Study of electron-beam propagation through preionized dense foam plasmas

The transport of an intense electron-beam produced by the Vulcan petawatt laser through dense plasmas has been studied by imaging with high resolution the optical emission due to electron transit through the rear side of coated foam targets. It is observed that the MeV-electron beam undergoes strong filamentation and the filaments organize themselves in a ringlike structure. This behavior has been modeled using particle-in-cell simulations of the laser-plasma interaction as well as of the transport of the electron beam through the preionized plasma. In the simulations the filamentary structures are reproduced and attributed to the Weibel instability.

Plasma ionization through wave-particle interaction in a capacitively coupled radio-frequency discharge

Phase resolved optical emission spectroscopy, with high temporal resolution, shows that wave-particle interactions play a fundamental role in sustaining capacitively coupled rf plasmas. The measurements are in excellent agreement with a simple particle-in-cell simulation. Excitation and ionization mechanisms are dominated by beam-like electrons, energized through the advancing and retreating electric fields of the rf sheath. The associated large-amplitude electron waves, driven by a form of two-stream instability, result in power dissipation through electron trapping and phase mixing. (c) 2007 American Institute of Physics.

Evidence for Strong Breit Interaction in Dielectronic Recombination of Highly Charged Heavy Ions

Resonant strengths have been measured for dielectronic recombination of Li-like iodine, holmium, and bismuth using an electron beam ion trap. By observing the atomic number dependence of the state-resolved resonant strength, clear experimental evidence has been obtained that the importance of the generalized Breit interaction (GBI) effect on dielectronic recombination increases as the atomic number increases. In particular, it has been shown that the GBI effect is exceptionally strong for the recombination through the resonant state [1s2s(2)2p(1/2)](1).

Unusual voltammetry of the reduction of O-2 in [C(4)dmim][N(Tf)(2)] reveals a strong interaction of O-2(center dot-) with the [C(4)dmim](+) cation

Voltammetric studies of the reduction of oxygen in the room temperature ionic liquid [C(4)dmim][N(Tf)(2)] have revealed a significant positive shift in the back peak potential, relative to that expected for a simple electron transfer. This shift is thought to be due to the strong association of the electrogenerated superoxide anion with the solvent cation. In this work we quantitatively simulate the microdisc electrode voltammetry using a model based upon a one-electron reduction followed by a reversible chemical step, involving the formation of the [C(4)dmim](+)center dot center dot center dot O-2(center dot-) ion-pair, and in doing so we extract a set of parameters completely describing the system. We have simulated the voltammetry in the absence of a following chemical step and have shown that it is impossible to simultaneously fit both the forward and reverse peaks. To further support the parameters extracted from fitting the experimental voltammetry, we have used these parameters to independently simulate the double step chronoamperometric response and found excellent agreement. The parameters used to describe the association of the O-2(center dot-) with the [C(4)dmim](+) were k(f) = 1.4 x 10(3) s(-1) for the first-order rate constant and K-eq = 25 for the equilibrium constant.

Ruthenium complexes of the 1,4-bis(diphenylphosphino)-1,3-butadiene-bridged diphosphine 1,2,3,4-Me-4-NUPHOS: Solvent-dependent interconversion of four- and six-electron donor coordination and transfer hydrogenation activity

