Fundamental studies on a planar-cathode direct current glow discharge. Part I: characterization via laser scattering techniques

A laser-scattering-based instrument was used to study an argon d.c. planar-diode glow discharge. The gas-kinetic temperature (T_g) was determined via Rayleigh scattering and the electron number density (n_e), electron temperature (T_e), and shape of the electron energy-distribution function were determined by Thomson scattering. Axial profiles of these parameters were obtained as the discharge current, voltage, and pressure were varied. Trends in the profiles of T_g and in the other parameters show the interdependence of these plasma species and properties. The results will be compared with current theoretical computer models in order to improve our understanding of the fundamental processes in glow discharges sustained under conditions appropriate for spectrochemical analysis.     

Some considerations on the microwave electrodeless discharge: Plasma diagnostics in argon,helium or nitrogen atmospheres

Plasma diagnostics of several microwave plasmas are determined by making electrical (with double floating probes) and optical measurements in pure Ar, He or N₂ plasmas, and also in Ar plasmas containing various metals, i.e. Cs, Tl or Zn; plasma parameters, such as, electric field (E), electron (j_e) and ion (j_i) current densities, electron density (n_e), electron temperature (T_e) electron conductivity (σ_e), ion density (n_i), electron mean free path (λ_e) electron (μ_e) and ion (μ_i) mobilities and electron [(v_e)_drift] and ion [(v_i)_drift] volocities are either directly measured or calculated. The reversal temperature (T_r) of excited (0.96 eV lower level) thallium atoms is measured, and the steady-state conditions of the plasma are analyzed by the energy balance equation. The experimental measurements indicate that the electric field strength E decreases as the space charge decreases (ionization extent) increases. Although the plasma appears to be under steady-state conditions, it is not under local thermodynamic equilibrium conditions, i.e. T_e >T_r. In addition, the measurements indicate that there is a deficiency of electrons in the plasma (n_e < n_i), probably due to electron affinity processes; and the plasma has a small positive space charge.     

An approximate quantitative analysis of non-equilibrium plasma transport for high density plasmas

For the evaluation of transport in high density plasmas numerical models have been developed in which simultaneously the conservation laws for mass, momentum and energy are solved. For high density plasmas, which are not too far from equilibrium the commonly used thermodynamic quantities are, electron temperature T_e, electron density n_e, heavy particle temperature and neutral density (or pressure). In this contribution an alternative formulation is described in which the plasma state is described by electron density ne and total pressure p and two non-equilibrium parameters: the deviation from Saha equilibrium of the neutral ground state (δb₁ = n₁/n₁ ^saha−1) and the deviation from thermal equilibrium between electrons and heavy particles δΘ = 1−T_h/T_e. The latter two parameters are zero in local thermodynamic equilibrium.

The advantage of this formulation is, that the transport coefficients and radiative properties can be reformulated as function of mainly n_e (at constant pressure), as the influences of non zero δb₁ and δΘ are small or can be explicitly given. As a result a simpler approximate formulation of the transport problem can be obtained. As an example the procedure is illustrated for atmospheric argon plasmas and for one aspect a comparison is made with work from e.g. E. Pfender.

     

Effect of an inductively coupled plasma mass spectrometry sampler interface on electron temperature,electron number density,gas-kinetic temperature and analyte emission intensity upstream in the plasma

Thomson scattering, Rayleigh scattering and line-of-sight emission intensities of Ca ion and Sr ion from the inductively coupled plasma were measured in the presence and in the absence of an inductively coupled plasma mass spectrometry sampler interface. When present, the sampler interface was located 13 mm above the load coil (ALC); optical measurements were made 6, 7 and 8 mm ALC. The experimental results suggest that both the electron temperature (T_e) and gas-kinetic temperature (T_g) dropped in the presence of the sampler interface, with the change in T_g seemingly greater than that in T_e, suggesting a faster cooling process for the heavy particles. In contrast, electron number density (n_e) seemed to be generally increased in the outer regions of the discharge but went down in the central channel, a reflection that n_e is possibly dominated by ambipolar diffusion which becomes less efficient as T_e drops. Assuming these results, the plasma decays more gradually ALC and deviates from local thermodynamic equilibrium even more significantly in the presence of the sampler interface. Analyte line emission intensity was either depressed or enhanced in the presence of the interface, depending on the element being observed and the operating conditions. In addition, the change in emission intensity caused by the sampler interface became much more dramatic when a matrix element, such as Li or Zn, was introduced.     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

