Transfer and ionization in collisions of positrons with atoms

Kinematic singularities in certain few-body reactions may provide special features that are both conceptually simple and observable. Particle transfer is such a reaction. At high velocities, it is characterized by singularities that are manifested by peaks and ridges in differential cross sections. For electrons captured by protons, some of these singular features have been observed. For capture by positrons, i.e. positronium formation, new features are predicted.     

Transfer ionization in isocharge sequence ion and Ne collisions

The dependence of the ratio R1 for transfer ionization to single capture for Cq+, Nq+, Oq+, Neq+ ions on Ne target upon the electronic structure of the projectile is studied. For Aq+-Ne collisions the ratio R1 decreases as the atomic number Z of the projectile increases for q=4,5,6,7 sequences which provides strong evidence for the increase of the binding energy of the target valence electron after single electron capture. The increase in binding energy depends both upon the atomic number of the projectile and the target atom.     

Transfer ionization of the isocharge sequence ion and Ar collisions

The ratios of transfer ionization to single capture for isocharge C^{q+}, N^{q+}, O^{q+}, Ne^{q+} ions on Ar atoms are measured by using position-sensitive and time-of-flight techniques. It is found that the ratio R_{1} increases with nuclear charge Z in a q=4 sequence, and the ratio R_{1} sequences have the minimum values at Z=7 and Z=8 in q=5 and q=6 sequences, respectively, and the ratio R_{1} decreases with the increase in the nuclear charge Z in a q=7 sequence. The results may be explained within the frame of the statistical model. It may be concluded that the transfer ionization depends remarkably upon the electronic structure of the projectile. The projectiles used in this study have the same charge state and velocity, but different electronic structures. The dependence of the ratio R_{1} on q is studied for Ne^{q+}-Ar collisions. The ratio R_{1} is found to increase as q increases for Ne^{q+}－Ar collisions. The measured dependence of the ratio R_{1} on q is compared with the calculation using the molecular Coulomb over-barrier model.     

L-shell ionization in proton-atom collisions

The cross section of ionization of atoms by protons is calculated in the plane-wave approximation. It is shown that the wave function of the ejected electron must be orthogonal to all wave functions of the atom. A method taking into account a finite number of wave functions in the calculation of the ionization amplitude is proposed. The calculated cross sections of ionization of nitrogen, oxygen, and neon atoms by protons agree well with the experimental data and results of calculations carried out by other authors.     

Transfer ionization cross-sections measured in collisions of highly charged argon ions with neon target

: Multiple electron transfer processes are studied for Arq+n- Ne (q= 8, 9, 11, 12) collisions by using multi-parameter coincidence techniques. Various electron transfer processes are identified experimentally and the related cross-sections are measured. The dependence of transfer ionization cross-sections on the recoil charge states is compared with the results from the modified molecular classical overbarrier model. It is found that the modified model described the experimental results reasonably.     

Radiative ionization in slow ion-atom collisions

Bremsstrahlung from the ionization of inner shell electrons in slow ion atom collisions is considered. The yield predicted by the binary encounter approximation is small compared with experiment in 12–33 MeV O + Zr and O + Au collisions.     

Induced electron capture in field-assisted energetic atomic collisions

We present model considerations for the process of the electron capture in energetic nonrelativstic collisions of light atomic particles in the presence of a relatively weak low-frequency external electromagnetic field. The field is treated as an elliptically polarized quantum single-mode field. Establishing validity of the dipole approximation to the electron transfer (where the total momentum of all emitted or absorbed photons can be well above the typical inneratomic momenta of an electron in the initial and final states) and neglecting the Doppler and aberration effects, we give a fully nonrelativistic treatment for the field-assisted collisions and show that the capture cross section is invariant with respect to the Galilean transformations. The model consideration suggests that the field can substantially influence the capture dynamics and considerably change the capture cross section compared to the field-free collisions. This is especially the case if the “resonance” conditions nω≈±v ²/2 are satisfied, with nω being the energy transferred to or absorbed from the electromagnetic field and v the collision velocity.     

