Ionization dynamics of helium dimers in fast collisions with He++

By employing the cold target recoil ion momentum spectroscopy technique, we have investigated the (He+, He+) breakup of a helium dimer (He2) caused by transfer ionization and double capture in collisions with alpha particles (E = 150 keV/u). Surprisingly, the results show a two-step process as well as a one-step process followed by electron exchange. In addition, interatomic Coulombic decay [L.?S. Cederbaum, J. Zobeley, and F. Tarantelli, Phys. Rev. Lett. 79, 4778 (1997).] is observed in an ion collision for the first time.     

Ionization of hydrogen and helium atoms by highly charged fast ions in collisions with small momentum transfer

Ionization of hydrogen and helium atoms is studied for the case of “soft” collisions with highly charged fast ions with v≲Z≪v² and v≫v ₀, where Z is the ion charge, v is the collision velocity, and v ₀∼1 is the characteristic velocity of the electron in the ground state of the atom. Analytical expressions are derived for the singly and doubly differential cross section for ionization of a hydrogen atom accompanied by the ejection of a slow electron v _e≲v ₀, where v _e is the velocity of the ejected electron with respect to the recoil ion). The results are generalized to the case of single ionization of helium. It is shown that soft collisions provide the main contribution to the hydrogen ionization cross section and for all practical purposes determine the cross section for single ionization of helium. The asymmetry in the angular distribution of the ejected slow electrons and the properties of momentum exchange in such collisions are discussed. Finally, a formula for the cross section for single ionization of helium is proposed. Zh. éksp. Teor. Fiz. 112, 1966–1977 (December 1997)     

Ionization in collisions between slow atoms

Atomic collisions involving transitions between states in the discrete and continuous spectra are discussed in terms of the adiabatic approximation, using the time-independent perturbation theory. The ionization and recombination cross sections are found by a semiempirical method for the He + He He + He⁺ + e reaction.     

Ionization of K-shell electrons by positron impact

We have investigated the universal scaling behavior for cross sections of the single K-shell ionization by positron impact. The calculations are performed within the framework of non-relativistic perturbation theory, taking into account the one-photon exchange. Special emphasis is laid on the near-threshold energy domain. The results obtained are applicable for wide family of atomic targets.     

Ionization rates in a bose-einstein condensate of metastable helium

We have studied ionizing collisions in a BEC of metastable He. Measurements of the ion production rate combined with measurements of the density and number of atoms for the same sample allow us to estimate both the two- and three-body contributions to this rate. A comparison with the decay of the atom number indicates that ionizing collisions are largely or wholly responsible for the loss. Quantum depletion makes a substantial correction to the three-body rate constant.     

Penning collisions of laser-cooled metastable helium atoms

We present experimental results on the two-body loss rates in a magneto-optical trap of metastable helium atoms. Absolute rates are measured in a systematic way for several laser detunings ranging from -5 to -30 MHz and at different intensities, by monitoring the decay of the trap fluorescence. The dependence of the two-body loss rate coefficient β on the excited state ( 2³ P ₂) and metastable state ( 2³ S ₁) populations is also investigated. From these results we infer a rather uniform rate constant K _sp = (1±0.4)×10^-7 cm³/s. Received 8 September 2000 and Received in final form 19 December 2000     

Zeeman relaxation of CaF in low-temperature collisions with helium

The collision-induced Zeeman relaxation rate for collisions of CaF X2Sigma(v(')=0) with 3He is measured to be Gamma(Z)=(7.7+5.4/-2.5)x10(-15) cm(3)/s at 2 K. This rate is a direct measurement of the influence of spin-rotation coupling on Zeeman relaxation in the first rotational level of CaF. The relationship of this rate to known molecular constants is consistent with recent theory of cold molecular collisions and outlines the (2)Sigma molecules conducive to magnetic trapping.     

K-vacancy creation and positron emission in heavy-ion collisions

The production of K-shell vacancies during a heavy-ion collision is calculated in the adiabatic monopole approximation. For (U, U) collisions we present the cross section for positron emission in the transient supercritical field, based on this K-vacancy production probability.     

Ultracold collisions in the system of three helium atoms

The Faddeev differential equations for a system of three particles with a hard-core interaction are described. Numerical results on the binding energies of the ⁴He₃ and ³He⁴He₂ trimers and on ultracold collisions of ^3,4He atoms with ⁴He₂ dimers obtained with the help of those differential equations are reviewed. The results obtained for the hard-core model using the Faddeev equations are compared with analogous results obtained by alternative methods.     

Elastic and inelastic positron-helium scattering

Elastic and inelastic positron--helium scatterings have been investigated with the coupled-channel optical method (CCO). Ionization continuum and positronium formation channels are included via a complex equivalent-local optical potential. Calculations are reported of cross sections of elastic scattering, total excitation and n=2, 3, and 4 excitations of ground-state helium for incident energies from 30~eV to 400~eV. The present calculation shows that the ionization and Ps-formation channels significantly affect the cross sections of elastic and inelastic positron--helium scatterings.     

Ionization of heavy ions in relativistic collisions with neutral atoms

The problem of ionization of ions in ion-ion and ion-neutral relativistic collisions is considered. Formulas for ionization cross sections are derived in the Born approximation in terms of the momentum transfer without allowance for magnetic interactions. Using these formulas implemented in the LOSS-R code, the ionization cross sections are calculated for the K shells of neutral atoms colliding with protons and also for 1s and 2p electrons of multiply charged heavy ions (nuclear charge Z = 80−90) colliding with bare nuclei and neutral atoms. The calculation results are compared with experimental data and calculations of other authors.     

The theory of low-energy positron collisions with molecules

This review begins by describing the theory of the collisions of low-energy positrons with molecules. It then goes on to describe in detail the calculations, both phenomenological and ab initio, which have been carried out to date to predict the results of low-energy positron-molecule scattering experiments. The review conludes with a discussion of possible future developments.     

Ionization loss of relativistic structural heavy ions in collisions with atoms

A nonperturbative theory of energy loss in collisions between structural, highly charged heavy ions moving at relativistic velocities and atoms is developed. A simple formula for effective deceleration is derived. By structural ions are meant ions containing partially filled electron shells. It is such ions characterized, as a rule, by a significant charge (for example, partially “stripped” uranium ions) that are used in numerous experiments involving the use of modern heavy-ion accelerators.