J. P. Grandin

Defect creation in alkali-halides under dense electronic excitations: experimental results on NaCl and KBr☆

We present the results of simultaneous in situ luminescence and optical absorption studies of alkali-halides, submitted to very dense electronic excitations induced by swift heavy ion irradiations at low temperatures (15–200 K). X-ray irradiations performed in the same experimental conditions permit the necessary comparison with ion irradiations. The luminescence of the self-trapped excitons is only slightly modified by increasing the electronic stopping power, while new features appear for the point defect creation. For instance, in NaCl at 15 K, F centres are efficiently created by Ne, Zn and Xe irradiations and F2 centres only by Zn and Xe irradiations. We discuss the evolution of the yields of defect creation as a function of the temperature and of the electronic stopping power.

FAST ION-INDUCED CO MOLECULE FRAGMENTATION IN THE STRONG INTERACTION REGIME

A coincident time of flight technique is used to investigate fast ion-induced CO molecule fragmentation with swift and multicharged heavy ions. The results concern the strong interaction regime for which no data have yet been reported. Kinetic energy release distributions, branching ratios and multi-electron removal cross sections are determined for each dissociation channel of ions (Q = 2-9). The general trend, from the perturbative regime to the strong interaction regime, is analysed in the light of recent ion-atom advances. The use of recoil ion momentum spectroscopy to study one dissociative single ionization channel - with simultaneous excitation - greatly improves the experimental KER resolution. Furthermore, it allows a coincident measurement of the angular and kinetic energy release distributions. This technique is proved promising for molecular spectroscopy studies.

Observation of dynamical substate mixing of fast ions in solids

Using high-resolution x-ray spectroscopy, we have measured, as a function of target thickness, the relative intensities of the fine-structure components of the Balmer line emitted by fast hydrogen-like krypton ions propagating through thin carbon and copper targets. Our results are in clear disagreement with the predictions of a rate-equation model accounting for collisional l mixing. On the other hand, good agreement is found with a model taking solely into account a wake field-induced Stark mixing of degenerate n,l,j substates. Within this model, the values obtained for the electric field agree well with those deduced from measured total stopping power, which indicates that the effect of core electrons must be considered. Furthermore, off-diagonal density matrix elements of the initial capture process to the n = 3 states are inferred from the experimental intensities. A comparison, for carbon targets, with available (gas-phase) calculations of these matrix elements, reveals important differences.

Heavy ion track potentials in solids probed by electron yield measurements

Abstract We studied the electronic energy deposition by swift heavy ions in matter by means of electron yield measurements as a function of the projectile energy and atomic number. The dependence of electron yields from carbon induced by ions of about 10 MeV/u on the projectile atomic number is incompatible with the predictions of a recent theoretical model dealing with the induced track charge in the ions wake and the influence of the induced track potential on electron emission.

Fluence dependent electron emission as a measure of surface modification induced by swift heavy ions

Abstract From the fluence dependence of low energy electron yields induced by swift heavy ions, one can deduce a cross section for surface transformation by electronic processes. For a clean metal surface exposed to a contaminant gas in ultrahigh vacuum, this cross section is related to the electronic desorption. In general, this cross section is also related to structural modification of the surface. The dependence of electron yields, surface transformation cross section and secondary ion yields for carbon foils show a different dependence on the electronic stopping power which is a hint for different ejection mechanisms involved.

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.

Dynamics of Xe44+(6.7MeV/A) +Ar and He collisions

Abstract The momentum vector of recoiling Ar and He target atoms ionized by Xe 44+ (6.7 MeV/ A ) impact has been determined using high resolution recoil ion momentum spectroscopy. From the angle of emission and recoil velocity distributions of each recoil ion charge state q ( q = 1to7 for argon ions, q = 1and2 for helium ions), one deduces that the recoil ions are mostly emitted backward with respect to the projectile beam direction. We show that this behavior is due to the electrons which are mainly emitted in the forward direction and thus prove, in the case of single and multiple ionization, the importance of the role played by the electrons in the momentum balance of such a collision.

Self-trapped exciton luminescence under dense electronic excitations: Ion-induced transient thermal effects

We have measured the low temperature luminescence of the self-trapped excitons, STE, in NaCl submitted to very dense electronic excitations induced by swift heavy ions. We have compared the features of the excitonic luminescence excited by X-ray and by high energy ion irradiations. The luminescence yield results from a competition between radiative (temperature independent) and non-radiative (temperature dependent) recombinations. For swift heavy ion irradiations, the high density of the deposited energy is supposed to induce a very significant transient temperature increase in the vicinity of the ion path. In this paper we have studied the influence of a thermal spike on the STE luminescence yield. Using numerical computations based on equilibrium thermodynamics, we predict the local increase of the temperature as a function of time t, and of the distance r, from the ion path. At every (r, t) we calculate the temperature induced decrease of the STE non-radiative recombination lifetimes, and we deduce the corresponding changes in the STE luminescence yields. It appears that the numerical predictions of the luminescence yield are very sensitive to the radial extent of the energy deposition, and consequently of the STE concentration. The role of the different parameters and the hypotheses of the calculations are discussed.

Pre-equilibrium charge state distributions of Ni26+ (74 MeV/u) in carbon foils

Abstract We measured pre-equilibrium charge state distributions of Ni26+ ions at 74 MeV/u traversing carbon foils as a function of target thickness (28–49000 μg/cm2). We also present the evolution of the mean final charge for this collision system and for Cu25+ at 9.6 MeV/u. The experimental data agree well with numerical simulations performed with the new computer code ETACHA written by Rozet et al. [Nucl. Instr and Meth. B 107 (1996) 67].

Single and double ionisation of He by HCI — a recoil ion momentum study☆

Abstract Single and double ionisation of He by highly charged ions have given rise to a huge amount of experimental as well as theoretical studies. The advent of the new generation of recoil ion spectrometers, which begin to provide experimental results in the field of ion-atom collisions, allow us to obtain a new insight into the ionisation mechanisms. We report for the first time angular and energy differential cross sections for the production of He recoil ions. These results were obtained in the intermediate energy range using a 6.7 MeV/u Xe44+ projectile beam and a high resolution (better than 0.1 a.u.) recoil ion momentum spectrometer. They show strong differences in the momentum distributions of the recoil ions corresponding to the single and double ionisation processes.