Statistical simulation of complete events in energetic nuclear collisions

A statistical model is developed for the generation of complete multifragment events in medium-energy nuclear collisions. Based on simple geometrical considerations, the collision system is divided into a few (participant/spectator) sources that disassemble independently. The sufficiently excited sources quickly explode into pions, nucleons, and composite, possibly particle unstable, nuclei. The different final states compete according to their microcanonical weight. The less excited sources, and the unstable explosion products, deexcite by sequential light-particle emission. The model has been implemented as a Monte Carlo computer code that is sufficiently efficient to permit generation of large event samples. The analysis of such multifragment events is addressed and some illustrative applications are discussed.     

Associative ionization in slow collisions of atoms

An analysis of the modern theory of associative ionization (AI) is performed, and ways of its further development are discussed. The threshold behavior of the cross section of endothermic AI reactions is considered and it is shown that, in the quantum case, its dependence on the above-threshold energy E is strictly linear, i.e., significantly different from the E ^3/2 law ensuing from the semiclassical theory. This has a simple explanation, since the matrix elements of the scattering operator are finite at E = 0 due to the tunneling effect. The possibility of describing the dynamics of an elementary AI event within the framework of the diffusion approximation is substantiated, and the conditions of applicability of the theory of “quantum chaos” to treating the spectrum of highly excited Rydberg molecules are examined. The quantum properties of the exothermic processes of AI and Penning ionization in the case of the states of the interacting atoms being autoionizing are discussed in detail. A comparison with the semiclassical theory is presented.     

Two-jet production inp\bar p collisions

The production of two quark/gluon jets in \(p\bar p\) collisions is analyzed. We determine the cross section dependence on the transverse momentum, the rapidities, and on the two-jet invariant mass, and we evaluate the sensitivity of the results on the theoretical input assumptions and the experimental cuts. We extrapolate these results into the TeV energy region.     

Bilepton production in $ e^-\gamma $ collisions

We study the production of bileptons, new gauge bosons of lepton number two, in the minimal 3-3-1 model in high energy collisions. If the bilepton masses are in the range of 300 GeV the reaction will give observable cross-sections in future colliders.Received: 5 August 2003, Published online: 23 March 2004Dang Van Soa: On leave of absence from Department of Physics, Hanoi University of Education, Hanoi, VietnamHoang Ngoc Long: On leave of absence from Institute of Physics, NCST, P.O. Box 429, Bo Ho, Hanoi 10000, Vietnam     

On Drawin's formula for ionization in atom-atom collisions

Drawin's semiclassical formula for ionization in collisions of neutral atoms or molecules is examined. Both, from an analysis of its derivation and from a comparison with additional experimental data, it appears that considerable errors can be expected in general applications of this formula.     

Inclusive particle production inp\bar p collisions

We calculate the inclusive production of charged hadrons in collisions at the next-to-leading order in the QCD improved parton model using a new set of fragmentation functions for charged pions and kaons. We predict transverse-momentum distributions and compare them with experimental data from the CERN Collider and the Fermilab Tevatron.     

The photon structure and exclusive production of vector mesons in \gamma\gamma collisions

The process of exclusive vector meson production, , is studied for almost real photons. This process may be reduced to the photoproduction of off the meson. We discuss the possibility of extracting the gluon distribution of and of the photon from such a measurement. Predictions are also given for the reaction for various cms energies typical for LEP and for the future linear colliders. Received: 13 December 2000 / Revised version: 6 January 2001/ Published online: 6 April 2001     

Symmetric scattering ine ±-H ionization collisions

Triple differential cross-sections for ionization of hydrogen atoms by electrons and positrons have been calculated for symmetric coplanar geometry following a multiple scattering method suggested and used earlier by the authors. Results show single binary peaks exactly at 45° and double binary peaks exactly at 135° for higher energies as are expected from an analysis of Briggs [3]. At lower energies there are certain deviations from these values. An analysis of scattering mechanism at peaks are also given. This supports Briggs’ explanation.