J.-Y. Chesnel

Fragmentation of H2^2+ ions following double electron capture in slow Xe23+ and O5+ + H2 collisions

The energy distributions of H+ fragments produced in 345 keV Xe23+ + H2 and 75 keV O5+ + H2 collisions have been investigated experimentally as a function of the detection angle. For both systems, the experiment shows strong deviations from the molecular Coulomb explosion. Two- and three-body model calculations have also been performed to understand the energy spectra. In the collisions involving the heaviest projectile Xe23+ , the two-body calculations reproduce the experimental data satisfactorily, indicating that the recoil energy transferred to the target plays an important role in the fragment-energy distributions. On the other hand, for the system O5+ + H2, the deviations between calculations and experiment suggest that the interaction between the outgoing projectile and each fragment plays the major role.

Continuous electron spectra from 150 keV/u C+ + He, Ne, Ar collisions at electron emission angles from 0° to 180°

Electron spectra in the 20–550 eV energy range and in the full angular range of 0–180° were measured by the impact of 150 keV/u C+ ions on He, Ne and Ar atoms. Double differential cross sections for electron emission have been determined. We observed an unexpected, broad structure around 300 eV electron energy at backward emission angles relative to the beam direction. Our calculations support the hypothesis that the new structure is due to double scattering of the target electrons on the screened fields of the projectile and the target. The calculations also show that both electron-emitting partners are multiply ionized in the collision.

Double-electron capture and radiative stabilization processes in slow ion-atom collisions

Abstract Mechanisms for double-electron capture producing projectile doubly excited states in O6++He and Ne10++He collisions are studied. Emphasis is given to slow collisions with projectile energies of a few keV. At these impact energies the production of configurations nln′l′ of nonequivalent electrons (n′≫n) is dominant. It is shown that the creation of nonequivalent electron states O4+(1s22pn′l′) and Ne8+(3ln′l′) with n′⩾6 originates from dielectronic processes involving electron–electron interaction. After the collision the populated excited states decay either by Auger electron emission or by radiative (photon) emission. Individual contributions to radiative stabilization of the Ne8+(nln′l′) states (n=3–4) are studied in the impact-energy range from 1–150 keV. The relative importance of the different contributions to stabilization changes significantly with varying collision energy. At 1 keV the major contribution is due to the decay of the configurations 3lnl′ (n⩾6) created by collisional dielectronic processes.

Interaction of nucleobase clusters with multiply charged ions: Insight into base pairing

We have studied the collision of low energy multiply charged ions with mixed nucleobases clusters. Mass spectra of the products show selectivity between particular nucleic bases. We could explain this enhanced stability considering base pairing occurring in DNA or RNA.

Experimental evidence for Young's interference effects in autoionization following 30 keV He2++H2 collision

The emission of electrons from autoionizing He ** outgoing projectiles formed in a double capture 30 keV He 2+ +H2 collision has been analysed at detection angles ranging from 90 o up to 162 o . The autoionization cross section differential in the angle is found to oscillate. This result is attributed to a Young interference mechanism produced by the postcollisional interaction of the emitted electron with the two-centre exploding H + + H + residual target.

K-shell ionization cross sections following 0.6-4-keV e{sup -}+H{sub 2}O collisions

Collisions between electrons and a H{sub 2}O vapor target at projectile energies ranging from 600 eV to 4 keV have been investigated experimentally and theoretically, in order to determine cross sections for the production of a K-shell vacancy of the H{sub 2}O molecule. The electrons originating from direct ionization and from autoionization were detected at angles in the range 30 degree sign -130 degree sign with respect to the incident beam direction. From the spectra, cross sections for the emission of a target electron following the production of a K-shell vacancy in oxygen are determined as a function of the projectile energy. The present experimental results are compared with semiempirical formula that have been extensively used previously for electron-atom collisions, and with model calculations using plane wave Born approximation. The ratio between K-shell and total ionization cross sections is found to be similar to that determined for an O atomic target, except at the highest projectile energies, for which it is equivalent to that determined for Ne. We show that this ratio is essentially governed, for sufficiently high collision energies, by the ratio between the outer- and inner-shell ionization potentials.

Energy distribution of protons following electron capture in collisions of N7+ ions with H2 at sub keV impact energies

The energy distributions of H+ fragments produced in N7+ + H2 collisions at projectile energies ranging from 1.4 keV down to 32 eV have been investigated experimentally as a function of the detection angle. At 1.4 keV, two groups of peaks are clearly visible, especially at forward angles. The structure centered at energies lower than ∼ 20 eV, corresponds to a double capture at relatively large impact parameters, whereas the highly energetic protons result from capture events at very small impact parameters. The results from a quasiclassical calculation method show a good agreement with our experimental data for the fragment energy distributions. At the lowest projectile energy, the fragment energy is found to be independent on the detection angle.

Dielectronic processes producing radiative stabilization in 150 keV Ne10+ + He collisions

Abstract Different contributions to radiative stabilization in the collision 150 keV Ne10+ + He are discussed. Stabilization is shown to represent 30% of the total double capture. The main contribution (0.51) to stabilization follows from the population of near-equivalent electron configurations 3 l n l ′ (n = 4, 5) and 4 l n l ′. Further stabilization is due to the decay of configurations 3 l n l ′ of nonequivalent electrons which are populated by means of dielectronic processes during the collision (0.26). The contributions due to configuration interaction in the postcollisional and asymptotic regions are relatively small.

Influence of the environment on the fragmentation of amino acids provoked by low-energy ions

With highly charged ions at low energy, molecules can be ionised on fs timescale at large distances without appreciable energy transfer. Their interaction with small amino acids leads to the fragmentation by cleavage of the weakest bond similarly to the other radiation induced fragmentation. A protective effect of the environment is observed when the molecules are embedded in a cluster of amino acids. The molecular cluster acts as a buffer dissipating the excess energy.

Multiple-Scattering Phase Distortion in the Ionization of Molecules

We theoretically analyze the multiple-scattering effects that might occur in the ionization of a molecule by the impact of photons or massive particles, by calculating the series to all orders within a muffin-tin description. We find a large sensitivity on the final state and a sizable momentum-dependent distortion of the phase-shift and frequency of the interference oscillations, that are not replicated by non-scattering or single-scattering approximations. Furthermore, our results do not validate the existence of any discernible harmonic oscillations.