Generation of an intense low-velocity metastable-neon atomic beam

A metastable neon (1S ₅) atomic beam with a velocity of 394 m/s (373 K) and an intensity of 1.1×10¹⁵ s^–1 sr^–1 is produced with a novel construction using a dc glow discharge.     

Excitation of the 7s2P1/2, 3/2 levels of Xe+ due to charge exchange in the negative glow of a glow discharge in a neon-xenon mixture

It is shown that in the negative glow of a glow discharge in mixtures of neon with xenon (xenon contents of 5.5·10^–3 to 2% of the total pressure P of the mixture) with P=15 torr and discharge currents i=40 mA (cathode diameter 30 mm), the 7s²P_{1/2, 3/2} levels of the Xe⁺ ion are excited by the charge exchange reaction Xe ^m + Ne⁺ Xe^+* + Ne.The effective cross section Q_3/2 for excitation of the 7s²P_3/2 level is estimated to lie between 10^–14 and 10^–15 cm² and Q_3/26.5 × Q_1/2, where Q_1/2 is the effective cross section for excitation of the 7s²P_1/2 level.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 96–99, March, 1979.     

Anomalous plasma phenomena of hollow cathode arc discharge

Magnetically confined argon plasma produced by hollow cathode arc discharge has been studied in different experimental conditions, with discharge current from 10–50 A, vessel argon pressure between 10^–3 and 10^–4 torr (1 torr=133·32 Pa) and axial magnetic field up to 0·12 T. The plasma density measured by a cylindrical Langmuir probe is found to be 10¹⁹ to 4 × 10¹⁹ m^–3 and the electron temperatureT _e varies between 2·5 and 4·8 eV. When an external axial magnetic field is applied the plasma temperature decreases with the increase in the magnetic field intensity until it reaches a minimum value at 0·075T and then increases with the same rate. This has been interpreted as high frequency waves excitation due to electron beam-plasma interaction, which explains the electron density jumps with the magnetic field intensity. Enhanced plasma transport across the magnetic field is studied and classified as anomalous diffusion.     

Endoergic and resonant charge transfer excitation in He-Cu discharge

The intensity of Cu-II lines with upper level energies near and above those of the He ion was measured as a function of He pressure in a Cu hollow cathode tube. In this tube at low pressures the negative glow could expand above the cathode. The maximum intensity of the Cu-II 493.1 nm line was found in the low voltage, high pressure hollow cathode discharge region in accordance with a resonant charge transfer excitation process. Enhancement of the intensity of the Cu-II 436.5 nm and 417.9 nm lines was observed in the cathode glow at low pressures. Excitation of these lines is attributed to endoergic charge transfer collisions between He ions accelerated by the 2 kV tube voltage and ground state Cu atoms. The cross-section for this reaction exciting the 436.5 nm line was estimated to be of the order of 10^–17 cm².     

Luminescence of rare gas crystals at high excitation densities for VUV laser applications

Exciton densities of the order of 10¹⁸ cm^–3 are generated in 0.1–0.3 mm thick surface layers in an area of 10×20 mm² of optically clear rare gas crystals. The quantum efficiencies at 126 nm (Ar), 145 nm (Kr), and 172 nm (Xe) remain near 0.5 even for the highest excitation densities. The corresponding gain coefficients of 2.6 cm^–1 (Ar) to 18 cm^–1 (Xe) exceed those of high pressure gas lasers by a factor of 20. Stimulated emission is inferred by observing the line narrowing, the dependence of intensities and time courses on excitation density and amplification measurements. The net gain coefficient is reduced however to 0.5–1 cm^–1 by transient absorption of excited centers and scattering by irradiation induced defects. The results are analysed by a system of rate equations for the excitation, relaxation, quenching, and amplification processes. A peculiar temperature dependence of the quantum efficiencies and time courses is attributed to electron trapping at grain boundaries.     

Hollow-Cathode Low-Pressure Arc Discharges and Their Application in Plasma Generators and Charged-Particle Sources

This paper presents the results of a study of hollow-cathode arc discharges which generate gas-discharge plasmas of densities 10¹⁰–10¹² cm^–3 in large volumes (1 m³) at low pressures (10^–2–1 Pa) and at discharge currents of up to 200 A. Consideration is given to the design and peculiarities of hot-cathode and cold-cathode discharge systems. The parameters of plasma generators and charged-particle sources where use is made of arc discharges are cited and the problems of the most efficient application of such systems in technological processes of solid surface modification are discussed.     

