)$ and $(3,-3)$ states where high-density clouds should be long-lived.

The Fine Structure of Alkali Metal Atoms in Inelastic Collisions with Hydrogen

A method is proposed for calculating the rate constants of inelastic processes occurring during collisions of alkali metal atoms and their positive ions with hydrogen atoms and negative ions, taking into account the sublevels of the fine structure of alkali metal levels. The results of the application of the proposed method are presented in this paper based on the example of the KH and RbH collisional systems.

A magnetic spectrometer for electron‐positron pair spectroscopy in storage rings

We report an analysis of electron‐optical properties of a toroidal magnetic sector spectrometer and examine parameters for its implementation in a relativistic heavy‐ion storage ring, for example the High Energy Storage ring (HESR) at the future Facility for Antiproton and Ion Research (FAIR) facility. For studies of free–free pair production in heavy‐ion atom collisions, this spectrometer exhibits very high efficiencies for coincident e+–e− pair spectroscopy over a wide range of momenta of emitted lepton pairs. The high coincidence efficiency of the spectrometer is the key for stringent tests of theoretical predictions for the phase space correlation of lepton vector momenta in free–free pair production.

Pressure balance in a lower collisionality, attached tokamak scrape-off layer

Previously the gyrokinetic neoclassical code XGCa found that the pressure balance (or momentum balance) in the diverted scrape-off layer (SOL) does not follow that from the fluid description based on the Chew–Goldberger–Low (CGL) theory. In this paper a gyrocenter parallel momentum moment equation is derived for pressure balance (or momentum balance) in a tokamak SOL, for use in interpreting this difference. The new gyrocenter-fluid pressure balance equation allows identifying from the XGCa code results which physical processes dominate the setting of pressure variation in the scrape-off layer. This pressure balance equation is applied to the simulation of a DIII-D H-mode discharge, with a lower ion collisionality in the SOL, and the Coulomb and atomic collisions are not strong enough to yield a detached divertor plasma. It is found that the total pressure balance is much better matched using the gyrocenter parallel momentum moment equation. Electrons are shown to be dominantly adiabatic, while ions have multiple contributions to pressure balance, including terms originating from particle drifts. These results show that in strong gradient, low collisionality regions of the SOL, the typical fluid reductions miss important effects captured in the gyrokinetic model.

Two-center dielectronic recombination in slow atomic collisions

A free electron can form a bound state with an atomic center $A$ upon photo emission (radiative recombination). In the presence of a neighboring atom $B$, such a bound state can, under certain conditions, be also formed via resonant transfer of energy to $B$, with its subsequent relaxation through radiative decay (two-center dielectronic recombination). This two-center process is very efficient in the 'static' case where $A$ and $B$ form a weakly bound system, dominating over single-center radiative recombination up to internuclear distances as large as several nanometers. Here we study its dynamic variant in which recombination occurs when a beam of species $A$ collides with a gas of atoms $B$ and show that, even though the average distance between $A$ and $B$ in collisions is orders of magnitude larger than the typical size of a bound system, the two-center recombination can still outperform the single-center radiative recombination.

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.

Topical Issue on atomic cluster collisions

The rapidly emerging field of atomic cluster collisions attracts strong research interest of many experimental and theoretical groups worldwide. Being a highly interdisciplinary field, it has numerous links with atomic, molecular and optical physics, astrophysics, plasma physics, biophysics, physical chemistry, solid state physics and even molecular biology. This Topical Issue presents a state-of-the-art description of current research activities in the field of atomic cluster collisions and atomic cluster science. The contributions to this Issue represent experimental, theoretical and computational studies both at the fundamental level of elementary mechanisms and at a more applied level which is necessary in numerous applications of nano- and biotechnology, materials science and medicine.

