Slow Atomic Collisions Research Articles

Temperature dependence of the formation rates of hydrogen-helium mesic molecules in collisions of slow hydrogen atoms with helium

The rates of molecular muon transfer from the ground-state muonic hydrogen to helium isotopes are calculated in an improved adiabatic approximation. The results obtained by us at various temperatures are compared with the available experimental data.

Theoretical study of electronic energy loss recently observed in impact collision ion scattering of a low energy Li + from a TaB 2(0 0 0 1) surface

Based on the Firsov model of electronic excitations in slow atomic collisions, a method to calculate the electronic energy loss is proposed. In this method, molecular orbital theory is employed in order to obtain the electronic excitation probability of a quasi-molecule between colliding atoms. Using this method, the electronic stopping power has been calculated for the case of the Li–B and Li–Ta collision pair, and the usefulness of this method has been verified by using the calculated stopping power in molecular dynamics simulations of an ion scattering experiment. The experimental observations are well explained by the theory.

Electronic energy loss of slow projectiles evaluated by molecular orbital theory

Based on the Firsov model of electronic excitations in slow atomic collisions, a method for calculations of the electronic energy loss is proposed. In this method, molecular orbital theory is employed in order to obtain the electronic excitation probability of a quasi-molecule between colliding atoms. By this method, the electronic stopping power has been calculated for the case of the Li–B and Li–Ta collision pairs. Using this calculated electronic stopping power, molecular dynamics simulations have been performed in order to analyze the energy loss observed in a recent experiment, in which 1-keV Li + ions were scattered from a TaB 2 (0 0 0 1) surface. These experimental observations are well explained by this theory.

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.

Solutions of the Schrödinger equation for an attractive1/r6potential

Mathematical methods are presented that give pairs of linearly independent solutions for an attractive ${1/r}^{6}$ potential. These solutions represent a different class of special functions that are important to the understanding of molecular vibration spectra near the dissociation threshold and slow atomic collisions.

Radial-decoupling excitation mechanism in slow atomic collisions studied using zero-range model potentials

One-dimensional zero-range model potentials are employed to investigate the recently proposed radial-decoupling excitation mechanism in ion-atom collisions. The expression for the total (i.e. excitation plus ionization) radial-decoupling probability, obtained in the adiabatic limit, agrees with the result of our earlier, tentative approach (Pieksma and Ovchinnikov 1994 J. Phys. B: At. Mol. Opt. Phys. 27 4573) to second order in the collision velocity v. In fact, at low collision energies the cross section just scales with . This is in accordance with the adiabatic theorem as formulated by Born and Fock (1928 Z. Phys. 51 165), which also implies a power law for the cross section as the collision velocity becomes sufficiently low. The dependence is a purely quantal effect, and it is argued that it represents the correct low-energy cross section behaviour, even though it seems to contradict the exponential law that follows from the conventional theory of adiabatic transitions.

Hidden crossings approach to inner-shell ion-atom collision processes

The recently developed hidden crossings approach to slow atomic collisions is applied to inelastic processesinvolving inner-shell electrons. The hidden crossing theory reveals new mechanisms for inner-shell vacancy creation and transfer which are associated with the S- and Q-superseries of hidden crossings of adiabatic energy surfaces in the complex plane of internuclear distance. The properties of these new electron transition mechanisms are explored and compared with some of the conventional mechanisms (rotational, Demkov and Landau-Zener coupling). The theory is illustrated by probability and cross section calculations for vacancy creation and transfer in specific collision systems.

Theory of slow-atom collisions.

A general theory of slow-atom collisions is presented with special emphasis on the effects of nuclear statistics and atomic fine and/or hyperfine structures. Symmetry properties of the collision complex and correlations between the molecular states and the separated-atom states are carefully examined. The frame transformations between various angular momentum coupling schemes are derived, which, in combination with the multichannel quantum defect theory, provides a solid foundation for the computation and the physical interpretation of slow-atom collision processes. The theory reduces to those of Stoof {ital et} {ital al}. [Phys. Rev. B {bold 38}, 4688 (1988)] and Zygelman {ital et} {ital al}. [Phys. Rev. A {bold 49}, 2587 (1994); {bold 50}, 3920 (1994)] in their respective ranges of validity. {copyright} {ital 1996 The American Physical Society.}

Quasiclassical expression for inelastic energy losses in atomic particle collisions below the Bohr velocity.