Nitrogen atom detection in low-pressure flames by two-photon laser-excited fluorescence

Nitrogen atoms have been detected in stoichiometric flat premixed H₂/O₂/N₂ flames at 33 and 96 mbar doped with small amounts of NH₃, HCN, and (CN)₂ using two-photon laser excitation at 211 nm and fluorescence detection around 870 nm. The shape of the fluorescence intensity profiles versus height above the burner surface is markedly different for the different additives. Using measured quenching rate coefficients and calibrating with the aid of known N-atom concentrations in a discharge flow reactor, peak N-atom concentrations in these flames are estimated to be on the order of 10¹²–5×10¹³ cm^–3; the detection limit is about 1×10¹¹ cm^–3.     

Effect of Selective Excitation of Gold Atoms in a Nonequilibrium Oxygen Plasma of a Combined Glow Discharge

The effect of selective excitation of gold atoms in a nonequilibrium oxygen plasma of a combined glow discharge (CGD) has been revealed. When working with alumina oxide as a matrix where the content of aluminum is about 30 wt.%, the atomic lines of Al are not seen at all in the CGD spectrum. But the atomic lines of gold have a very high intensity. Even when the aluminum and gold atomic lines overlap, gold can be detected at a content of 10^–7%. This effect has been registered both in synthetic and natural ore samples.     

Luminescence of AgCl excited by u.v. laser light

The spectral distribution and decay of the blue-green luminescence of AgCl has been measured at very low temperatures, using u.v. laser excitation. A structure was observed in the emission band at 77°K. The decay consists of three processes: a) a very fast process which follows the laser pulse (10^–8 sec), b) a fast exponential process which is also observed with low intensity u.v. excitation,(10^–4–10^–5 sec) and, finally, c) a slow exponential process (seconds).     

General intense electron beams by means of a contracted arc discharge

Two types of contracted arc discharge are investigated with a view to generating intense electron beams over a wide pressure range (1–10^–3 Pa). For an arc discharge with a hollow cathode and anode, an electron beam corresponding to a current of up to 300 A and a pulse length of 25 µsec is obtained at a pressure of 1–10^–1 Pa in the accelerating gap with an accelerating voltage of up to 15 kV. At pressures of 10^–2–10^–3 Pa, emitting plasma is created by a low-pressure arc discharge on the basis of a Penning cell. Three discharge systems operating in parallel are used to increase the working life of the cathode and improve the current density distribution of the beam. An electron beam of diameter 200 mm with a current of up to 125 A and a pulse length of 50 µsec is obtained.Institute of High-Power Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshkikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 76–82, March, 1994.     

Emission spectroscopy of atmospheric pressure plasmas for bio-medical and environmental applications

The paper demonstrates several ways of use of the UV-vis optical emission spectroscopy of medium resolution for the diagnostics of atmospheric pressure air and nitrogen plasmas relevant to bio-medical and environmental applications. Plasmas generated by DC discharges (streamer corona, transient spark, and glow discharge), AC microdischarges in porous ceramics, and microwave plasma were investigated. Molecular (OH, NO, CN) and atomic (H, O, N) radicals, and other active species, e.g. N₂ (C, B, A), (B), were identified. The composition of the emission spectra gives insight in the ongoing plasma chemistry. Rotational, i.e. gas, and vibrational temperatures were evaluated by fitting experimental with simulated spectra. Streamer corona, transient spark and microdischarges generate cold, strongly non-equilibrium plasmas (300-550 K), glow discharge plasma is hotter, yet non-equilibrium (1900 K), and microwave plasma is very hot and thermal (∼3000-4000 K). Electronic excitation temperature and OH radical concentration were estimated in the glow discharge assuming the chemical equilibrium and Boltzmann distribution (9800 K, 3 × 10¹⁶ cm⁻³). Optical emission also provided the measurement of the active plasma size of the glow discharge, and enabled calculating its electron number density (10¹² cm⁻³).     

Spectroscopic studies of a helium plasma generated near ceramic surface at pressures below 1000 hPa

Spatially resolved line intensity measurements from a plasma generated near ceramic surfaces have been performed. Disk-shaped helium plasmas of diameter 20 mm and thickness 0.9 mm have been studied in a pressure range of 2×10³–10⁵ Pa. On the basis of line intensity measurements and applying an appropriate collisional-radiative model for a helium plasma, the distributions of electron density and electron temperature have been evaluated.     

Population of the higher-lying vibrational levels of the nitrogen B3π state in a glow discharge

The absolute populations of the higher-lying vibrational levels of the B³ state of the nitrogen molecule in a glow discharge have been determined for pressures from 0.1 to 1.0 torr and current densities from 80 to 200 mA/cm². The absolute concentrations of molecules at levels v = 5, 6, ..., 12 under these conditions are of the order of 10⁹–10¹¹ cm^–3. The most probable processes leading to a population of the B³ state are thought to be direct electronic impact, cascade transitions, and collisions of normal molecules with molecules in the singlet state.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 2, pp. 57–63, February, 1971.     