Studies of Thallium Line Spectra in Thallium – Mercury Discharge

In this paper, we present a study of broadening of thallium emission spectral line shapes in the Tl-Hg discharge. The spectral lines were emitted from high frequency electrodeless lamps, containing Tl, Hg, Ar mixtures and measured by means of Fourier transform spectrometer. The deconvolution procedure, by means of ill posed inverse task solution, was performed to obtain the real (without instrumental function) profiles for further analyze. The solution was implemented using Tikhonov regularization algorithm. The Tl 276.8 nm, 351.9 nm, 352.9 nm spectral lines were analyzed in detail in dependence on the discharge power. The additional broadening of Tl 276.8 nm and 351.9 nm lines were observed due to the excitation transfer in collisions of ground state Tl atoms with excited Hg atoms.

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.

Studying 630 nm atomic oxygen emission sources during strong magnetic storms in the night mid-latitude ionosphere

We analyze significant increases in 630 nm atomic oxygen night emissions during very strong geomagnetic storms, using optical measurements, theoretical modeling, and magnetogram inversion technique (MIT) data. It is shown that during strong magnetic storms when electron precipitation equatorial boundary at the night sector expands up to ~40°, the interaction of energetic electron flux with thermospheric components may cause extreme increases in the 630 nm emission intensity. Model calculations of the red line intensity show good agreement with observational data. Using the November 20, 2003 magnetic storm as an example, we have found that oxygen atom collisions with thermal Maxwell and superthermal electrons make a major contribution to the integral emission intensity. Thermospheric density variations during the magnetic storm significantly affect the red line generation.

Изучение источников эмиссии атомарного кислорода 630 нм во время сильных магнитных бурь в ночной среднеширотной ионосфере

We analyze significant increases in 630 nm atomic oxygen night emissions during very strong geomagnetic storms, using optical measurements, theoretical modeling, and magnetogram inversion technique (MIT) data. It is shown that during strong magnetic storms when electron precipitation equatorial boundary at the night sector expands up to ~40°, the interaction of energetic electron flux with thermospheric components may cause extreme increases in the 630 nm emission intensity. Model calculations of the red line intensity show good agreement with observational data. Using the November 20, 2003 magnetic storm as an example, we have found that oxygen atom collisions with thermal Maxwell and superthermal electrons make a major contribution to the integral emission intensity. Thermospheric density variations during the magnetic storm significantly affect the red line generation.

Sodium atom beam collisions with the liquid glycerol surface: Mass effects of deuteration

Gas-phase sodium atom collisions with liquid glycerol were investigated by atomic beam scattering from normal and deuterated surfaces. The scattering signal of the recoiling effusive Na atom beam was monitored by a mass spectrometer. Because all thermally equilibrated Na atoms ionize into the liquid surface, this system was uniquely suited to interrogate the subtle kinematic effect of glycerol deuteration on the collisional energy transfer process within the impulsive scattering regime. An interplay of kinematic and vibrational effects explains the Na scattering behavior from variously deuterated glycerol surfaces, extending our fundamental understanding of collisional energy transfer, and ultimately, chemical reaction dynamics.

Application of Quantum Scattering Theory in Calculation of the Simplest Chemical Reactions (Dissociative Attachment, Dissociation, and Recombination)

Several parameters (cross section, reaction rate, etc.) of various processes in laser, atomic, and chemical physics are calculated using quantum scattering theory for a system of a few particles. Results of the study of electronic and atomic collisions with diatomic molecules and collisions of diatomic molecules in excited rovibrational states are discussed. Several approximations needed for calculations of real physical systems consisting of several bodies are analyzed. Such approximations can be used for simulation of direct reactions and reactions that yield intermediate transient complexes. Calculated cross sections of electronic and atomic collisions with diatomic molecules and collisions of diatomic molecules are compared with experimental data and results of alternative calculations.