Langmuir probe measurements of electron temperature in an inductively coupled plasma

The feasibility of using double Langmuir probes to measure electron temperature (T_e) in an Ar inductively coupled plasma (ICP) was evaluated. Experimental methods for probing the plasma and for reducing rf interference were devised. Despite these measures, the probe signal was noisy and erratic if the ICP had the normal analytical configuration with a hole through its center, so measurements were restricted to an ICP without an axial channel. Theoretical criteria indicated that Langmuir probe measurements in an atmospheric pressure ICP were in a borderline regime in which the measured T_e values may have been depressed somewhat (relative to the actual T_e values in the ICP) due to cooling of electrons as they approached the probe. The T_e values obtained from the center of the ICP were 7500 K at a forward power of 1.0 kW and 10 000 K at 1.25 kW for a measurement position 8 mm above the load coil. Electron density (n_e) measurements by the Langmuir probe method were comparable to or higher than n_e values calculated from the Saha equation at the measured T_es. The T_e and n_e values were high enough to indicate that, if electron cooling and ion-electron recombination occurred near the probes, these effects were not extreme and/or the use of two probes compensated for them in some fashion. The probe measurements also indicated that T_e increased with the potential difference between the probes. This latter observation provided tentative evidence that the electron kinetic energy distribution was non-Maxwellian with an excess of higher energy electrons relative to lower energy electrons.     

Measured Stark widths and shifts of the neutral argon spectral lines in 4s–4p and 4s–4p′ transitions

We investigate the Stark widths (W) and the shift (d), of the seven neutral argon (Ar I) spectral lines from the 4s–4p and 4s–4p′ transitions. The line shapes are measured in a linear, low-pressure, optically thin pulsed arc discharge at about 16 000 K electron temperature (T) and about 7.0 × 10²² m^− 3 electron density (N). The new data separates the electron width (W_e) and ion width W_i from the total Stark width (W_t), as well the separation of electron total Stark shift (d_t) on electron (d_e) and ion (d_i) parts. There are no theoretical predictions for these lines. Comparison to theoretical predictions for other lines within the same multiplets finds that the experimental data exhibits stronger influence by the ion contribution to the measured Ar I line shape. We have also deduced the ion broadening parameters which describe the influence of the ion static (A) and the ion–dynamical (D and E) effect on the width and the shift of the line shape.Applying the line deconvolution procedure, the basic plasma parameters i.e. electron temperature (T) and electron density (N) are recovered. The plasma parameters (T and N) are measured using independent diagnostics techniques as well. Good agreement is found among two sets of the N and T plasma parameters obtained from deconvolution procedure and independent diagnostics techniques.     

Characterization of laser-induced plasmas by emission spectroscopy with curve-of-growth measurements. Part II: Effect of the focusing distance and the pulse energy

Laser-induced plasmas generated with different focusing distances and pulse energies have been characterized by a method based in emission spectroscopy that includes the measurement and calculation of curves of growth. An infrared Nd:YAG laser is used to generated the plasmas from Fe–Ni samples placed in air at atmospheric pressure. The characterization method provides a reduced set of plasma parameters (N_e, T, N′l, αA) that describe the line emission in optically thin and optically thick conditions. For a pulse energy of 100 mJ, the plasma parameters for varying focusing distances are obtained. The apparent (population averaged) temperatures for neutral atoms and ions are shown to be different in the plasmas generated with all the focusing distances. For each pulse energy (in the range 20–100 mJ), the plasmas generated with the optimum focusing distance, which corresponds to a constant value of irradiance, have been investigated. In these conditions, simple laws have been obtained for the variation of the plasma parameters with the pulse energy E: the electron density N_e and the apparent temperature T are independent of E while linear relations with E are obtained for the parameters N′l, αA. These simple laws lead to a quadratic dependence on E of the line intensities in the optically thin limit and to a variation of the intersection concentration C_int that characterizes self-absorption as E^− 1.     