Characteristics of volume discharge in gaps with a small interelectrode separation

This paper examines the excitation conditions, ignition ranges, and also the energetic characteristics of high-pressure volume discharge. It is shown that, using the UV radiation of an auxiliary discharge to preionize the active medium, volume discharge turns out to be substantially more stable than if weak x-ray radiation is used, creating an initial electron concentration 10⁸ cm^–3. Data are cited which indicate that the energy entering the gas with the primary discharge increases in proportion to the increase of energy dissipated in the auxiliary discharge. Several results are reported about the effect of electric field homogeneity on the nature of the discharge progression.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 89–94, April, 1986.     

Generation of uniform atmospheric pressure argon glow plasma by dielectric barrier discharge

In this paper, atmospheric pressure glow discharges (APGD) in argon generated in parallel plate dielectric barrier discharge system is investigated by means of electrical and optical measurements. Using a high voltage (0–20 kV) power supply operating at 10–30 kHz, homogeneous and steady APGD has been observed between the electrodes with gap spacing from 0.5 mm to 2 mm and with a dielectric barrier of thickness 2 mm while argon gas is fed at a controlled flow rate of 1 l/min. The electron temperature and electron density of the plasma are determined by means of optical emission spectroscopy. Our results show that the electron density of the discharge obtained is of the order of 10¹⁶ cm^???3 while the electron temperature is estimated to be 0.65 eV. The important result is that electron density determined from the line intensity ratio method and stark broadening method are in very good agreement. The Lissajous figure is used to estimate the energy deposited to the glow discharge. It is found that the energy deposited to the discharge is in the range of 20 to 25 μJ with a discharge voltage of 1.85 kV. The energy deposited to the discharge is observed to be higher at smaller gas spacing. The glow discharge plasma is tested to be effective in reducing the hydrophobicity of polyethylene film significantly.     

Light Source with a Thermoionic Cathode

A light source with a thermoionic cathode has been investigated in helium at atmospheric pressure; the spectral radiance of the light source in the range 200–800 nm and the cathode temperature (3510 K) have been determined. Its employment as a light source in absorption spectroscopy for determination of the concentration of triplet and singlet metastable helium atoms in an atmospheric-pressure glow discharge has been demonstrated. The total concentration of metastable atoms was 3.5·10¹³ cm^–3.     

Spectral diagnostics of a unipolar high-frequency discharge excited in nitrogen and argon at pressures from 1 to 12 atm

By methods of spectral diagnostics, the temperature of neutral gas and the electron temperature and density have been determined in the channel of a unipolar high-frequency discharge excited at very high pressures. In nitrogen the h.f. discharge was excited at pressures of 1–5 atm, in argon at pressures of 1–12 atm. In the discharge excited in argon, the electron temperature does not change with increasing pressure and isT _e =(6–7)×10³ °K; the electron density increases with increasing pressure. It can be demonstrated that the electron velocity distribution is given by a Maxwellian distribution function although the plasma of a unipolar high-frequency discharge is non-isothermal (T _e –T _n 5×10³ °K).In conclusion, the author thanks Prof. Dr. V. Truneek for stimulating remarks and his kind interest in this work.     

Electron kinetics in dc CF4 + O2 mixture discharge

The electron distribution function, transport coefficients and rate coefficients for the dissociation of CF₄ and the excitation of a¹ _g and b¹ _g ⁺ states of oxygen have been calculated for a CF₄ + O₂ mixture discharge. The two-term approximation was used in solving the electron Boltzmann equation. The influence of the dissociation was ignored, so that the results are parametric in the density reduced electric fieldE/N and the ratio of the mixture components. Only slight dependence of the calculated quantities on the composition of the gas mixture has been observed. The critical electric field for the onset of the discharge was found to beE/N=1.1–1.25 × 10^–15 Vcm² and it is weakly decreasing with growth of the oxygen content. With a small amount of oxygen added to CF₄ the electron distribution function and the mean kinetic energy remain practically unchanged.     

Gain and saturation measurements in a discharge excited F2 laser using an oscillator amplifier configuration

The small-signal gain coefficient and the saturation intensity of a F₂ pulsed discharge molecular laser at 157 nm have been measured using two discharge devices in an oscillator-amplifier configuration. The small signal gain coefficient was measured to be 5.2±0.4% cm^–1 at 3 atm total pressure and 1.5 cm electrode spacing and 4.1±0.4% cm^–1 at 2 atm total pressure and 2 cm electrode spacing while the values of the saturation intensity were 5 MW/cm² and 4.6 MW/cm², respectively.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India