The applicability of actinometry for measuring the absolute concentration of O, N and F atoms in discharge plasma was studied. For this purpose, concentrations of these atoms were measured downstream of an ICP plasma by means of the actinometry method and of appearance potential mass-spectrometry (APMS). Comparison of the results showed good agreement between the two methods. Since the excitation cross sections of electron states O(3p 3P) and O(3p 5P) applied in actinometry are well tested, this allows using the APMS method for absolute calibration of the theoretical excitation cross sections for N and F atoms. As a result, total excitation cross sections of the atomic levels N(3p 4Po), F(3p 2Po) and F(3p 4Do) have been obtained for the first time. Since different types of electron energy distribution function (EEDF) were observed (Maxwellian, bi-Maxwellian and Druyvesteyn) the influence of these possible EEDF types on actinometric coefficients (X  =  O, N, F), that link the ratio of the atom and actinometer intensities with that of their concentrations [X]/[Ar], was also analyzed. It was shown that at the same ionization rate (effective electron temperature) the excitation rate constants are highly sensitive to the shape of EEDF, whereas actinometric coefficients depend on it only slightly. Dependence of actinometric coefficients on electron temperature is positive if the emitting level of the X-atom is lower than that of the actinometer, and negative if vice versa. The energy difference between the emitting states of O and Ar atoms is maximal (~3 eV), so that is not constant for a whole range of electron temperatures typical for discharge plasmas (~2–8 eV). For nitrogen atoms varies considerably with Te only when Te  <  4 eV. In the case of fluorine atoms the energy difference of emitting F and Ar states is only ~1 eV and coefficient is nearly constant in a wide region of Te  >  1.5 eV.

We present a detailed study of the density and kinetics of O2(b1Σg+) in steady-state and partially modulated DC positive column discharges in pure O2 for gas pressures of 0.3-10 Torr and 10-40 mA current. The time-resolved density of O2(b1Σg+) was determined by absolutely-calibrated optical emission spectroscopy (OES) of the A-band emission at 762 nm. Additionally, the O2(b1Σg+) density was determined by VUV absorption spectroscopy using the Fourier –Transform spectrometer at the DESIRS beamline at Synchrotron Soleil, allowing the absolute calibration of OES to be confirmed. The O(3P) atoms were detected by time-resolved sub-Doppler cavity ringdown spectroscopy (CRDS) using the O(3P2) -> O(1D2) transition at 630 nm. The CRDS measurements were synchronized to the discharge modulation allowing the O(3P) dynamics to be observed. As a function of gas pressure the O2(b1Σg+) density passes through a maximum at about 2 Torr. Below this maximum, the O2(b1Σg+) density increases with discharge current, whereas above this maximum it decreases with current. The gas temperature increases with pressure and current, from 300 to 800 K. These observations can only be explained by the existence of fast quenching process of O2(b1Σg+) by O(3P), with a rate that increases strongly with gas temperature, i.e. with a significant energy barrier. The data are interpreted using a 1D self-consistent model of the O2 discharge. The best fit of this model to all experimental data (including the O2(b1Σg+) average density as a function of pressure and current, the radial profiles, and the temporal response to current modulation) is achieved using a rate constant of kQ=10^-10∙exp(-3700/T) cm3/s.

N2 dissociation in pure nitrogen plasma has a long history of research. It seems to be a complex process which comprises many reactions involving various electronic and vibrational nitrogen states whose contributions can vary depending on conditions. In this paper, we studied N2 dissociation in the stationary N2 discharge both experimentally and theoretically. We used a DC glow discharge in a quartz tube in pure N2 at moderate pressures (5–50 Torr). The degree of dissociation, atomic nitrogen loss rate and gas temperature were measured by applying optical emission spectroscopy (OES) and as a result an ‘effective’ rate constant for nitrogen dissociation was obtained across a wide range of the reduced field E/N. The analysis of N2 dissociation was carried out using a specially developed 1D radial self-consistent model which takes into account the spatial inhomogeneities of species concentrations, E/N, electron energy distribution function, Tgas etc, together with fairly complete plasma-chemical kinetics and all the cross-sections known to date for electron kinetics. The model was successfully validated through the experimental results obtained for electric field, gas temperature and N atom density. Comprehensive analysis of closely coupled processes in nitrogen plasmas—gas heating, VDF formation and N2 dissociation—was carried out. Simulations reproduced the experimental data on well and allowed us to evaluate the different contributions of the various dissociation channels considered. It was shown that the nitrogen dissociation mechanism in the stationary N2 discharge is provided by direct electron impact via the excitation of the pre-dissociative states from the vibrationally excited nitrogen molecules N2(X, υ). The upper limit for the rate constant of the processes N2(A)  +  N2(14  ⩽  υ  ⩽  19)  →  N  +  N  +  N2 was estimated to be 5 · 10−14 cm3 s−1.

The O( 3 P) atom loss has been studied in O 2 RF plasma in the quartz tube in the intermediate pressure range (10-100 Torr) when the transition from the surface to volume loss is observed. Space-and time-resolved actinometry on Ar and Kr atoms was used to study O( 3 P) atom loss. The research has shown that such a transition actually takes place. However, it was revealed that the gas temperature plays a significant role in this. Gas temperature was measured spectroscopically using theIt was demonstrated that the gas temperature in the plasma volume was rather high (>1200 K) due to the high values of the specific input RF power which in turn led to a significant O( 3 P) loss rate decrease in the discharge volume. The atomic oxygen loss is limited by the O( 3 P) surface recombination as well as by the volume recombination in a thin layer near the wall. It leads to a low integral loss rate and provides a high oxygen dissociation degree. Analysis based on measured [O( 3 P)]/N, T gas , O 3 spatial profiles and surface loss model including recombination with chemisorbed and physisorbed atoms has revealed the importance of both surface and volume losses. High values of the specific input RF power also lead to increase of the gas temperature near the wall and the temperature of the internal tube surface. As a result, the O atom surface loss rate increases and the volume loss rate near the wall decreases, so overall the contributions of both O( 3 P) volume and surface recombination are comparable at pressures up to 100 Torr. The O( 3 P) loss kinetics at intermediate pressures appears to be a rather complex phenomenon and requires at least 1D or even 2D modeling for its correct description. Using global models under similar conditions may lead to dubious results in a detailed study of kinetic mechanisms and processes, but it can be useful for a simple analysis of experimental results.

The electron energy probability function (EEPF) probe measurements in cold plasma controlled by high-energy (0.5-1 keV) run-away electrons generated in an 'open-discharge' configuration are presented in this paper. High plasma stability along with the second harmonic lock-in measurement method provide a sufficiently high accuracy of the EEPF measurements which makes it possible to thoroughly study the features of both cold plasma and the probe method itself. The experiments have been carried out in pure gases: Ar, He, O 2 and H 2 . The run-away electron beam is revealed to produce a lot of cold electrons in each gas and the EEPF is Maxwellian with T e about few tens of meV (which is slightly higher than the gas temperature). It is shown that an EEPF can be correctly measured under these conditions if the modulation amplitude and, accordingly, the energy resolution of the method is less than the electron temperature, T e . At the same time, integration of the measured EEPF always underestimates the electron density, n e , due to the 'depletion effect' for low-energy electrons, regardless of the energy resolution of the measurements. It is demonstrated that the open discharge allows obtaining a sufficiently dense and cold, in fact thermal, plasma, which may be of interest for research and various applications.