Thomson scattering on argon surfatron plasmas at intermediate pressures: Axial profiles of the electron temperature and electron density

The axial profiles of the electron density n_e and electron temperature T_e of argon surfatron plasmas in the pressure range of 6–20 mbar and microwave power between 32 and 82 W have been determined using Thomson Scattering of laser irradiation at 532 nm. For the electron density and temperature we found values in the ranges 5 × 10¹⁸ < n_e < 8 × 10¹⁹ m^− 3 and 1.1 < T_e < 2.0 eV. Due to several improvements of the setup we could reduce the errors of n_e and T_e down to 8% and 3%, respectively. It is found that n_e decreases in the direction of the wave propagation with a slope that is nearly constant. The slope depends on the pressure but not on the power. Just as predicted by theories we see that increasing the power leads to longer plasma columns. However, the plasmas are shorter than what is predicted by theories based on the assumption that for the plasma-wave interaction electron–atom collisions are of minor importance (the so-called collisionless regime). The plasma vanishes long before the critical value of the electron density is reached. In contrast to what is predicted by the positive column model it is found that T_e does not stay constant along the column, but monotonically increases with the distance from the microwave launcher. Increases of more than 50% over 30 cm were found.     

Investigation of the Expansion of an Oxygen Microwave Remote Plasma for the Growth of Functional Oxide Thin Films

The expansion of an oxygen low-pressure microwave plasma was investigated in order to determine the optimal plasma parameters for the growth of functional oxide semiconductors. Langmuir probe measurements show that the electron density (n _e ) increases with the injected power up to a saturation value of 3.0 × 10⁹ cm^?3 determined at 10 mTorr while electron temperature (T _e ) remains constant at a value of 1.5 eV. When pressure is varied, n _e shows a maximum value at a range from 12 to 20 mTorr while T _e decreases monotonously with increasing pressure. In addition, both n _e and T _e decrease with the axial distance from the plasma source. These effects were discussed through the loss mechanisms in the remote plasma. For a pressure of 13 mTorr and at a substrate temperature of 500 °C, plasma enhanced oxidation of pure metallic Ti thin films lead to the formation of a pure TiO₂ anatase phase compared to a mixed phase of TiO₂ and TiO in the absence of plasma activation. For Mn thin films, the exposure to oxygen remote plasma led to the formation of MnO₂ as opposed to obtaining Mn₃O₄ when oxidation is performed in the oxygen gas ambient. Remote plasma processing was thus found to provide selective pathways to control oxidation states, stoichiometry and phase composition of technologically attractive oxide thin films.     

Characterization of laser-induced plasmas by emission spectroscopy with curve-of-growth measurements. Part I: Temporal evolution of plasma parameters and self-absorption

Laser-induced plasmas have been characterized by emission spectroscopy, including the measurement of curves of growth. The plasmas have been generated in air at atmospheric pressure using an infrared Nd:YAG laser from a set of Fe–Ni alloys with varying Fe concentrations. The procedure used provides, in addition to the apparent temperature T and electron density N_e, a parameter N′l (the atom number density for 100% concentration times the length of the plasma along the line-of-sight), relevant to obtain the self-absorption and the intensity of the emission lines. The temporal evolution of the plasma parameters has been deduced from the measurement and fitting of the curves of growth. A fast temporal decrease of N′l is obtained for ions, whereas a gradual increase takes place for neutral atoms. The temporal evolution of the line intensity in the optically thin limit and the self-absorption of neutral atom and ion lines have been obtained experimentally and calculated from the evolution of the plasma parameters. The usefulness of the curve-of-growth method in measurements with time integration, in spite of the fast variation of the plasma parameters, has been demonstrated.     

Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ? T_e ? 15000 K, 300 ? T_g ? 1000 K, 10¹¹ ? n_e ? 10¹² cm^?3,5 ? p ? 20 torr) increase with decreasing gas (T_g) and electron (T_e) temperatures and pressure (p) and with increasing electron density (n_e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.     

Measurements of Stark widths and shifts of Ne I lines using degenerate four-wave mixing and Thomson scattering methods

We report on measurements of Stark widths and shifts of four prominent Ne I lines of the 3s,3s′-3p transition arrays. The measurements were performed in an atmospheric-pressure arc discharge operated in argon–neon gas mixture.Sub-Doppler degenerate four-wave mixing technique was used to measure the line profiles, while Thomson scattering yielded the plasma parameters: electron density, n_e = (0.53–1.33) × 10²³ m^− 3, and electron temperature, T_e = 10,200–20,900 K. The measured profiles are symmetric within the uncertainty limits. The experimental Stark widths and shifts are compared with results of other experiments and theoretical calculations.     

