Barrier discharges (BDs) produce highly non-equilibrium plasmas in a controllable way at atmospheric pressure, and at moderate gas temperature. They provide the effective generation of atoms, radicals and excited species by energetic electrons. In the case of operation in noble gases (or noble gas/halogen gas mixtures), they are sources of an intensive UV and VUV excimer radiation. There are two different modes of BDs. Generally they are operated in the filamentary one. Under special conditions, a diffuse mode can be generated. Their physical properties are discussed, and the main electric parameters, necessary for the controlled BD operation, are listed. Recent results on spatially and temporally resolved spectroscopic investigations by cross-correlation technique are presented. BDs are applied for a long time in the wide field of plasma treatment and layer deposition. An overview on these applications is given. Selected representative examples are outlined in more detail. In particular, the surface treatment by filamentary and diffuse BDs, and the VUV catalyzed deposition of metallic layers are discussed. BDs have a great flexibility with respect to their geometrical shape, working gas mixture and operation parameters. Generally, the scaling-up to large dimensions is of no problem. The possibility to treat or coat surfaces at low gas temperature and pressures close to atmospheric once is an important advantage for their application.

The technique of spatially resolved cross-correlation spectroscopy (CCS) is used to carry out diagnostic measurements of the barrier discharge (BD) in air at atmospheric pressure. Quantitative estimates for electric field strength E(x, t) and for relative electron density n e (x, t)/n max e are derived from the experimentally determined spatio-temporal distributions of the luminosity for the spectral bands of the 0-0 transitions of the second positive system of N 2 (λ = 337.1 nm) and the first negative system of N + 2 (λ = 391.5 nm). These results are used to test the validity of some physical models of electrical breakdown in a BD. The influence of the spatio-temporal structure of the discharge on the chemical kinetics of ozone synthesis is studied by means of a semi-empirical method based on the results of spatially resolved CCS measurements.

The ratios of intensities of the spectral bands of molecular nitrogen corresponding to transitions N + 2 (B 2 + g , v = 0) → N + 2 (X 2 + g , v = 0), N 2 (C 3 u , v = 0) → N 2 (B 3 g , v = 0) and N 2 (C 3 u , v = 2) → N 2 (B 3 g , v = 5) as a function of the applied electric field strength were measured for air in the pressure range of 300 to 10 5 Pa. The non-self-sustaining dc discharge in a parallel-plane gap was used for excitation of gas molecules. The reduced field strength was varied in the range of (150-5000) × 10 −21 V m 2. The measured ratio of intensities as a function of electric field strength is compared with the theoretical estimates made by other authors. The obtained intensity ratio versus field strength curves can be used for field strength estimation in plasmas if the nitrogen molecules are excited dominantly from the ground state directly by the electron impact.

The techniques of spatially resolved cross-correlation spectroscopy (CCS) and current pulse oscillography were used to carry out systematic investigations of the barrier discharge (BD) in the binary gas mixtures N 2 /O 2 at atmospheric pressure. At very low oxygen concentrations (<500 ppm), the BD was observed in a so-called diffuse mode (also referred to as atmospheric pressure glow discharge, glow silent discharge or homogeneous BD). In the case of the BD filamentary mode, the spatio-temporal distributions of the BD radiation intensities were recorded for the spectral bands of the 0-0 transitions of the second positive (λ = 337 nm) and first negative system of molecular nitrogen (λ = 391 nm). In the case of the diffuse mode, the spectral bands λ = 337 nm, λ = 260 nm (0-3 transition of the γ-system of NO) and λ = 557 nm (radiation of ON 2 excimer) were used for this purpose. The velocities of the cathode-directed ionizing waves as well as the effective lifetimes of the excited states N 2 (C 3 u) υ =0 and N + 2 (B 2 + u) υ =0 were evaluated from the CCS data. Special attention was devoted to the investigation of the transition between the filamentary and diffuse modes of the BD, this transition being caused by the variation of oxygen content within the range 500-1000 ppm.

Diffuse barrier discharges (BDs) are characterized by the periodicity of their discharge current and by the uniform coverage of the entire electrode surface by the plasma. Up to now the discharge development, their appearance and dynamics cannot be adequately explained by elementary processes. Different processes are discussed in the literature controversially, in particular the importance of volume and surface processes on the pre-ionization (Penning-ionization, secondary (γ-) processes, role of surface charges). Diffuse BDs in nitrogen with small admixtures of oxygen are investigated by plasma diagnostics (current/voltage-oscillography, optical emission spectroscopy) and numerical modelling. Special attention is paid to the transition to the usual filamentary mode, characterized by the presence of micro-discharges and caused by the admixture of oxygen in the range of 0-1200 ppm (parts-per-million). This transition starts at low values of O 2 (about 450 ppm) and is introduced by an oscillative multi-peak mode. At higher admixtures (about 1000 ppm) the micro-discharges are generated. According to the results of numerical modelling, secondary electron emission by N 2 (A 3 u) metastable states plays a major role in discharge maintenance. Due to the much more effective quenching of these states by O 2 and NO than by N 2 the subsequent delivery of electrons will be decreased when the oxygen amount is increased.