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- ItemLaser-driven ion acceleration using isolated mass-limited spheres(College Park, MD : Institute of Physics Publishing, 2010) Sokollik, T.; Paasch-Colberg, T.; Gorling, K.; Eichmann, U.; Schnürer, M.; Steinke, S.; Nickles, P.V.; Andreev, A.; Sandner, W.We report on our experiments on laser-driven ion acceleration using fully isolated mass-limited spheres with a diameter down to 8μm for the first time. Two-dimensional (2D) particle-in-cell (PIC) and hydro-code simulations were used to show that the pre-plasma at both the front and rear sides of the target strongly affect the efficiency of the ion acceleration. The mechanism of the plasma flow around mass-limited targets has not yet been identified for laser-driven ion acceleration. Our models indicate that this effect is the cause of the observed limitation to the ion-beam energy in both previous experiments and in our own. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
- ItemTime-resolved investigation of transient charges in laser-produced nanoplasmas(Bristol : IOP Publ., 2015) Schütte, Bernd; Vrakking, Marc J. J.; Rouzée, ArnaudWe report on the observation of a transient C4+ ion charge state in nanoplasmas produced by the interaction of intense near-infrared (NIR) laser pulses with CH4 clusters. The underlying dynamics are studied by pump-probe spectroscopy, which reveals that the ion charge states are lowered by electron-ion recombination. Furthermore, we present direct evidence that autoionization of multiply-excited ions plays an important role in expanding nanoplasmas, in contrast to models that neglect quantum phenomena.
- ItemImaging Proton Transfer and Dihalide Formation Pathways in Reactions of F(-) + CH3I(Washington, DC : American Chemical Society, 2016) Carrascosa, Eduardo; Michaelsen, Tim; Stei, Martin; Bastian, Björn; Meyer, Jennifer; Mikosch, Jochen; Wester, RolandIon–molecule reactions of the type X– + CH3Y are commonly assumed to produce Y– through bimolecular nucleophilic substitution (SN2). Beyond this reaction, additional reaction products have been observed throughout the last decades and have been ascribed to different entrance channel geometries differing from the commonly assumed collinear approach. We have performed a crossed beam velocity map imaging experiment on the F– + CH3I reaction at different relative collision energies between 0.4 and 2.9 eV. We find three additional channels competing with nucleophilic substitution at high energies. Experimental branching ratios and angle- and energy differential cross sections are presented for each product channel. The proton transfer product CH2I– is the main reaction channel, which competes with nucleophilic substitution up to 2.9 eV relative collision energy. At this level, the second additional channel, the formation of IF– via halogen abstraction, becomes more efficient. In addition, we present the first evidence for an [FHI]− product ion. This [FHI]− product ion is present only for a narrow range of collision energies, indicating possible dissociation at high energies. All three products show a similar trend with respect to their velocity- and scattering angle distributions, with isotropic scattering and forward scattering of the product ions occurring at low and high energies, respectively. Reactions leading to all three reaction channels present a considerable amount of energy partitioning in product internal excitation. The internally excited fraction shows a collision energy dependence only for CH2I–. A similar trend is observed for the isoelectronic OH– + CH3I system. The comparison of our experimental data at 1.55 eV collision energy with a recent theoretical calculation for the same system shows a slightly higher fraction of internal excitation than predicted, which is, however, compatible within the experimental accuracy.