Higher-order modified Born approximation for potential scattering in the presence of a strong magnetic field

Summary The total cross-section for potential scattering in the presence of a strong magnetic field presents singularities and exhibits giant growth (cyclotron resonances) at values of the incident particle energy exactly matching the differences between the initial-and final-state Landau levels. In this paper (in cylindrical coordinates) a higher-order modified Born series has been derived that can be summed at all orders giving a nondivergent total cross-section at the Landau thresholds.     

Landau levels line widths in potential scattering in the presence of a strong magnetic field

Summary The FBA scattering cross-section in the presence of a strong magnetic field diverges at the Landau thresholds. Such divergences are eliminated by the introduction of a modified density of states, accounting for the finite Landau states lifetime of the electrons in a magnetized plasma.     

Electron scattering from an assembly of point scatterers in the presence of a quantizing magnetic field

Summary We have studied the scattering of electrons by a structured target in the presence of a quantizing static magnetic field, under the assumption that the presence of the field does not affect the behaviour of the massive target nuclei, but it influences only the motion of the incident electrons. In this case, the electron motion in the plane perpendicular to the magnetic field is confined within a typical distance given by the cyclotron radius ρ₀=(cℏ/|e|B)^1/2, that for particular values of the intensity of the magnetic field can be comparable with the distance between two scattering centres. The known field-free interference conditions are modified, depending both on the energy of the incident particle and on the intensity and the direction of the magnetic field. The general case of a three-dimensional scattering array has been derived in detail. Numerical results are given for the case of two scattering centres in perpendicular geometry. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Electron-molecule scattering processes

Summary The collisions of slow electrons (k ²<2 Ryd) with gaseous molecules provide a very powerful experimental tool for the study of molecular properties and for correlating their structural characteristics with the specific ways in which energy is exchanged and stored during each encounter between the impinging light charge and the various molecular degrees of freedom. In the present study a generalab initio approach is described for treating the various parts of the interaction occurring between the bound molecular electrons and those in the continuum of the probing beam. It is shown that the results found with this method are closely correlated with our knowledge of the molecular structure which thus bears great significance when interpreting collisional events involving simple molecular targets.

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Riassunto Le collisioni di elettroni lenti con molecole gassose forniscono un potente mezzo sperimentale per correlare specifiche proprietà molecolari con il comportamento dinamico del sistema e con i processi competitivi di deposizione selettiva di energia nei vari gradi di libertà molecolare. In questo studio si descrive un metodo completamente generale edab initio per trattare le varie componenti dell'interazione fra gli elettroni legati e quelli nel continuo del fascio incidente. Si mostra, per molecole polari come esempio iniziale, che tale metodo permette di correlare in modo chiaro le nostre conoscenze di struttura molecolare con il dettagliato comportamento dinamico del processo collisionale in questione.

     

Shape analysis of the proton-induced convoy electron peak

Summary The target thickness dependence of the shape of the convoy electron peak from protons (2.4 MeV/u) emerging from thin carbon foils ((2.2÷20)μg/cm²) is studied by a series expansion fitting method. Th results show nearly symmetric and target-thickness-independent behaviour close to the maximum of the peak. In the wings of the peak, however, a thickness-dependent asymmetry is observed. Supported in part by BMFT/Bonn     

Advanced adiabatic approximation for momentum distributions of ejected electrons in slow atomic collisions

The problem of determination of momentum distributions of ejected electrons in slow atomic collisions is studied within the impact-parameter method by using a dynamic adiabatic basis which takes into account the correct boundary conditions. An expression is obtained which relates the momentum distribution of the ejected electrons with a coherent sum of the delocalized dynamic adiabatic eigenstates (elementary wavepackets). The form of the momentum distribution exactly coincides with the form of the total wavepacket in configuration space. General formulas are applied to a model problem of electron detachment in the process in which the electron-atom interactions are described by the zero-range potentials. In the example considered, the momentum distribution of ejected electrons, in the center-of-mass frame, exhibits a maximum located in the scattering plane on the circle of radius (in atomic units), where v is the relative collision velocity and is the impact parameter. Received: 5 October 1998     

Classical study of and collisions in intense laser fields

In this paper, ion-atom and ion-ion collisions in the presence of intense laser fields are qualitatively studied by Classical Trajectory Monte Carlo (CTMC) simulations. It is found that in contrast to the field-free collisions, the colliding ion and the target nucleus could absorb energy from the applied laser fields when the electrons escape from the collision system. This result is explained in terms of Coulomb explosion induced by the enhanced ionization at the so-called critical internuclear distance. Also, the corresponding energy gain cross-sections are evaluated. Received: 7 October 1998 / Received in final form: 28 January 1999     

