M.F. Politis

X-ray reflectivities, at low and high order of reflection, of flat highly oriented pyrolytic graphite crystals

Abstract Integrated reflectivities of flat highly oriented pyrolytic graphite crystals have been measured at various energies (5.9 ≤E ≤ 15.10 keV) and for various orders of reflection n (1 ≤n ≤ 5). For low n values, the reflectivity is severely affected by secondary extinction; our results show that the theory of Zachariasen represents the experiment reasonably well, a conclusion reached by other authors for the case of n = 1 reflections. For high n values, the reflectivity is more sensitive to the structure factors temperature correction, a quantity not precisely known. Our measurements are in favour of the larger temperature correction within the range reported in the literature.

New investigation of saturation effect in ion-atom excitation

Excitation cross sections of Kr34+ ions colliding with various target atoms, (Zt between 6 and 40) at nu =35 au have been investigated both experimentally and theoretically. The experimental cross sections were obtained from those for production of projectile He-like and H-like Lyman X-rays. The combined experimental and theoretical analysis of H-like emission allowed for a determination of the competitive capture-ionization process, whose contribution to the Lyman production cross sections is important for near-symmetric collision systems and has to be subtracted. A saturation effect of the experimental excitation cross sections with Zt is analysed from the theoretical point of view through atomic and molecular coupled-state calculations.

Characteristics of single capture nl distributions and double capture probabilities in slow collisions of Al13+, Al12+ and Ne9+ ions with two-electron targets (He, H2)

Relative intensities of Lyman X-rays consecutive to one- or two-electron capture have been measured for collisions of 4 keV u-1 Al13+, Al12+, Ne9+ ions with He and H2 targets. A method is presented to extract separate information on single and double transfer processes. Preferential n states, proportion of p substrates and mean l values populated by single capture are measured, together with double capture probabilities. Comparison of experimental results with theoretical predictions for one-electron systems with near-crossing radii suggest that the location of these crossings may be more important than target internal structure to determine final l distributions. Finally, for the investigated systems with different ionic charges but near-crossing radii, double capture probabilities are found to be quite similar.

Doubly excited states populated in collisions of O8+ ions with He and H2 at 1.24 keV amu-1

Coincidence measurements between O6+(3, 3), (3, 4) (or (4, 4)) ions and their emitted X-rays have been performed in double capture collisions between fully stripped O8+ ions and He (or H2) targets at 1.24 keV amu-1. Average ratios of autoionization widths towards different continuum states have been extracted for this measurement and compared with predictions of new Feshbach calculations. A complete set of theoretical partial autoionization widths for the singlet (3, 3), (3, 4) and (4, 4) states of O6+ is also given. In most cases the results exclude a dominant population of high-L states by double capture in contrast with previously reported measurements at higher velocities.

Excitation of Kr34+ ions at intermediate velocity

Abstract Total and partial excitation cross sections for 34 MeV/amu Kr34+ ions colliding with various targets (6 ≤ Zt ≤ 40) have been determined from projectile X-ray measurements. The results were compared to the predictions of a method based on Schwingers variational principle.

Role of angular correlations and total angular momentum on partial autoionisation widths of O6+ (3,3) and (4,4) states

Partial widths for autoionisation to 2l and 3l sublevels have been calculated for O6+ (3,3) and (4,4) states by a configuration-interaction method. The Gamma (2s)/ Gamma (2p) branching ratios of states under the N=3 threshold are shown to exhibit a characteristic dependence upon the quantum numbers K and L used in the doubly excited-state basis classification. An interpretation is provided by an analysis of predominant configurations in these states.

Investigation of angular momenta and correlation patterns in doubly excited states populated by low velocity double charge exchange

Abstract Autoionization branching ratios of O 6+ doubly excited states are shown to depend sensitively on the K , T and L quantum numbers of these states. A measurement of the decay probabilities to 2s and 2p sublevels has been performed for the (3, 3) and (3, 4) doubly excited states populated in the 1.3 keV/u O 8+ → He collision. A new technique of X-ray, energy gain coincidence has been used. Results are discussed in terms of possible distributions K , L for the doubly excited states.

Single and double charge exchange processes with low energy multicharged ions

Abstract An original method is presented for extracting information on single and double capture processes in low velocity collisions with bare and one-electron projectiles. Experimental data are reported for P(l) distributions in single capture and for the percentage of double capture with respect to the single capture in 10q keV Ne9+ → H2 and Al13 → He systems (v ∼ 0.4 a.u.). The preferential n state populated by double capture (after autoionization) is also deduced.

A method for the determination of angular momenta in single and double capture by low velocity multiply charged ions using energy gain-X-ray coincidence technique☆

Abstract A new technique is presented providing information on the angular momentum distribution of electrons captured by multiply charged ions and particularly suitable for investigating weak channels: capture in non-dominant n states, double capture. This method is based on coincidence measurements between scattered ions having suffered a given energy gain and X-rays resulting from the decay of excited states formed during the collision. Preliminary results are given for the O 8+ + H 2 collision at 3 q keV.