T. G. Lee

Charge transfer and excitation in slow 20 eV-2 keV H + +D (1s) collisions

We used the recently developed hyperspherical close-coupling method to study H++D(1s) collisions at H+ impact energies between 20 eV and 2 keV. We showed that the cross sections for excitation and charge transfer to the 2p states are essentially identical over the whole energy range and stay relatively independent of energy from 2 keV down to about 150 eV. Below 150 eV the cross sections drop precipitously as the energy decreases. Electron capture to H(1s) cross section in this energy region is also calculated.

Strong double K-K transfer channel in near symmetric collision of Si+Ar at intermediate velocity range

We present a combined study of single and double K-K electron transfer cross sections along with the single and double K-shell ionization of Ar induced by Si projectiles in the energy range 0.9-4.0 MeV u-1 . The charge-state dependence of the normal and hypersatellite x-rays was used to derive the cross sections for the one- and two-electron processes, respectively. The enhancement in the fluorescence yields due to multiple vacancies was measured from the energy shifts and intensity ratios of the characteristic x-ray lines to derive K-shell vacancy production cross sections from x-ray production cross sections. The ratio of double to single K-K transfer cross sections is found to be quite large for this nearly symmetric collision system, whereas the ratio of double to single ionization cross sections is quite small. The measured single K-K transfer cross sections are reproduced very well by the two-centre close-coupling calculations whereas the double K-K transfer data are underestimated by the theory based on the independent-electron approximation (IEA). The K-shell ionization cross sections are found to deviate strongly from the calculations based on the continuum distorted wave eikonal initial state (CDW-EIS) and ECPSSR models. The CDW-EIS calculations along with the IEA model grossly underestimate the double ionization cross sections. It is stressed that in the case of two-electron processes the independent-electron model breaks down and the possible role of correlations between K-electrons is discussed.

Hyperspherical close-coupling calculations for charge transfer cross sections in Si4+ + H(D) and Be4+ + H collisions at low energies

Total and partial electron capture cross sections are calculated for Si4+ + H(1s)/D(1s) collisions from 1 meV amu−1 to 1 keV amu−1, and for Be4+ + H(1s) collisions from 2.5 eV amu−1 up to 1 keV amu−1, using the recently developed hyperspherical close-coupling (HSCC) approach. For both systems, our results are in good agreement with calculations based on the molecular orbital expansion method in reaction coordinates (RC). For Si4+ + D(1s) our total charge transfer cross sections agree well with experiment, but not the partial cross sections. For Be4+ + H, we show that partial wave cross sections from the RC method have more oscillatory structures than those from the HSCC method, even though the total cross sections agree. In comparison with the RC method, the HSCC approach does not require the introduction of any ad hoc external parameters in the formulation.

Experimental observation and theoretical calculations of triply excited 2 s 2 p 2 2 S e , 2 , 4 P e , 2 D e and 2 p 3 2 P o , 2 D o states of fluorine

An alternative experimental technique for efficient population of triply excited states in atoms and ions is presented. The states are produced by triple electron capture in energetic ion-atom collisions. The method is used to study autoionizing 2s2p 2 2 S, 2 P, 2 D, 4 P and 2p 3 2 P, 2 D states of fluorine. Differential cross sections, resonance energies, and Auger decay branching ratios were determined from the experiment. Results are compared to different calculations based on the hyperspherical close coupling method.

Experimental observation and theoretical calculations of triply excited 2s2p2 2s e, 2,4pe, 2Deand 2p3 2p0, 2D0 states of fluorine

An alternative experimental technique for efficient population of triply excited states in atoms and ions is presented. The states are produced by triple electron capture in energetic ion-atom collisions. The method is used to study autoionizing 2s2p 2 2 S, 2 P, 2 D, 4 P and 2p 3 2 P, 2 D states of fluorine. Differential cross sections, resonance energies, and Auger decay branching ratios were determined from the experiment. Results are compared to different calculations based on the hyperspherical close coupling method.

Absolute measurements and calculation of triple electron capture cross sections in fast 0.5–1.1 MeV/u C6+ on Ar collisions

Doubly excited KLL states were populated by triple electron capture in collisions of fast C6+ ions with Ar atoms. Measurements of the Auger electron emission at zero degrees with respect to the beam direction were used to determine the absolute single differential cross sections (SDCS) for the triple electron capture to all autoionizing KLL states. In addition, independent particle model was utilized for the calculation of cross sections for individual LJ states of populated intrashells. The agreement of theory and experiment promoted conclusions about the role of projectile screening and electron–electron correlation effects in multiple electron capture in fast, highly charged ion–atom collisions.

Resonant (RTE) and Non Resonant (NTE) Transfer Excitation in 4 MeV B4+ collisions with H2, He and Ar studied by zero‐degree Auger projectile electron spectroscopy

It is well known that for very asymmetric collisions between a highly charged ion and H2 near the RTE resonance, NTE is practically negligible. However, even though the interplay between RTE and NTE and their possible interference has been addressed formally and more recently by concrete calculations, experimentally very little work has been done on fast collision systems of increasing symmetry. We report on recent Auger electron RTE/NTE double differential cross section measurements of H‐like B4+ ions in 4 MeV collisions with H2, He and Ar gas targets. Calculations using an atomic orbital close coupling (AOCC) method indicate NTE to be negligible for B4+ collisions with H2, but possibly substantial (depending on screening considerations) for He and Ar targets. R‐matrix calculations are also presented for elastic (resonant and non‐resonant) electron scattering from B4+. Within the electron scattering model (ESM), the active target electron can be considered to scatter as a free particle with a momentum di...

Doubly‐Excited KLL States Formed in Triple Electron Capture

Doubly excited KLL states of C 3+ were populated by triple electron capture in collisions of fast bare C 6+ ions with Ar atoms. Measurements of the Auger electron emission at zero degree with respect to the beam direction were used to determine the absolute single differential cross sections for the triple electron capture to all autoionizing KLL states. In addition, independent particle model was utilized to predict the differential cross section for the population of all doubly excited states of carbon. Theoretical cross sections were critically compared with experimental results.