Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.

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Plasma-liquid interactions: A review and roadmap

The THEMIS plasma instrument is designed to measure the ion and electron distribution functions over the energy range from a few eV up to 30 keV for electrons and 25 keV for ions. The instrument consists of a pair of “top hat” electrostatic analyzers with common 180°×6° fields-of-view that sweep out 4π steradians each 3 s spin period. Particles are detected by microchannel plate detectors and binned into six distributions whose energy, angle, and time resolution depend upon instrument mode. On-board moments are calculated, and processing includes corrections for spacecraft potential. This paper focuses on the ground and in-flight calibrations of the 10 sensors on five spacecraft. Cross-calibrations were facilitated by having all the plasma measurements available with the same resolution and format, along with spacecraft potential and magnetic field measurements in the same data set. Lessons learned from this effort should be useful for future multi-satellite missions.

The THEMIS ESA Plasma Instrument and In-flight Calibration

It is shown that under the usual conditions of zero‐electron‐kinetic‐energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high‐lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.

Factors affecting lifetimes and resolution of Rydberg states observed in zero-electron-kinetic-energy spectroscopy

Purpose: TheGEANT4 general-purpose Monte Carlo simulation toolkit is able to simulate physical interaction processes of electrons, hydrogen and helium atoms with charge states ( H 0 , H + ) and ( He 0 , He + , He 2 + ), respectively, in liquid water, the main component of biological systems, down to the electron volt regime and the submicrometer scale, providing GEANT4 users with the so-called “GEANT4-DNA” physics models suitable for microdosimetry simulation applications. The corresponding software has been recently re-engineered in order to provide GEANT4 users with a coherent and unique approach to the simulation of electromagnetic interactions within the GEANT4 toolkit framework (since GEANT4 version 9.3 beta). This work presents a quantitative comparison of these physics models with a collection of experimental data in water collected from the literature. Methods: An evaluation of the closeness between the total and differential cross section models available in theGEANT4 toolkit for microdosimetry and experimental reference data is performed using a dedicated statistical toolkit that includes the Kolmogorov–Smirnov statistical test. The authors used experimental data acquired in water vapor as direct measurements in the liquid phase are not yet available in the literature. Comparisons with several recommendations are also presented. Results: The authors have assessed the compatibility of experimental data withGEANT4microdosimetry models by means of quantitative methods. The results show that microdosimetric measurements in liquid water are necessary to assess quantitatively the validity of the software implementation for the liquid water phase. Nevertheless, a comparison with existing experimental data in water vapor provides a qualitative appreciation of the plausibility of the simulation models. The existing reference data themselves should undergo a critical interpretation and selection, as some of the series exhibit significant deviations from each other. Conclusions: TheGEANT4-DNA physics models available in the GEANT4 toolkit have been compared in this article to available experimental data in the water vapor phase as well as to several published recommendations on the mass stopping power. These models represent a first step in the extension of the GEANT4 Monte Carlo toolkit to the simulation of biological effects of ionizing radiation.

Comparison of GEANT4 very low energy cross section models with experimental data in water.

Nonthermal secondary electrons with initial kinetic energies below 100 eV are an abundant transient species created in irradiated cells and thermalize within picoseconds through successive multiple energy loss events. Here we show that below 15 eV such low-energy electrons induce single (SSB) and double (DSB) strand breaks in plasmid DNA exclusively via formation and decay of molecular resonances involving DNA components (base, sugar, hydration water, etc.). Furthermore, the strand break quantum yields (per incident electron) due to resonances occur with intensities similar to those that appear between 25 and 100 eV electron energy, where nonresonant mechanisms related to excitation/ionizations/dissociations are shown to dominate the yields, although with some contribution from multiple scattering electron energy loss events. We also present the first measurements of the electron energy dependence of multiple double strand breaks (MDSB) induced in DNA by electrons with energies below 100 eV. Unlike the SSB and DSB yields, which remain relatively constant above 25 eV, the MDSB yields show a strong monotonic increase above 30 eV, however with intensities at least 1 order of magnitude smaller than the combined SSB and DSB yields. The observation of MDSB above 30 eV is attributed to strand break clusters (nano-tracks) involving multiple successive interactions of one single electron at sites that are distant in primary sequence along the DNA double strand, but are in close contact; such regions exist in supercoiled DNA (as well as cellular DNA) where the double helix crosses itself or is in close proximity to another part of the same DNA molecule.