Free electron-positron pair production in collisions of Au (10.8 GeV/u) with Cu , Ag and Au

Total and differential cross-sections for the production of free electron-positron pairs are calculated for the reactions Au⁷⁹⁺ (10.8 GeV/u) on Cu²⁹⁺, Ag⁴⁷⁺ and Au⁷⁹⁺. The methods used are the semiclassical approach and the solution of the time-dependent Dirac equation by a coupled channel method with free wavepakets describing the created fermions. The obtained total cross-sections are in good agreement with the experimental data. The differential cross-sections give informations about the correlation between the electron and positron. Received 21 December 1999 and Received in final form 6 March 2000     

Charge exchange collisions of H<Superscript>+</Superscript>/D<Superscript>+</Superscript> ions with alkaline Earth atoms (Ca,Mg)

The Classical Trajectory Monte Carlo (CTMC) Method has been used to calculate the differential, partial and total single electron capture cross sections for the collision of H⁺/D⁺ with Ca and Mg atoms in the energy range of 1–100 keV. The differential cross sections at angles near the diffraction limit (<0.1^○) in both systems show a forward peak followed by an asymptotic fall at higher angles. Total and partial capture cross sections are found to be in good agreement with the experimental observations. Oscillations in the partial capture cross sections have been explained due to the swapping of the field electron. Isotope effect in the electron transfer is reported to be negligible.     

Dynamics of ionization mechanisms in relativistic collisions involving heavy and highly-charged ions

The dynamics of mechanisms associated with the ionization of inner-shell electrons in relativistic collisions involving heavy and highly-charged ions is investigated within a nonperturbative approach formulated explicitly in the time domain. The theoretical treatment is based on the exact numerical solution of the time dependent Dirac equation for two Coulomb centers on a lattice in momentum space. We present results for ionization in encounters between 100 MeV/u Au⁷⁹⁺ projectile ions impinging on a hydrogen-like uranium target. By directly visualizing the collision dynamics we identify a new ionization mechanism in which electrons are emitted from the internuclear region preferentially in the transverse direction with respect to the projectile trajectory. A striking characteristic of this ionization mechanism is that the velocity of the electron is higher than the projectile velocity. Received 26 June 2001 and Received in final form 27 November 2001     

Classical impact parameter approximation: Application to atomic collisions with n independent electrons and to recoil analysis

We show that a classical Impact Parameter Method may be derived when taking fully into account the smallness of the ratio between the electron and nuclear masses. It allows to calculate, exactly as in the quantum version, projectile scattering and therefore recoil momenta required for the interpretation of recent measurements. We prove an additivity theorem which allows, in particular, to reduce the n-non-interacting electron problem to a set of n one-electron problems. Consequences for the interpretation of target recoil measurements are discussed. Received 25 June 1999     

Self-consistent field theory of collisions I. Scattering channels

The conventional Hartree and Hartree-Fock approaches for treating many-electron bound systems have been extended recently to positive energy scattering problems, in which both the bound and continuum orbitals are determined by the requirement of full self-consistency. Serious consequences of such a theory are that the target orbitals become energy dependent and the asymptotic boundary conditions are satisfied only approximately, in lowest order. It is important therefore to test the theory for its convergence under configuration mixing. This self-consistent field (SCF) theory for scattering has been tested here for scattering from hydrogenic target as a model where the target function is determined dynamically. Penetration of the projectile inside the bound target orbital is manifest through the SCF for the bound state. Our results show that the theory converges to the correct amplitudes and to the exact boundary conditions as more configurations are added. The use of the amputated functions and the weak asymptotic condition (WAC) upon which the SCF theory is based, is justified as the WAC converges to the correct limit. It is then applied to the positron-helium and electron-helium scattering systems where the helium function is calculated simultaneously together with the scattering function. The resulting phase shifts and the SCF target functions are compared with those obtained with the pre-determined target functions in the conventional approaches. Received 22 September 1999     

Loss estimation of electrostatically trapped ND3 molecules

Losses of electrostatically trapped molecules are caused by the Majorana transition or the inelastic collisions between trapped molecules. The loss rate of electrostatically trapped ND₃ molecules in the (J=1,K=1,M=1, A) state was estimated: ND₃ molecules in this state have actually been trapped. When trapping cold molecules, using ¹⁵ND₃ molecules (fermion) with a minimum electric field higher than 10 kV/cm yields high stability.     