Ionization equation of state for the dusty plasma including the effect of ion–atom collisions

The ionization equation of state (IEOS) for a cloud of dust particles in the low-pressure gas discharge under microgravity conditions is proposed. IEOS relates pairs of the parameters specific for the charged components of dusty plasma. It is based on the modified collision enhance collection model adapted for the Wigner–Seitz cell model of the dust cloud. This model takes into account the effect of ion–atom collisions on the ion current to the dust particles and assumes that the screening length for the ion–particle interaction is on the same order of magnitude as the radius of the Wigner–Seitz cell. The included effect leads to a noticeable decrease of the particle charge as compared to the previously developed IEOS based on the orbital motion limited model. Assuming that the Havnes parameter of the dusty plasma is moderate, one can reproduce the dust particle number density measured in experiments and, in particular, its dependence on the gas pressure. Although IEOS includes no fitting parameters, it can ensure a satisfactory precision in a wide range of dusty plasma parameters. Based on the developed IEOS, the threshold relation between the dusty plasma parameters for onset of the lane formation in binary dusty plasmas is deduced.

The response of nonequilibrium nonstationary plasma created by an intense femtosecond UV laser pulse in rare gases to the emission of the THz frequency band

We suggest a new approach for the analysis of the response of plasma created in the process of multiphoton gas ionization by means of an intense UV laser pulse to the terahertz radiation. It is based on the kinetic Boltzmann equation for the temporal behavior of the electron velocity distribution function (EVDF) in an arbitrary nonmonochromatic external electric field and allows us to take into account both the temporal retardation of the EVDF evolution with respect to the external electric field and the effect of relaxation of the strong nonequilibrium electron energy spectrum due to elastic electron—atomic collisions during the external field action. The suggested consideration provides the basis for the theoretical analysis of short and ultra-short (up to one or two periods of oscillations) THz pulse transportation in plasma waveguides formed by high-intensity laser pulses.

Inelastic H and D atom scattering from Au(111) as benchmark for theory.

Efficient transfer of translational energy to electron-hole pair excitation involving multiple collisions dominates H atom collisions with metal surfaces. For this reason, H atom interaction with metal surfaces cannot be modeled within the commonly used Born-Oppenheimer approximation (BOA). This fact makes H atom scattering from metal surfaces an ideal model system for dynamics that go beyond the BOA. We chose the H/Au(111) system as a model system to obtain a detailed dataset that can serve as a benchmark for theoretical models developed for describing electronically nonadiabatic processes at metal surfaces. Therefore, we investigate the influence of various experimental parameters on the energy loss in detail including isotopic variant, incidence translational energy, incidence polar and azimuthal angles, and outgoing scattering angles.

Reaction blockading in a reaction between an excited atom and a charged molecule at low collision energy.

Recent advances have enabled studies of atom-ion chemistry at unprecedentedly low temperatures, allowing precision observation of chemical reactions and novel chemical dynamics. So far, these studies have primarily involved reactions between atoms and atomic ions or non-polar molecular ions, often in their electronic ground state. Here, we extend this work by studying an excited atom-polar-molecular-ion chemical reaction (Ca* + BaCl+) at low temperature in a hybrid atom-ion trapping system. The reaction rate and product branching fractions are measured and compared to model calculations as a function of both atomic quantum state and collision energy. At the lowest collision energy we find that the chemical dynamics differ dramatically from capture theory predictions and are primarily dictated by the radiative lifetime of the atomic quantum state instead of the underlying excited-state interaction potential. This reaction blockading effect, which greatly suppresses the reactivity of short-lived excited states, provides a means for directly probing the reaction range and also naturally suppresses unwanted chemical reactions in hybrid trapping 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.

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.

Numerical analysis of heat removal from gas phase clusters during condensation of Cu nanoparticles under Ar atmosphere

Abstract The statistical analysis of the molecular dynamics modeling for the nucleation of copper (Cu) vapors under argon (Ar) atmosphere in the evaporation-condensation technology was carried out. The model was used to calculate the nucleation in a cell containing 5000 atoms of Cu and 10,000 atoms of Ar. The probabilities of various outcomes in the collision of Ar and Cu atoms with Cu clusters were determined. It was found that pair collisions have a greater probability than the triple collisions. The heat removal from clusters during a collision with Ar and Cu atoms was analyzed to find the probabilities of formation of long-lived small clusters in a variety of conditions, attempting to estimate the magnitude of heat transfer from the clusters of different sizes. It is found that the pair collisions caused a very excited dimer, whereas, the triple collisions formed a more stable dimer.