In chloroform, [RuCl2(nbd)(py)(2)] (1) (nbd = norbornadiene; py = pyridine) reacts with 1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadiene (1,2,3,4-Me-4-NUPHOS) to give the dimer [Ru2Cl3(eta(4)-1,2,3,4-Me-4-NUPHOS)(2)]Cl (2a), whereas, in THF [RuCl2(1,2,3,4-Me-4-NUPHOS)(PY)(2)] (3) is isolated as the sole product of reaction. Compound 2 exists as a 4:1 mixture of two noninterconverting isomers, the major with C, symmetry and the minor with either C, or C-2 symmetry. A single-crystal X-ray analysis of [Ru2Cl3 (eta(4)-1,2,3,4-Me-4-NUPHOS)(2)] [SbF6] (2b), the hexafluoroantimonate salt of 2a, revealed that the diphosphine coordinates in an unusual manner, as a eta(4)-six-electron donor, bonded through both P atoms and one of the double bonds of the butadiene tether. Compounds 2a and 3 react with 1,2-ethylenediamine (en) in THF to afford [RuCl2(1,2,3,4-Me-4-NUPHOS)(en)] (4), which rapidly dissociates a chloride ligand in chloroform to give [RuCl(eta(4)-1,2,3,4-Me-4-NUPHOS)(en)] [Cl] (5a). Complexes 4 and 5a cleanly and quantitatively interconvert in a solvent-dependent equilibrium, and in THF 5a readily adds chloride to displace the eta(2)-interaction and re-form 4. A single-crystal X-ray structure determination of [RuCl(eta(4)-1,2,3,4-Me-4-NUPHOS)(en)][ClO4] (5b) confirmed that the diphosphine coordinates in an eta(4)-manner as a facial six-electron donor with the eta(2)-coordinated double bond occupying the site trans to chloride. The eta(4)-bonding mode can be readily identified by the unusually high-field chemical shift associated with the phosphorus atom adjacent to the eta(2)-coordinated double bond. Complexes 2a, 2b, 4, and 5a form catalysts that are active for transfer hydrogenation of a range of ketones. In all cases, catalysts formed from precursors 2a and 2b are markedly more active than those formed from 4 and 5a.

Breit-Pauli R-matrix calculation of fine-structure effective collision strengths for the electron impact excitation of Mg V

Context. Electron-impact excitation collision strengths are required for the analysis and interpretation of stellar observations.
Aims. This calculation aims to provide effective collision strengths for the Mg V ion for a larger number of transitions and for a greater temperature range than previously available, using collision strength data that include contributions from resonances.
Methods. A 19-state Breit-Pauli R-matrix calculation was performed. The target states are represented by configuration interaction wavefunctions and consist of the 19 lowest LS states, having configurations 2s22p4, 2s2p5, 2p6, 2s22p33s, and 2s22p33p. These target states give rise to 37 fine-structure levels and 666 possible transitions. The effective collision strengths were calculated by averaging the electron collision strengths over a Maxwellian distribution of electron velocities.
Results. The non-zero effective collision strengths for transitions between the fine-structure levels are given for electron temperatures in the range = 3.0 - 7.0. Data for transitions among the 5 fine-structure levels arising from the 2s22p4 ground state configurations, seen in the UV range, are discussed in the paper, along with transitions in the EUV range – transitions from the ground state 3P levels to 2s2p5?3P levels. The 2s22p4?1D–2s2p5?1P transition is also noted. Data for the remaining transitions are available at the CDS.

Observation and characterization of laser-driven Phase Space Electron Holes

The direct observation and full characterization of a phase space electron hole (EH) generated during laser-matter interaction is presented. This structure, propagating in a tenuous, nonmagnetized plasma, has been detected via proton radiography during the irradiation with a ns laser pulse (I?2 ˜ 1014 W/cm2) of a gold hohlraum. This technique has allowed the simultaneous detection of propagation velocity, potential, and electron density spatial profile across the EH with fine spatial and temporal resolution allowing a detailed comparison with theoretical and numerical models.

