Relative (e,2e) triply differential cross sections (TDCS) are measured for the ionization of the helium atom and the hydrogen molecule in coplanar asymmetric geometry at a scattered electron energy of 500 eV and ejected electron energies of 205, 74 and 37 eV. The He experimental results are found to be in very good agreement with convergent close-coupling calculations (CCC). The H2 experimental results are compared with two state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. Both models yield an overall good agreement with experiments, except for some intensity deviations in the recoil region. Similar (e,2e) works were recently published on H2 with contrasted conclusions to the hypothesis that the two H nuclei could give rise to an interference pattern in the TDCS structure. Murray (2005 J. Phys. B: At. Mol. Opt. Phys. 38 1999) found no evidence for such an effect, whereas Milne-Brownlie et al (2006 Phys. Rev. Lett. 96 233201) reported its indirect observation. In this work, based on a direct comparison between experimental results for He and H2, we observe an oscillatory pattern due to these interference effects, and for the first time the destructive or constructive character of the interference is observed, depending on the de Broglie wavelength of the ejected electron wave. The experimental finding is in good agreement with the theoretical prediction by Stia et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L257).

We present a combined experimental and theoretical study of fragmentation of small Cn clusters (n = 5,7,9) produced in charge transfer collisions of fast (nu = 2.6 a.u.) singly charged Cn+ clusters with He. Branching ratios for all possible fragmentation channels have been measured. Comparison with microcanonical Metropolis Monte Carlo simulations based on quantum chemistry calculations allows us to determine the energy distribution of the excited clusters just after the collision.

Measurements of the (e,2e) triply differential cross sections (TDCS) are presented for the ionization of the nitrogen molecule in coplanar asymmetric geometry at an incident energy of about 600 eV and a large energy transfer to the target. The experimental results are compared with state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. The experimental TDCS are characterized by a shift towards larger angles of the angular distribution with respect to the momentum transfer direction, and by a large intensity in the recoil region, especially for ionization of the 'inner' 2σ g molecular orbital. Such shifts and intensity enhancement are not predicted by the model calculations which rather yield a TDCS symmetrically distributed around the momentum transfer direction.

Abstract. We present a new experiment on (quasi) symmetric collision systems at low-velocity, namely Ar 17+ ions (v = 0.53 a.u.) on gaseous Ar and N 2 targets, using low-and high-resolution X-ray spectroscopy. Thanks to an accurate efficiency calibration of the spectrometers, we extract absolute X-ray emission cross sections combining low-resolution X-ray spectroscopy and a complete determination of the ion beam -gas jet target overlap. Values with improved uncertainty are found in agreement with previous results [1]. Resolving the whole He-like Ar 16+ Lyman series from n = 2 to 10 with our crystal spectrometer enables to determine precisely the distribution {P n } of the electron capture probability and the preferential n pref level of the selective single-electron capture. Evaluation of cross sections for this process as well as for the contribution of multiple-capture is carried out. Their sensitivity to the ℓ-distribution of n levels populated by single-electron capture is clearly demonstrated, providing a stringent benchmark for theories. In addition, the hardness ratio is extracted and the influence of the decay of the metastable 1s2s 3 S 1 state on this ratio is discussed.

Dissociative and non-dissociative charge transfer cross sections in high velocity (v = 2.6 au) collisions between ionic carbon clusters Cn+ (n = 2–10) and helium atoms have been measured. The sum of the cross sections has been found to increase significantly with n. Measurements of branching ratios for all fragmentation channels of excited Cn clusters are reported. The summed branching ratios associated with a given number of emitted fragments exhibit odd–even alternations reflecting the higher stability of the species having an odd number of atoms. From an analysis of the summed branching ratios within the statistical microcanonical metropolis Monte Carlo model, and knowing the temperature of the incident clusters, deposited energy distributions due to the charge transfer process are deduced (n = 5–9). These distributions, of similar characteristics whatever n, peak around 4–5 eV and exhibit a large percentage of superexcited states situated above the continuum.

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We have measured fragmentation branching ratios of neutral C(n)H and C(n)H(+) cations produced in high velocity (4.5 a.u) collisions between incident C(n)H(+) cations and helium atoms. Electron capture gives rise to excited neutral species C(n)H and electronic excitation to excited cations C(n)H(+). Thanks to a dedicated setup, based on coincident detection of all fragments, the dissociations of the neutral and cationic parents were recorded separately and in a complete way. For the fragmentation of C(n)H, the H-loss channel is found to be dominant, as already observed by other authors. By contrast, the H-loss and C-loss channels equally dominate the two-fragment break up of C(n)H(+) species. For these cations, we provide the first fragmentation data (n>2). Results are also discussed in the context of astrochemistry.

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