Time dependence in quantum mechanics

It is shown that the time-dependent equations (Schr?dinger and Dirac) for a quantum system can be derived from the time-independent equation for the larger object of the system interacting with its environment, in the limit that the dynamical variables of the environment can be treated semiclassically. The time which describes the quantum evolution is then provided parametrically by the classical evolution of the environment variables. The method used is a generalization of that known for a long time in the field of ion-atom collisions, where it appears as a transition from the full quantum mechanical perturbed stationary states to the impact parameter method in which the projectile ion beam is treated classically. Received 25 October 1999     

Levinson theorem for Dirac particles in one dimension

The scattering of Dirac particles by symmetric potentials in one dimension is studied. A Levinson theorem is established. By this theorem, the number of bound states with even(odd)-parity, n₊ (n_-), is related to the phase shifts [] of scattering states with the same parity at zero momentum as follows: The theorem is verified by several simple examples. Received 26 August 1998 and Received in final form 18 January 1999     

Reaction and inelastic processes in the collision O () + O ()

A reduced dimensionality model is used to study the reaction OO O ₃ ( X1A1 ) + O( ³ P ) by means of time-dependent and time-independent quantum-mechanical methods. State-selected probabilities and rate constants are obtained for the reactive process as well as for the inelastic collision in which the vibrationally excited oxygen loses one or more quanta. It is found that the experimentally observed jump in depletion rates above a critical value of v could be partially explained by the vibrational relaxation rather than reaction. Reaction only becomes important for relatively high translational energies and therefore the calculated rates are too small at the temperatures of interest. It is concluded, however, that the reaction saddle point region in the potential energy surface plays a crucial role in the enhancement of vibrational relaxation. Received: 3 February 1998 / Revised: 27 March 1998 / Accepted: 15 May 1998     

Cooling efficiency in collisions between Pd and He,Ne, Ar and Kr

The cooling of the metal cluster Pd₁₃ in an atmosphere of rare gas has been studied by means of computer simulation. By simulation, the average energy transfer in collisions between one cluster and one gas atom has been obtained. Emphasis has been placed on conditions when the temperatures of the colliding species are almost equal. All modes of motion, inclusive the translation, must be considered in order to obtain vanishing energy transfer at equilibrium. A simulation scheme is presented by which the energy transfer is zero to the cluster when the gas and the cluster temperatures are equal. At equilibrium the energy transfer does however not vanish for all impact parameters. In the collisions with Pd₁₃, the cluster is heated by collisions with a small impact parameter but equally cooled by collisions with a large impact parameter. Argon and krypton are found to cool Pd₁₃ equally efficiently while neon and helium are less efficient cooling agents. Received 28 September 2001 / Received in final form 8 August 2002 Published online 12 November 2002 RID="a" ID="a"e-mail: JanW@phc.gu.se     

Non-partial-wave Coulomb-Born theory for the excitation of many-electron atomic ions II: Numerical description and application

A non-partial-wave Coulomb-Born theory is recently formulated to treat the excitation of many-electron atomic ions for impact by an arbitrary charged particle [Y.B. Duan et al., Phys. Rev. A 56, 2431 (1997)]. The multiple expansion of the transition matrix element is decomposed into the target form factor and the projectile form factor. These are the matrix elements of the tensor operators between quantum states so that any complicated wave function for the target ion can be employed. In this formal theory, an infinitesimally small positive quantity is introduced artificially to guarantee the convergence of integrals. As a supplementary part of the theory, we discuss how to choose the value of . It is found that the should be taken as functions of the momentum transfer and multipolarity . Illustrations are carried out by calculating the cross-sections for some typical transitions n _a l _a -n _b l _b of hydrogen-like ions for impact by electron, positron, and proton, respectively. The resulting cross-sections are in good agreement with ones produced by using a method available for ion targets with Slater-type orbitals [N.C. Deb, N.C. Sil, Phys. Rev. A 28, 2806 (1993)]. Comparisons demonstrate that the Coulomb-Born theory with non-partial wave analysis provides a powerful method to treat the excitation of many-electron atomic ions impact by an arbitrary charged particle. Received 6 April 1999