Low-Energy Electron Attachment to the Dichlorodifluoromethane (CCl2F2) Molecule

Results from a joint experimental study of electron attachment to dichlorodifluoromethane (CCl2F2) molecules in the gas phase are reported. In a high resolution electron beam experiment involving two versions of the laser photoelectron attachment method, the relative cross section for formation of the dominant anion Cl- wits measured over the energy range 0.001-1.8 eV at the gas temperature T-G = 300 K. It exhibits cusp structure at thresholds for vibrational excitation of the v(3)(a(1)) mode due to interaction with the attachment channels. With reference to the thermal attachment rate coefficient k(T-e;T-G = 300 K) = 2.2(8) x 10(-9) cm(-3) s(-1) (fitted average from several data), a new highly resolved absolute attachment cross section for TG = 300 K was determined. Partial cross sections for formation of the anions Cl-, Cl-2(-), F-, ClF-, and CCl2F- were measured over the range 0-12 eV, using three different electron beam experiments of medium energy resolution. The dependence of the attachment rate coefficient k(T-e;T-G = 300 K) on electron temperature T-e wits calculated over the range 50-15 000 K, based on a newly constructed total cross section for anion formation at T-G = 300 K. R-matrix Calculations for Cl- production have been carried out for comparison with the experimental data. The R-matrix results are in line with the main experimental observations and predict the dependence of the DEA cross section on the initial vibrational level v(3) and on the vibrational temperature. Furthermore, the cross section for I Vibrational excitation of the v(3) mode hits been computed.

Generation and optimization of electron currents along the walls of a conical target for fast ignition

The interaction of an ultraintense laser pulse with a conical target is studied by means of numerical particle-in-cell simulations in the context of fast ignition. The divergence of the fast electron beam generated at the tip of the cone has been shown to be a crucial parameter for the efficient coupling of the ignition laser pulse to the precompressed fusion pellet. In this paper, we demonstrate that a focused hot electron beam is produced at the cone tip, provided that electron currents flowing along the surfaces of the cone sidewalls are efficiently generated. The influence of various interaction parameters over the formation of these wall currents is investigated. It is found that the strength of the electron flows is enhanced for high laser intensities, low density targets, and steep density gradients inside the cone. The hot electron energy distribution obeys a power law for energies of up to a few MeV, with the addition of a high-energy Maxwellian tail.

Breit interaction in dielectronic recombination of hydrogenlike uranium

Absolute rate coefficients for dielectronic recombination (DR) of H-like U91+ ions have been measured. The electron-ion merged-beam technique at a heavy-ion storage ring was employed using a stochastically cooled ion beam. Thereby, the previously accessible electron-ion collision energies could be greatly extended to the range 63-90 keV. High-resolution DR spectra were measured covering all KLL and KLM resonances. For the resonance strengths, excellent agreement between relativistic theory and experiment is found only if the Breit contribution to the electron-electron interaction is included in the calculations. For the KL1/2L1/2 and KL1/2M1/2 groups the Breit contribution amounts to 44% of their total resonance strengths.

MeV negative ion generation from ultra-intense laser interaction with a water spray

MeV negative oxygen ions are obtained from a water spray target irradiated by high intensity (5 x 10(19) W/cm(2)) and ultrashort (50 fs) laser pulses. Generation of negative ions is ascribed to electron-capture processes that the laser-accelerated high-energy positive ion experiences when it interacts with atoms in the spray. This mechanism implies the existence of a large number of MeV neutral oxygen atoms, which is consistent with indirect experimental evidence. (C) 2011 American Institute of Physics. [doi:10.1063/1.3622664]

Alkynyl-Bridged Ruthenium(II) 4 '-Diferroceny1-2,2 ':6 ',2 ''-terpyridine Electron Transfer Complexes: Synthesis, Structures, and Electrochemical and Spectroscopic Studiese

The novel ligand 4'-diferrocenylallcyne-2,2':6',2 ''-terpyridine (7; Fc-C C-Fc-tpy; tpy = terpyridyl; Fc = ferrocenyl) and its Ru2+ complexes 8-10 have been synthesized and characterized by single-crystal X-ray diffraction, cyclic voltammetry, and UV-vis and luminescence spectroscopy. Electrochemical data and UV absorption and emission spectra indicate that the insertion of an ethynyl group causes delocalization of electrons in the extended pi* orbitals. Cyclic voltammetric measurements of 7 show two successive reversible one-electron-oxidation processes with half-wave potentials of 0.53 and 0.78 V. The small variations of the E-1/2 values for the Fe2+/Fe3+ redox couples after the coordination of the Ru2+ ion suggest a weak interaction between the Ru2+ and Fe2+ centers. After insertion of an ethynyl group, UV-vis absorption spectra show a red shift of the absorption peak of the (1)[(d(pi)(Fe))(6)]->(1)[(d(pi)(Fe))(5)(pi*(Ru)(tpy))(1)] MMLCT of the Ru2+ complexes. The Ru2+ complex 8 exhibits the strongest luminescence intensity (lambda(em)(max) 712 nm, Phi(em) = 2.63 x 10(-4), tau = 323 ns) relative to analogous ferrocene-based terpyridine Ru(II) complexes in H2O/CH3CN (4/1 v/v) solution.

