A. Cassimi

Diode pumping of LNA lasers for helium optical pumping

Abstract Neodymium-doped LNA laser crystals (LaxNd1-xMgAl11O19) have been pumped by laser diode arrays emitting around 800 nm. With longitudinal pumping using two 200 mW diode arrays focussed on the crystal end, 10 mW of power was obtained at 1.054 μm. With the insertion of a Lyot filter and a thin etalon, tuning across the 1.082 μm band was possible. Tuning the LNA laser output to the resonance transition of helium-4 (23S1-23P1) at 1.083 μm enabled us to optically pump the metastable helium atoms in a discharge cell and record magnetic resonance signals. Transverse pumping of the LNA crystals by a high-power, quasi-cw array was also successfully demonstrated.

K-shell photoionization of CO and N2: is there a link between the photoelectron angular distribution and the molecular decay dynamics?

We have used COLTRIMS to measure the angular distribution of electrons released from the K-shell of N2 and the carbon K-shell of CO by absorption of one linear polarized photon. For each ionization event which leads to two charged fragments we determine the angle of the photoelectron with respect to the fragment ion momenta. In addition we determine the charge state and energy of the molecular fragments. We find a breakdown of the axial recoil approximation for CO for kinetic energy releases below 10.2 eV, whereas for N2 that approximation is found to be valid for all fragment energies. Furthermore, the photoelectron emission spectrum for N2 is found to be the same for the molecular breakup channels producing N + N + and N + N ++ . (Some figures in this article are in colour only in the electronic version)

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.

Multiple ionization in the earlier stages of water radiolysis

We have studied the fragmentation of water vapour molecules induced by collision with a Xe44+ beam at 6.7 MeV/u. From the measurement of the fragment time of flight, we show that the amount of fragmentation due to multiple ionization is very large. In the case of single ionization, we are able to reproduce accurately the experimental cross sections by calculating for each molecular level the single-ionization cross section in the framework of the CDW-EIS theory and with a diagram of dissociation modified with respect to the diagram obtained in the case of dipolar ionization. By using qualitative arguments based on the ability of the medium to neutralize a charged species, we tentatively extend our result to liquid water. From our analysis, we show that ionizations involving three or more ejected electrons could enhance the oxygen production. For the physicochemical phase we estimate that the rate of oxygen production by multiple ionization represents approximately 18% of the OH rate produced by single ionization.

Ion-induced molecular fragmentation: beyond the Coulomb explosion picture

The fragmentation of the CO molecule by O7+ ion impact is investigated in two different energy regimes by fragment ion momentum spectroscopy. The improved resolution of the present kinetic energy release measurement together with application of a time-dependent wavepacket dynamics method used in conjunction with new high-level computations of a large number of dication potential energy curves enables one to unambiguously assign each line to an excited state of the transient molecular dication produced during the collision. This is the first direct experimental evidence of the limitations of the Coulomb explosion model to reproduce the molecular fragmentation dynamics induced by ion impact. Electron removal due to a capture process is shown to transfer less excitation to the target than direct ionization. At low collision velocity, the three-body interaction between the projectile and the two fragments is also clearly highlighted.

Triply differential single ionization cross sections in coplanar and non-coplanar geometry for fast heavy ion-atom collisions

We have performed a kinematically complete experiment and calculations on single ionization in 100 MeV/amu C6+ + He collisions. For electrons ejected into the scattering plane (defined by the initial and final projectile momentum vectors) our first- and higher-order calculations are in good agreement with the data. In the plane perpendicular to the scattering plane and containing the initial projectile axis a strong forward-backward asymmetry is observed. In this plane both the first-order and the higher-order calculations do not provide good agreement neither with the data nor amongst each other.

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.

Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2

Modification of surface and bulk properties of solids by irradiation with ion beams is a widely used technique with many applications in material science. In this study, we show that nano-hillocks on CaF2 crystal surfaces can be formed by individual impact of medium energy (3 and 5 MeV) highly charged ions (Xe22+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy xenon ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy (Ep) while for swift heavy ions a minimum electronic energy loss per unit length (Se) is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via Se the Ep-threshold for hillock production can be lowered substantially. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, which can be visualized in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to the case where both kinetic and potential energies are deposited into the surface.

Single- and double-electron capture in low-energy Ne10+-He collisions

We present a combined experimental/theoretical study of Ne10+-He collisions in the 50?E?150?keV impact energy range using improved-resolution recoil ion momentum spectroscopy. Total and differential cross sections for single-electron capture onto the levels n = 3,4,5 and 6 of Ne9+(nl) and double-electron capture onto series of doubly excited states 3, n and 4, n with n = 4, 5 and 6 of Ne8+ have been obtained. They are found to be in good agreement with close-coupling calculations. Population mechanisms for the dominantly populated double-capture channels have been deduced from this work. Special emphasis is brought to the analysis of the partial stabilization ratios to obtain a new insight into the velocity dependence of total stabilization ratios.