Statistical Treatment of Ionization in Atomic Collisions

The production rate of negative ions in collisions between neutral, singly and multiply charged ions with clusters or small molecules is a crucial parameter to study the fragmentation of the targets as a function of the excitation energy (CIDEC method). We report on high production yields of negative ions in collisions between Fq+(q = 1-3) ions and fullerene C60 and adenine molecules. The dynamics of the formation of the negative ion in such collisions is discussed.

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.

A crossed-beam technique incorporating time-of-flight spectroscopy together with electron-ion and ion-ion coincidence counting of the collision products has been used to study the ionisation of He by H+, He2+ and Li3+ ions. Cross sections for single and double ionisation have been determined with high precision at energies within the range 50-2380 keV amu-1. Cross sections for single ionisation have been compared with the predictions of the first Born approximation, while cross sections for double ionisation have been considered in terms of the 'two-step' and 'shake-off' mechanisms. The authors have also measured the corresponding cross sections for one- and two-electron capture and for the transfer-double-ionisation process; the latter is shown to be the main He2+ production mechanism at the lower impact energies.

Electron-ion collisions: Dielectronic recombination and ionization

Classical (CTMC) calculations of single electron capture and single ionization cross sections are carried out for collisions of H+ and C6+ collisions with H2O molecules in the impact energy range 25<E<5000 keV/amu. The calculation employs a multi‐center model potential for the interaction of the active electron with the H2O+ core. The results agree with the experimental data when the independent electron event model is applied to evaluate total electron capture and ionization probabilities from the corresponding one‐electron ones.

The single and double ionization of helium atoms by fast nuclei and multicharged ions

The absolute cross section of associative ionization in the collision of C+ with D− has been measured in a merged beam experiment for collision energies ranging from 0.01 to 2 eV. The measured cross section is found to be compatible with an E−1 law at low energies and presents a relative maximum between 0.8 and 1 eV.

Translational energy spectroscopy in an apparatus previously developed in this laboratory has been used to study one-electron capture by 4-28 keV N4+ ions in collisions with helium. The main excited N3+ product channels have been clearly identified and the relative cross sections determined. The two main channels are found to involve core rearrangement. While the N3+ 2p2 1 S product channel is dominant, it is shown that other channels involving large energy defects contribute significantly at the higher energies thereby resulting in an essentially energy invariant total one-electron capture cross section: MCLZ calculations, which have also been carried out, satisfactorily predict the main 2p2 1 S product channel but fail to account for the other channels arising from curve crossings at small internuclear separations.

Cross sections have been determined for electron transfer, direct excitation, and ionization in collisions between protons and $\text{H}(1s)$ atoms at proton energies from 1 keV to 16 MeV, extending the pioneering work by Shakeshaft [R. Shakeshaft, Phys. Rev. A 14, 1626 (1976); Phys. Rev. A 18, 1930 (1978)] with a much smaller basis and energy range, and following the author's work for other collisional systems [T. G. Winter, Phys. Rev. A 76, 062702 (2007)]. Transfer as well as excitation cross sections into individual states up to $3d$ (excitation up to $4f$ at the higher energies) have been determined with several coupled-Sturmian-pseudostate bases, and tests of basis sensitivity have been carried out. These and ionization cross sections have been compared with experimental, other coupled-state, and numerical results. Ionization and excitation cross sections have also been compared with corresponding Born results at higher energies.

Cross sections have been determined for electron transfer, direct excitation, and ionization in collisions between protons and $\text{H}(1s)$ atoms at proton energies from 1 keV to 16 MeV, extending the pioneering work by Shakeshaft [R. Shakeshaft, Phys. Rev. A 14, 1626 (1976); Phys. Rev. A 18, 1930 (1978)] with a much smaller basis and energy range, and following the author's work for other collisional systems [T. G. Winter, Phys. Rev. A 76, 062702 (2007)]. Transfer as well as excitation cross sections into individual states up to $3d$ (excitation up to $4f$ at the higher energies) have been determined with several coupled-Sturmian-pseudostate bases, and tests of basis sensitivity have been carried out. These and ionization cross sections have been compared with experimental, other coupled-state, and numerical results. Ionization and excitation cross sections have also been compared with corresponding Born results at higher energies.