Single, double, and multiple double strand breaks induced in DNA by 3-100 eV electrons.

We report absolute differential cross sections for charge transfer scattering of 0.5-5 keV O + ( 4 S) ground state and O + ( 2 D, 2 P) metastable ions by atomic oxygen at angles between 0.04° and 3° in the laboratory frame. Absolute total charge transfer cross sections, derived from these measurements, are compared with previously published data. The measurements were made using a flowing gas target, which consisted of a mixture of atomic and molecular oxygen produced by passing O 2 through a microwave discharge. The atomic oxygen cross sections were obtained by appropriate subtraction of the signal due to molecular oxygen from that due to the mixture of O and O 2 . These data are relevant to model calculations of energetic O + precipitation into the upper atmosphere and to the diffusion of low-energy O + ions in the ionosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JA000437

Charge transfer of keV O+ ions with atomic oxygen

Calculated and experimental differential direct, charge-transfer, and total scattering cross sections are presented for collisions of ${\mathrm{H}}^{+}$ with ${\mathrm{O}}_{2}$. Experimental cross sections are obtained for projectile energies of 0.5, 1.5, and $5.0\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, and scattering angles of 0.15\ifmmode^\circ\else\textdegree\fi{} to 5\ifmmode^\circ\else\textdegree\fi{} while the theoretical results extend from $0.5\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}25\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and encompass scattering angles from 0.01\ifmmode^\circ\else\textdegree\fi{} to 180\ifmmode^\circ\else\textdegree\fi{}. We find good agreement between the calculated and measured differential and total cross sections. Also, a diffraction slit effect, which is target orientation dependent, is found in the differential scattering cross section. A theoretical explanation of the effect is presented.

Absolute differential and total cross sections for direct and charge-transfer scattering of keV protons by O 2

Absolute differential cross sections are reported for electron capture and loss by (1-5)-keV oxygen atoms incident on He, H{sub 2}, N{sub 2}, and O{sub 2} for scattering angles between 0.02 deg. and 1.73 deg. in the laboratory frame. The form of the differential cross sections is seen to vary significantly with energy and between different targets. Differences between the present O-atom electron-loss cross sections and those for H atoms reported previously imply that the underlying physical mechanism may not be the same. The integral cross sections, also reported here, are consistent with most previous studies. The consensus of the available electron-loss data suggests that the cross sections of Fogel et al. [Soviet Phys. JETP 35, 601 (1959)] are in error.

Electron capture and loss by kilo-electron-volt oxygen atoms in collisions with He, H 2 , N 2 , and O 2

Absolute differential and total cross sections for charge transfer of O + with H 2

Absolute differential cross sections are reported for electron capture and loss by 1–5 keV H atoms incident on CH4 for laboratory scattering angles up to 1.62°, and for charge transfer of 0.5–5 keV H+ with CH4 for scattering angles up to 2.09°. Electron-loss collisions are seen to result in comparatively large scattering angles and a very clear similarity exists between the present differential cross sections and those reported for other molecular targets. The present charge-transfer differential cross sections are consistent with that of Gao et al (1990 Phys. Rev. A 41 5929–33) but not with the calculations of Kimura et al (1995 Phys. Rev. A 52 1196–205). Prior experimental studies of electron-loss and charge-transfer are generally in good accord with the integral values reported here as are the calculations of Kusakabe et al (2000 Phys. Rev. A 62 062715).

http://iopscience.iop.org/article/10.1088/0953-4075/38/12/012/pdf

B. G. Lindsay

Papers

Charge transfer of keV O+ ions with atomic oxygen

Absolute differential and total cross sections for direct and charge-transfer scattering of keV protons by O 2

Electron capture and loss by kilo-electron-volt oxygen atoms in collisions with He, H 2 , N 2 , and O 2

Absolute differential and total cross sections for charge transfer of O + with H 2

Electron transfer in collisions of keV hydrogen atoms and ions with methane