Electron-impact excitation of CrII: A theoretical calculation of effective collision strengths for optically allowed transitions.

In this paper, we present electron-impact excitation collision strengths and Maxwellian averaged effective collision strengths for the complicated iron-peak ion Cr II. We consider specifically the allowed lines for transitions from the 3d(5) and 3d(4)4s even parity configuration states to the 3d(4)4p odd parity configuration levels. The parallel suite of R-Matrix packages, RMATRX II, which have recently been extended to allow for the inclusion of relativistic effects, were used to compute the collision cross sections. A total of 108 LS pi/280 J pi levels from the basis configurations 3d(5), 3d(4)4s, and 3d(4)4p were included in the wavefunction representation of the target including all doublet, quartet, and sextet terms. Configuration interaction and correlation effects were carefully considered by the inclusion of seven more configurations and a pseudo-corrector (4d) over bar type orbital. The 10 configurations incorporated into the Cr II model thus listed are 3d(5), 3d(4)4s, 3d(4)4p, 3d(3)4s(2), 3d(3)4p(2), 3d(3)4s4p, 3d(4)(4d) over bar, 3d(3)4s (4d) over bar, 3d(3)4p (4d) over bar, and 3d(3)(4d) over bar (2), constituting the largest Cr II target model considered to date in a scattering calculation. The Maxwellian averaged effective collision strengths are computed for a wide range of electron temperatures 2000-100,000 K which are astrophysically significant. Care has been taken to ensure that the partial wave contributions to the collision strengths for these allowed lines have converged with "top-up" from the Burgess-Tully sum rule incorporated. Comparisons are made with the results of Bautista et al. and significant differences are found for some of the optically allowed lines considered.

Micron-scale fast electron filaments and recirculation determined from rear-side optical emission in high-intensity laser-solid interactions

The transport of relativistic electrons generated in the interaction of petawatt class lasers with solid targets has been studied through measurements of the second harmonic optical emission from their rear surface. The high degree of polarization of the emission indicates that it is predominantly optical transition radiation (TR). A halo that surrounds the main region of emission is also polarized and is attributed to the effect of electron recirculation. The variation of the polarization state and intensity of radiation with the angle of observation indicates that the emission of TR is highly directional and provides evidence for the presence of mu m-size filaments. A brief discussion on the possible causes of such a fine electron beam structure is given.

Recent fast electron energy transport experiments relevant to fast ignition inertial fusion

A number of experiments have been undertaken at the Rutherford Appleton Laboratory that were designed to investigate the physics of fast electron transport relevant to fast ignition inertial fusion. The laser, operating at a wavelength of 1054 nm, provided pulses of up to 350 J of energy on target in a duration that varied in the range 0.5-5 ps and a focused intensity of up to 10(21) W cm(-2). A dependence of the divergence of the fast electron beam with intensity on target has been identified for the first time. This dependence is reproduced in two-dimensional particle-in-cell simulations and has been found to be an intrinsic property of the laser-plasma interaction. A number of ideas to control the divergence of the fast electron beam are described. The fractional energy transfer to the fast electron beam has been obtained from calibrated, time-resolved, target rear-surface radiation temperature measurements. It is in the range 15-30%, increasing with incident laser energy on target. The fast electron temperature has been measured to be lower than the ponderomotive potential energy and is well described by Haines' relativistic absorption model.