Fundamental processes of electron transfer, ionization, and excitation in ion-atom and ion-ion collisions are studied. Attention is focussed on one- and two-electron systems and, more recently, quasi-one-electron systems whose electron-target-ion core can be accurately modeled by one-electron potentials. The basic computational approaches can then be taken with few, if any, approximations, and the underlying collisional mechanisms can be more clearly revealed. At intermediate collision energies (e.g., proton energies for p-He[sup +] collisions on the order of 100 kilo-electron volts), many electronic states are strongly coupled during the collision, a coupled-state approach, such as a coupled-Sturmian-pseudostate approach, is appropriate. At higher collision energies (million electron-volt energies) the coupling is weaker with, however, many more states being coupled together, so that high-order perturbation theory is essential.

The cross sections for single and multiple ionization of uranium by electron impact for energies 7.5 to 500 eV have been measured using a modulated crossed-beam experiment. The absolute total ionization cross section was found by comparing the cross section for ionization to the cross section for associative ionization in the reaction U + O/sub 2/ ..-->.. UO/sub 2//sup ts+/+e. The total ion number cross section has a maximum value of (5.8 +- 0.9) x 10/sup -16/ cm/sup 2/ occurring at an energy of about 50 eV. The single ionization cross section has a maximum of (4.80 +- 1.24) x 10/sup -16/ cm/sup 2/ at an energy of 27 +- 5 eV. The cross sections for higher-order ionization have values at 500 eV that are 49%, 33%, and 17% that of single ionization for n = 2, 3, and 4, respectively. These ratios are significantly higher than those for other elements for which data are available.

On qualitative features of generalized oscillator strengths and ionization cross sections in ion-atom collisions

We show that in relativistic heavy-ion collisions the independent-electron model can be used to predict cross sections for multiple inner-shell ionization in a single collision. Charge distributions of 430- and 955-MeV/amu ${\mathrm{U}}^{90+}$, ${\mathrm{U}}^{89+}$, ${\mathrm{U}}^{83+}$, and ${\mathrm{U}}^{68+}$ beams emerging from thin solid targets were used to obtain single- and multiple-electron stripping cross sections. The probabilities of stripping electrons from the K, L, or M shells were calculated using the semiclassical approximation and Dirac hydrogenic wave functions. The data generally agree with theory. An influence of the Auger effect is seen in ${\mathrm{U}}^{68+}$ collisions.

Classical trajectory Monte Carlo calculations have been used to elucidate the collision mechanisms responsible for the differences in the single- and double-ionization cross sections of He atoms by protons and antiprotons in the energy range from 250 to 1000 keV. The calculations employed the Bohr helium-atom model which includes the $\frac{1}{{r}_{12}}$ electron-electron interaction. The unexpected large observed difference [Andersen et al., Phys. Rev. Lett. 57, 2149 (1986)] in the ratio $R$ of double to single ionization for proton and antiproton collisions is reproduced by the calculations. The calculations indicate that the antiproton double-ionization cross section is larger than that for protons because of two effects. The first effect is that the antiproton can push one electron into the other from larger impact parameters than for protons which must be between the nucleus and the first electron in order to pull it into a trajectory which collides with the second electron. The most important effect, however, is that at small impact parameters the antiproton screens the helium nucleus causing a Coulomb explosion while the proton increases the binding of the two electrons. The calculations also show a component of the difference in the ratio $R$ at low energies ($E&lt;~500$ keV) is due to the single-ionization cross sections with protons predicted to have a larger cross section than antiprotons (36% difference at 250 keV).