The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.

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Gas temperature determination from rotational lines in non-equilibrium plasmas: a review

Electron scattering by molecules II. Experimental methods and data

Publisher Summary The polarization propagator is the double-time Green's function that describes the propagation of a density disturbance through an interacting system. The focus of this chapter is to show how the polarization propagator can be used advantageously in practical calculations of excitation properties of atomic, molecular, and metallic systems. The approximate polarization propagator through any order in electronic repulsion is symmetric under a transformation E → –E, that is, it can be represented in a spectral form. This is not the case for the corresponding particle-hole propagator for which only the positive energy spectrum is treated consistently through the required order. This deficiency of the particle-hole propagator has very little influence on the (positive) excitation energies, whereas the transition moments are rather poor in methods where the E→ – E symmetry of the propagator is not preserved. The particle-hole propagator can be compared directly with the diagrammatic perturbation calculation of the proper particle-hole self-energy. The chapter also discusses the second-order response properties and sum rules, and the ground-state correlation energies.

Polarization Propagator Calculations

Dc-excited discharges generated in water at the tip of a tungsten wire which is located at the orifice of a quartz capillary are investigated by time-averaged optical emission spectroscopyTwo distinctive discharge modes are observed For small conductivities of the liquid the discharge is a streamer-like discharge in the liquid itself (liquid mode) For conductivities above typically 45 µS cm−1 a large vapour bubble is formed and a streamer discharge in this vapour bubble is observed (bubble mode)Plasma temperatures and electron densities are investigated for both modes The gas temperature is estimated from the rotational temperature of N2(C–B) and is 1600 ± 200 K for the bubble mode and 1900 ± 200 K for the liquid mode The rotational temperature of OH(A–X) is up to 2 times larger and cannot be used as an estimate for the gas temperature The rotational population distribution of OH(A), ν = 0 is also non-Boltzmann with a large overpopulation of high rotational states This discrepancy in rotational temperatures is discussed in detailElectron densities are obtained from the Stark broadening of the hydrogen Balmer beta line The electron densities in the liquid mode are of the order of 1021 m−3 In the bubble mode electron densities are significantly smaller: (3–4) × 1020 m−3 These values are compared with the Stark broadening of the hydrogen alpha and gamma lines and with electron densities obtained from current density measurements The chemical reactivities of the bubble and liquid modes are compared by means of the hydrogen peroxide production rate

https://iopscience.iop.org/article/10.1088/0963-0252/18/2/025017/pdf

Characterization of a direct dc-excited discharge in water by optical emission spectroscopy

Low temperature plasmas have been used in various plasma processing applications for several decades. But it is only in the last thirty years or so that sources generating such plasmas at atmospheric pressure in reliable and stable ways have become more prevalent. First, in the late 1980s, the dielectric barrier discharge was used to generate relatively large volume diffuse plasmas at atmospheric pressure. Then, in the early 2000s, plasma jets that can launch cold plasma plumes in ambient air were developed. Extensive experimental and modeling work was carried out on both methods and much of the physics governing such sources was elucidated. Starting in the mid-1990s, low temperature plasma discharges have been used as sources of chemically reactive species that can be transported to interact with biological media, cells, and tissues and induce impactful biological effects. However, many of the biochemical pathways whereby plasma affects cells remain not well understood. This situation is changing rather ...

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Perspective: The physics, diagnostics, and applications of atmospheric pressure low temperature plasma sources used in plasma medicine

Dissociative excitation of water by electron impact

Absolute emission cross sections (σem) of Balmer-α,β, γ radiation have been measured in the case of electrons (0–2000 eV) incident on H2 and D2. The H Balmer σem's are larger than the corresponding D Balmer σem's for every electron impact energy. It has been found that this isotope effect varies strongly as a function of the incident electron energy below 100 eV. The isotope effect will be interpreted using the H2 and D2 potential energy curves.

Emission cross sections of balmer-α,β,γ radiation for electrons (0–2000 eV) on H2 and D2

Emission cross sections of the H2(3p3Πu→2s3Σ+g transition for electron impact on H2

Measurements of the radiative lifetimes of the vibrational CO+ (A2Πi) states

Radiative lifetimes have been measured for the CO a′ 3 Σ + (ν′=4–9), b 3 Σ + (ν′= 0), c 3 Π(ν′=0), d 3 Δ(ν′=1–16) and B 1 Σ + (ν′= 0) states. Our experimental values, arranged in the same order, are 7–10 μs, 56 ns, 16 ns, 3–7.5 μs, 34 ns. Some of these values disagree with the results of previous experiments. To our opinion this is due to an incomplete identification of the emission spectrum in regions where many bands may overlap, dependent on the applied spectral resolution. For the a′Σ + −a 3 Π and d 3 Δ−a 3 Π emissions effective cross sections for quenching by CO molecules are given. In connection with the identification of the spectrum, absolute emission cross sections for electrons incident on CO have been measured for the b 3 Σ + −a 3 Π and c 3 Π−a 3 Π transitions. For an electron energy, corresponding to the maximum of the excitation function we find cross sections of 5.94 (∓1.2) × 10 −18 cm 2 and 0.630 (∓ 0.13) × 10 −18 cm 2 , respectively.

G.R. Möhlmann

Papers

Dissociative excitation of water by electron impact

Emission cross sections of balmer-α,β,γ radiation for electrons (0–2000 eV) on H2 and D2

Emission cross sections of the H2(3p3Πu→2s3Σ+g transition for electron impact on H2

Measurements of the radiative lifetimes of the vibrational CO+ (A2Πi) states

Radiative lifetimes of the CO a′3Σ+, b3Σ+, c3Π, d3Δ and B1Σ+ states. Absolute emission cross sections for t