A quasiclassical formula for inelastic energy losses in collisions of slow ions and atoms has been derived. The electronic stopping power is assumed to be due to a momentum transfer during scattering of the target atom's electrons in the screened Coulomb field of the projectile and vice versa. The Coulomb repulsion between the nuclei of colliding particles is shown to decrease greatly the electronic stopping in the limiting case of small, reduced energies, E\ensuremath{\ll}1. This leads to inelastic energy losses being no longer linearly dependent on the particle velocity, as predicted by the linear-response theory. The latter effect explains the experimentally observed strong dependence of the kinetic electron emission well below the threshold on the energy of ions bombarding a solid surface. At relatively large energies (E>1) the expression obtained reduces to the result found earlier by Lindhard and Scharff [Phys. Rev. 124, 128 (1961)]. The total energy losses of slow atoms calculated on the basis of the formula derived for the inelastic stopping power and the expression for the elastic energy losses available in the literature are in good agreement with the experimental data.

Temporal rearrangement of electronic densities in slow atomic collisions

AbstractElectronic rearrangement is a common occurrence in interactions of an atom with other atoms or with solid surfaces and in molecular photodissociation by visible and UV light. In this contribution, the emphasis is on the way properties change over time, as a system progresses from its initial to its final state in a collision. An overview is presented of our recent work describing electronic rearrangement in terms of coupled differential equations for electronic orbitals and nuclear degrees of freedom. An eikonal/(time‐dependent Hartree–Fock) approximation is described and a new compact formulation is given in terms of operators in Liouville space, for both small and extended molecular systems. A method is described for treating the coupling of degrees of freedom with very different time scales. The method is illustrated with selected results for temporal atomic population changes in ion–atom and ion–solid surface interactions. © 1994 John Wiley & Sons, Inc.

Time-dependent many-electron approach to slow ion-atom collisions: The coupling of electronic and nuclear motions.

We describe the coupling of electronic and nuclear motions in slow atomic collisions using a combination of the eikonal and time-dependent Hartree-Fock (TDHF) approximations. Starting with an eikonal representation of the total wave function, a wave function is constructed from classical trajectories in a way suitable for describing atomic collisions with velocities down to a fraction of an atomic unit. The TDHF formulation is developed in terms of its density operator. The differential equations coupling the density operator to the nuclear motions have been solved with a procedure developed to account for the coupling of fast (electronic) and slow (nuclear) degrees of freedom. This is based on a local-interaction picture and on a temporal linearization of the equations, allowing for the integration of the electronic density over large time intervals. Density-matrix equations are derived in a basis of traveling atomic orbitals, and numerical results are presented for ${\mathrm{H}}^{+}$+H and ${\mathrm{He}}^{2+}$+H. Good agreement is found with experimental results for ${\mathrm{H}}^{+}$+H, comparing integral electron transfer cross sections from 2 to 2000 eV. In addition, an analysis of the time dependence of atomic orbital populations provides insight on electronic rearrangement during collisions and shows that even very small contributions from the driving forces of the nuclei on the electrons have a cumulative effect on the density operator that can substantially change final populations.

Nonadiabatic formulation of the slow-atomic-collision problem in the finite electronic basis.

The resonating-group method developed for nuclear collisions is used to obtain equations describing the collisions of slow atoms. On one hand, these equations correctly take into account the indistinguish- ability of electrons and scattering boundary conditions and therefore are free from the drawbacks of conventional equations in the adiabatic electronic basis. On the other hand, they retain the form of the latter equations and therefore are in agreement with the generally accepted picture of heavy-particle motion in the fields of adiabatic electronic potentials accompanied by nonadiabatic transitions. The general theory is illustrated by considering the interaction of two ground-state hydrogen atoms in the Heitler-London electronic basis.

