R Morgenstern

Interaction of slow multicharged ions with solid surfaces

Abstract The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on “Interaction of Slow Highly Charged Ions with Solid Surfaces”, which has been carried out during the last three years. After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of “hollow atoms” formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understanding of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface.

Angular dependent post-collision interaction in auger processes

We have reformulated the theory of post-collision interaction (PCI) for Auger-decay following inner-shell photoionisation in order to take the time into account with the Auger-electron need to overtake the slow electron. The energy-shift of the Auger-electron due to PCI is calculated by solving in a reasonable approximation the classical equation of motion for the Auger electron. In contrast to the theory of Russek and Mehlhorn we derive analytical expressions for the transition amplitude, the line shape and the line shift of the Auger-electrons. If in our model the Auger electron and the slow electron are treated uncorrelated in direction our analytical expressions agree well with the numerical results of Russek and Mehlhorn. However if we account for directional electron-electron correlations, we show that deviations from the theory of Russek and Mehlhorn are to be expected. The possibility of detecting these deviations is discussed.

Cq+-induced excitation and fragmentation of uracil : effects of the projectile electronic structure

Ionization and fragmentation of the RNA base uracil (C4H4N2O2) by means of Cq+ ions (q = 1-6) has been studied for ion kinetic energies ranging from ;2 to 120 keV. Whereas for Cq+ (q = 1, 3, 4, 5, 6) very similar fragmentation yields are observed which increase with the projectile velocity v, C2+ ions not only induce a fundamentally different fragmentation pattern but also lead to a decrease of fragmentation with ion velocity v. At low velocities, even almost complete fragmentation is observed. Our findings can be explained by differences in the electronic structures of the Cq+ ions as well as by employing the electronic stopping model previously applied to ion-fullerene interactions.

Electronic versus vibrational excitation in Heq+ collisions with fullerenes

Abstract Collisions of Heq+ ions with neutral fullerenes have been studied as a function of projectile velocity (v ≈ 0.1–1.0 a.u.) and charge state (q = 1, 2). With increasing velocity, two trends are observed for both charge states: The yield of C60−2mr+ clusters decreases with 1/v, as expected for quantities related to direct vibrational excitation. The relative cross section for multifragmentation increases linearly with v and can be associated with electronic excitations. The additional potential energy of He2+ with respect to He+ manifests in increased direct ionization cross sections (high v) and multifragmentation cross sections (low v), respectively, revealing information about coupling times between electronic and vibrational excitation.

Collisions of with neutral : Charge transfer and fragmentation

Fragmentation of fullerenes by collisions with multiply charged ions has been studied experimentally for collision energies of . For high projectile charges the potential energy of the projectiles is mainly responsible for the processes induced: the fragment spectra are dominated by multiply charged parent ions and small fragment ions such as and . With decreasing projectile charge, fragmentation induced by the kinetic projectile energy becomes increasingly important, leading to a decreasing yield of surviving ions and an increasing number of fragments with n in the range 2-12. Widths and shapes of the fragment peaks in the measured time-of-flight spectra allow us to determine the kinetic energy release for various fragmentation processes and to distinguish between evaporation of neutral molecules from vibrationally excited ions on the one hand, and super-asymmetric fission into two charged fragments, on the other.

State-selective electron capture in collisions of He2+ with H

Charge transfer into He+ (nl) states (n)5) in collisions (2-13 keV amu-1) of He2+ on atomic hydrogen has been studied by means of photon emission spectroscopy. State selective electron capture cross sections into the degenerate He+(4l) states have been determined by deconvoluting the intensity distribution of the He II (n=4 to n=3) emission along the total ion-beam path inside the collision chamber. They are compared with theoretical results, especially with the large-basis set extended atomic orbital calculations of Fritsch (1988). Generally good agreement with these extended calculations is found. However, the structures in the theoretical cross sections for capture into the non-dominantly populated n=4 states are not observed experimentally.

State-selective electron-capture cross section measurements for low-energy collisions of He-like ions on H2

We measured state-selective absolute electron capture cross sections for collisions of He-like C4+, N5+ and O6+ ions with molecular hydrogen by means of photon emission spectroscopy at impact energies ranging from 4 keV amu(-1) down to energies as low as 5 eV amu(-1). In this low-energy region the total cross sections are found to be weakly dependent on the projectile energy, whereas the state-selective ones change drastically with varying impact energy. Up to now no experiments were available to benchmark theoretical predictions. In our method we combine for the first time decelerated projectile beams with the possibility of measuring state-selective electron capture. Compared with existing measurements for state-selective cross sections our deduced cross sections are in perfect agreement in the overlapping energy region (E much greater than 100 eV amu(-1)). Good agreement is also found on comparing our results with existing total cross sections. In comparison with theoretical calculations some remarkable and unexpectedly large discrepancies of orders of magnitude are found towards lower impact energies.

Inner- and outer-shell electron dynamics in proton collisions with sodium atoms

p+Na collisions have been investigated theoretically and experimentally at impact energies in the keV regime. We present results for capture and ionization processes, and, in particular, analyse the role of initial inner-shell electrons, whose active participation is identified in the experiments through the analysis of recoil-ion momentum spectra. Quantum-mechanical calculations within the independent particle model have been carried out for all active electrons. A very good overall agreement between the theoretical and experimental results is found. The calculations support the observation that capture from inner shells is an important reaction channel even at relatively low impact energies, and dominates total capture above 40 keV.

Laser cooled targets and recoil ion momentum spectroscopy for fundamental physics studies

In this contribution the plans for the TRIμP facility (Trapped Radioactive Isotopes: microlaboratories for fundamental Physics) at KVI are outlined. The status of the development of a magneto optical trap as a target for ion-atom collisions and the subsequent detection of recoiling target particles is described.

DISSOCIATION OF CO INDUCED BY HE2+ IONS : I. FRAGMENTATION AND KINETIC ENERGY RELEASE SPECTRA

The dissociation of ions produced in collisions of ions with CO has been studied by time-of-flight measurements. Both singles and coincidence time-of-flight techniques have been used to determine the kinetic energy release of the dissociating CO molecules. We describe the method to transform the singles and coincidence time-of-flight spectra into total kinetic energy distributions and discuss these distributions. They represent kinetic energy release distributions which clearly exhibit various contributions associated with different dissociation channels. In comparison with other ionization methods similarities but also clear differences are noted.