Charge transfer and dissociation in collisions of metal clusters with atoms

We present a combined theoretical and experimental study of charge transfer and dissociation in collisions of slow Li31(2+) clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is found only when the exact experimental time of flight and initial cluster temperature are taken into account in the theoretical modeling. We demonstrate the validity of the simple physical image that consists in explaining evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer.     

Determination of fireline intensity by oxygen consumption calorimetry

Fireline intensity is one of the most relevant quantities used in forest fire science. It helps to evaluate the effects of fuel treatment on fire behavior, to establish limits for prescribed burning. It is also used as a quantitative basis to support fire suppression activities. However, its measurement is particularly tricky for different reasons: difficulty in measuring the weight of the fuel consumed in the active fire front, difficulty to evaluate the rate of spread of the fire front, and uncertainty on combustion efficiency. In this study, an innovative and original approach to directly measure the fireline intensity at laboratory scale is proposed. Based on the oxygen consumption calorimetry principle, this methodology is applied here in case of spreading fires, for the first time. It allows for directly measuring the heat released by the fire front. The results are then used to test the famous Byram’s formulation that is generally applied to determine the fireline intensity. Combustion efficiency and effective heat of combustion results are provided. The uncertainty and the use of a full scale calorimeter instead of a bench scale calorimeter for this study are discussed.     

Absolute charge transfer and fragmentation cross sections in He2+-C60 collisions

We have determined absolute charge transfer and fragmentation cross sections in He2++C60 collisions in the impact-energy range 0.1-250 keV by using a combined experimental and theoretical approach. We have found that the cross sections for the formation of He+ and He0 are comparable in magnitude, which cannot be explained by the sole contribution of pure single and double electron capture but also by contribution of transfer-ionization processes that are important even at low impact energies. The results show that multifragmentation is important only at impact energies larger than 40 keV; at lower energies, sequential C2 evaporation is the dominant process.     

Experimental study of electron impact excitation of multiply charged ions

We report on measurements of angular differential cross sections for the excitation of multiply charged ions by electron impact. An ion beam is crossed by an electron beam; electrons which are inelastically scattered at different angles are identified by their energy loss due to the excitation process. Absolute excitation cross sections are obtained by comparing the signals of the elastic and the inelastic electron-ion scattering. Results obtained for the 3s→3p excitation of Ar⁷⁺ are discussed.     

Molecular view of charge exchange in ion–C60 collisions

We present a theoretical study of charge transfer in H⁺+C₆₀ and He²⁺+C₆₀ collisions using an extension of the molecular time‐dependent method of ion–atom collisions. Energy‐correlation diagrams have been evaluated for the corresponding (C₆₀–H)⁺ and (C₆₀–He)²⁺ quasi‐molecules. Single and double charge‐transfer cross sections in C₆₀+He²⁺ collisions are reported for the first time. The results show that double charge‐transfer cross sections are only one order of magnitude smaller than single charge‐transfer cross sections. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Statistical fragmentation of hot atomic metal clusters

Fragmentation processes of highly excited neutral and charged atomic metal clusters are studied in the framework of an equilibrium statistical model. In the particular case of hot (near and above melting) neutral and charged sodium clusters of 100 and 200 atoms, a microcanonical Metropolis sampling is used to compute mass (or charge) correlation functions as a function of the excitation energy. This method allows to take the strong anharmonicities in the internal phonon spectrum realistically into account which are linked to the internal structural changes like melting. It is found that, at high enough excitation energy, the system exhibits a phase transition. This phase transition is specific for fragmenting finite systems. From the shape of the caloric curve one sees that the two phases involved are connected by a van der Waals loop characterizing a first order phase transition. Here we observe an enhanced fission and multifragmentation into two or more charged clusters with more than 10 atoms each. Various fragment correlations are studied.     

Antihydrogen formation via antiproton scattering on positronium between the $e^{-} +\bar {H}(n = 2)$ and e−+H̄(n=3)$e^{-}+\bar {H}(n = 3)$ thresholds

The charge exchange reaction \(\bar {\mathrm {p}} + \text {Ps} \rightarrow \mathrm {e}^{-} + \bar {\mathrm {H}} \), of interest for the future experiments (GBAR, AEGIS, ATRAP, ...) aiming to produce antihydrogen atoms, is investigated in the energy range between the \(\mathrm {e}^{-}+\bar {\mathrm {H}}(n = 2)\) and \(\mathrm {e}^{-}+\bar {\mathrm {H}}(n = 3)\) thresholds. An ab-initio method based on the solution of the Faddeev-Merkuriev equations is used. Special focus is put on the impact of the Feshbach resonances and the Gailitis-Damburg oscillations, appearing in the vicinity of the \(\bar {\mathrm {p}} +\text {Ps}(n = 2)\) threshold, on the \(\bar {\mathrm {H}}\) production cross section.     

Laser induced ultrafast demagnetization in diluted magnetic semiconductor nanostructures

We present a dynamical model that reproduces the observed time evolution of the magnetization in diluted magnetic semiconductor films after weak laser excitation. Based on a many-particle expansion of the exact p–d exchange interaction, our approach goes beyond the usual mean-field approximation. Numerical results demonstrate that the hole spin relaxation plays a crucial role for explaining the ultrafast demagnetization processes observed experimentally. The influence of the laser power on the magnetization dynamics is also investigated.     

Fission of doubly charged iron and aluminium clusters

We present a statistical fragmentation study of doubly charged iron and aluminum clusters of less than 60 atoms. At low excitation energies we find that the evaporation of one charged monomer is the most probable decay channel (asymmetric fission). When the excitation energy increases there is a competition between evaporation of a charged monomer and a charged dimer. For higher energies the number of channels increases and the multifragmentation mode appears at about 2.5 eV/atom. Presented by M.E. Madjet at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.