Sub-nanosecond secondary geminate recombination in mercury halides HgX 2 (X = I, Br) investigated by time-resolved x-ray scattering

We present a set of self-consistent cross sections for electron transport in gaseous tetrahydrofuran (THF), that refines the set published in our previous study [1] by proposing modifications to the quasielastic momentum transfer, neutral dissociation, ionisation and electron attachment cross sections. These adjustments are made through the analysis of pulsed-Townsend swarm transport coefficients, for electron transport in pure THF and in mixtures of THF with argon. To automate this analysis, we employ a neural network model that is trained to solve this inverse swarm problem for realistic cross sections from the LXCat project. The accuracy, completeness and self-consistency of the proposed refined THF cross section set is assessed by comparing the analyzed swarm transport coefficient measurements to those simulated via the numerical solution of Boltzmann's equation.

Accurate modelling of electron transport in plasmas, plasma-liquid and plasma-tissue interactions requires (i) the existence of accurate and complete sets of cross-sections, and (ii) an accurate treatment of electron transport in these gaseous and soft-condensed phases. In this study we present progress towards the provision of self-consistent electron-biomolecule cross-section sets representative of tissue, including water and THF, by comparison of calculated transport coefficients with those measured using a pulsed-Townsend swarm experiment. Water-argon mixtures are used to assess the self-consistency of the electron-water vapour cross-section set proposed in de Urquijo et al (2014 J. Chem. Phys. 141 014308). Modelling of electron transport in liquids and soft-condensed matter is considered through appropriate generalisations of Boltzmann's equation to account for spatialtemporal correlations and screening of the electron potential. The ab initio formalism is applied to electron transport in atomic liquids and compared with available experimental swarm data for these noble liquids. Issues on the applicability of the ab initio formalism for krypton are discussed and addressed through consideration of the background energy of the electron in liquid krypton. The presence of self-trapping (into bubble/cluster states/solvation) in some liquids requires a reformulation of the governing Boltzmann equation to account for the combined localised-delocalised nature of the resulting electron transport. A generalised Boltzmann equation is presented which is highlighted to produce dispersive transport observed in some liquid systems.

The drift velocity and first Townsend ionization coefficient of electrons in gaseous tetrahydrofuran are measured over the range of reduced electric fields 4-1000 Td using a pulsed-Townsend technique. The measured drift velocities and Townsend ionization coefficients are subsequently used, in conjunction with a multi-term Boltzmann equation analysis, as a further discriminative assessment on the accuracy and completeness of a recently proposed set of electron-THF vapor cross sections. In addition, the sensitivity of the transport coefficients to uncertainties in the existing cross sections is presented. As a result of that analysis, a refinement of the momentum transfer cross section for electron-THF scattering is presented, along with modifications to the neutral dissociation and dissociative electron attachment cross sections. With these changes to the cross section database, we find relatively good self-consistency between the measured and simulated drift velocities and Townsend coefficients.

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A transport theory that explicitly incorporates loss of flux due to annihilating collisions is developed and applied to low-energy positron diffusion and annihilation. The use of more complete momentum transfer and annihilation cross sections for helium has resulted in improved descriptions of the time dependence of Z eff for positrons injected into gaseous helium. Similarly, the variation of Z eff versus E/n 0 for experiments where the annihilation region is immersed in an electric field is in closer agreement with experimental data. Inclusion of loss of flux due to annihilation was found to have a very small effect on the derived Z eff (t) for helium.

The pulsed-Townsend (PT) experiment is a well known swarm technique used to measure transport properties from a current in an external circuit, the analysis of which is based on the governing equation of continuity. In this paper, the Brambring representation (1964 Z. Phys. 179 532) of the equation of continuity often used to analyse the PT experiment, is shown to be fundamentally flawed when non-conservative processes are operative. The Brambring representation of the continuity equation is not derivable from Boltzmann's equation and consequently transport properties defined within the framework are not clearly representable in terms of the phase-space distribution function. We present a re-analysis of the PT experiment in terms of the standard diffusion equation which has firm kinetic theory foundations, furnishing an expression for the current measured by the PT experiment in terms of the universal bulk transport coefficients (net ionisation rate, bulk drift velocity and bulk longitudinal diffusion coefficient). Furthermore, a relationship between the transport properties previously extracted from the PT experiment using the Brambring representation, and the universal bulk transport coefficients is presented. The validity of the relationship is tested for two gases Ar and SF 6 , highlighting also estimates of the differences.

We have calculated the thermalisation time for an electron swarm in gaseous xenon using a multi-term time-dependent Boltzmann equation (BE), for a range of instantaneously applied reduced electric fields 1 Td<E/N< 1000 Td. Starting from a Maxwellian electron energy distribution function (EEDF) at room temperature for a given E/N, the time-evolution of the EEDF and associated electron swarm parameters (drift velocity W e , mean energy 〈ε〉, ionisation coefficient k i , excitation coefficient k ex ) are followed as they converge to steady-state values. For all values of E/N considered, the individual swarm parameters are found to converge at different rates. For E/N>5 Td, they converge in order W e (fastest), 〈ε〉, k ex , and k i (slowest). The time taken for the slowest swarm parameter to converge to an acceptable level (e.g. to within 10% of its steady-state value) is used universally as the benchmark for evaluating the thermalisation time τ th . This time is found to be strongly dependent on the value of the reduced electric field E/N, dropping by almost 5 orders of magnitude for increasing E/N fields 1 Td<E/N< 1000 Td. As a key outcome from this work, the calculated thermalisation times τ th •p are expressed as a general formula, as a function of both the reduced electric field E/N and a user defined convergence level between 1% and 20%. We show that ballpark estimates of thermalisation times, based on the inverse of the collision frequency for energy dissipation 1/ν e (ε) at typical average electron energies, are likely to be unreliable if applied to the heating phase. We also undertake a brief analysis of the cooling phase when the electric field is instantaneously removed from the plasma (i.e. field-free) after it evolves to steady-state conditions during the previous heating phase. Finally, we compare calculated thermalisation times with the typical risetimes of the voltage pulse waveforms for several experimental 'nanosecond' pulse excited plasma discharge devices.