Reply to "Origin of 'fast electrons' from slow atomic collisions"

We show reasons why negative ions cannot contribute significantly to our observation of fast electrons in slow atomic collisions [Phys. Rev. 45, 5286 (1992)]. We point out that the opposite suggestion by Yasui [preceding Comment, Phys. Rev. A 48, 1711 (1993)] results from multiple errors in his analysis, including inadequate consideration of energy conservation, the use of nonapplicable data from the literature, neglect of decay in flight of metastable negative ions, nonconsideration of detection efficiency, and the inconsistent fit of experimental data.

Origin of ‘‘fast electrons’’ from slow atomic collisions

We have analyzed the experimental results of ``fast electrons'' from slow atomic collisions [Baragiola et al., Phys. Rev. A 45, 5286 (1992)]. It is suggested that the observed negatively charged particles are not electrons, but sputtered or reflected ions.

Application of the advanced adiabatic approach to charge exchange in slow collisions between H and O8+

We study charge exchange in slow atomic collisions within the framework of the advanced adiabatic approach [Grozdanov and Solov'ev, Phys. Rev. A 42, 2703 (1990)]. In this approach, nonadiabatic transitions near hidden or avoided crossings are related to branch points connecting adiabatic potential curves at complex values of the internuclear distance. A program package has been developed that automatically takes into account all relevant nonadiabatic transition amplitudes for the calculation of charge-transfer cross sections. The present method allows the inclusion of a large number of molecular-orbital basis functions and turns out to be extremely fast. As an example, partial and total cross sections are calculated for charge exchange in slow ${\mathrm{O}}^{8+}$+H(1s) collisions. The results are compared to previous theoretical and experimental data.

Excitation and ionization processes in slow collisions of ground-state and excited hydrogen atoms with protons.

The collision dynamics in the ${\mathrm{H}}^{+}$+H system is considered at low relative velocities within the concept of hidden adiabatic energy crossings in the complex plane of internuclear distance. Detailed information is provided on the topology of all series of hidden crossings in this system which are involved in the transitions between the states with principal quantum numbers n\ensuremath{\le}4 and in the promotion of these states into the continuum. This information is used to perform cross-section calculations for the 1s\ensuremath{\rightarrow}nl (n=2--4), n=2\ensuremath{\rightarrow}n=3,4, and n=3\ensuremath{\rightarrow}n=4 excitation transitions and for ionization from the n=1, 2, and 3 levels in the adiabatic energy region.

Spectra of electrons ejected in autoionization of quasimolecules

Model complex quasimolecular potential curves are used to reproduce characteristic features in the energy spectra of electrons ejected in slow atomic collisions. The parabolic approximation for the time dependence of the potential curves (and its modifications) allows one to describe the contribution to the spectrum of the electrons ejected, when the atomic nuclei are near the position of closest approach. The new model proposed includes both united-atom and separated-atom limits. It accounts also for the possibility of steep variation of the quasimolecular state width with internuclear distance. The angular distribution of the ejected electrons is shown to be important when the interference oscillations in the spectrum are analysed.

Fast electrons from slow atomic collisions.

We report observations of extremely energetic electrons produced in collisions between keV ${\mathrm{He}}^{+}$ and ${\mathrm{Ar}}^{+}$ and different solid surfaces, with energies up to approximately 40% of the center-of-mass energy. This emission can result from the decay of many-electron excitations produced in nearly head-on collisions.

The force on atoms in intense standing wave laser fields: dipole force with doppleron structure

The dressed atom approach is used to investigate the evolution of atoms moving in an intense standing wave laser field. When coherences between the dressed states can be neglected, a simple expression is obtained for the radiative force acting on the atoms. The Landau-Zener model, well-known in the theory of slow atomic collisions, is here used to include coherences in an analytic treatment. At high velocities, where the simplest approach breaks down, the results are in good agreement with exact numerical calculations. Applications outside the field of laser cooling are discussed.

The contribution of the saddle point mechanism to the total ionization cross section in slow atomic collisions

Approximation analytic expressions for the total ionization cross section for H+-H and He2+-H collisions are obtained within the framework of the theory of non-adiabatic transitions in slow atomic collisions. Theoretical predictions for the cross section of the saddle point mechanism are presented. It is shown that this ionization channel contributes significantly to the total ionization. For both collision systems the calculated total ionization cross section agrees well with experimental data obtained by Shah et al. (1987).