The Existence of Non-negatively Charged Dust Particles in Nonthermal Plasmas

Particles in nonthermal dusty plasmas tend to charge negatively. However several effects can result in a significant fraction of the particles being neutral or positively charged, in which case they can deposit on surfaces that bound the plasma. Monte Carlo charging simulations were conducted to explore the effects of several parameters on the non-negative particle fraction of the stationary particle charge distribution. These simulations accounted for two effects not considered by the orbital motion limited theory of particle charging: single-particle charge limits, which were implemented by calculating electron tunneling currents from particles; and the increase in ion current to particles caused by charge-exchange collisions that occur within a particle’s capture radius. The effects of several parameters were considered, including particle size, in the range 1–10 nm; pressure, ranging from 0.1 to 10 Torr; electron temperature, from 1 to 5 eV; positive ion temperature, from 300 to 700 K; plasma electronegativity, characterized in terms of n ₊/n _e ranging from 1 to 1000; and particle material, either SiO₂ or Si. Within this parameter space, higher non-negative particle fractions are associated with smaller particle size, higher pressure, lower electron temperature, lower positive ion temperature, and higher electronegativity. Additionally, materials with lower electron affinities, such as SiO₂, have higher non-negative particle fractions than materials with lower electron affinities, such as Si.     

The use of different plasma gases (Ar, N 2 and air) for CMP-OES: figures of merit and temperature measurements

A comparative study of a 600 W capacitively coupled microwave plasma (CMP) operated with different plasma gases (Ar, N₂ and air) with respect to the achieved detection limits for Fe, Cr, Zn, Ca and Mg have been carried out. Radially and axially resolved rotational temperatures (T_rot), excitation temperatures (T_exc) and electron number densities (n_e) of these plasmas have been determined using OH (T_rot), Fe (T_exc) and Mg (n_e) as thermometric species. The influence of different gas flow rates on T_rot, T_exc and n_e, and of Li as an easily ionized element on T_exc has been investigated.     

Electron density spatial profiles of the DCP source

Electron densities are measured in the high current, analytical and intervening zones of a DCP whose operating parameters are systematically varied. Detailed N_e distribution profiles are obtained for various sleeve flow, nebulizer flow, arc current and matrix concentration regimes. Flowing argon is found to establish a thermal pinch in the high current zone and to steepen gradients in plasmas employed for spectrochemical analysis. The distinctive electron density distributions in the DCP are more sensitive to modulation of gas flow variables than to changes in arc current. Magnetic pressure has no discernible role in pinch formation. Electron densities in spectroscopic regions are minimally affected by easily ionized or other matrix constituents at usual analytical concentrations.     

Spatially resolved emission using a geometry-dependent system function and its application to excitation temperature profile measurement

As typical emission spectroscopy involves chord integration along the line of sight, a local measurement with high spatial resolution is attempted using simple lens optics in this work. In the experiment, chord integrated optical plasma emission profile was measured by moving a scanning lens located outside the plasma. The measured emission intensities were spatially reconstructed by employing a geometry-dependent system function, and the local (i.e., only from the lens focal point) emission intensities were obtained with all out-focused emissions subtracted. The 34 different Ar I emission lines spatially reconstructed in this way were used to determine excitation temperature (T_exc) of the argon plasma by the Boltzmann plot method. Being different from the plasma driven at 13.56 MHz where a rather uniform profile was obtained, the spatial profile of T_exc from the plasma driven at 90 MHz showed a hollow profile, which is similar to that of the electron temperature (T_e) measured by a Langmuir probe. This hollow profile is attributed from the electromagnetic phenomena such as skin effect and standing wave effect. The similar spatial tendency of T_exc and T_e implies that T_exc can be a representative of T_e. This is particularly useful for the cases in which conventional Langmuir probe measurements are limited, such as in large size plasmas.     

Plasma polymerization of tetrafluoroethylene. IV. Comparison of ethylene and tetrafluoroethylene by measurement of electron temperature and density of positive ions

The double probe method was applied to plasma of tetrafluoroethylene (TFE) and ethylene and the electron temperature (T_e) and density of positive ions (n_p) were measured at various discharge wattages. The probe current-probe voltage diagrams for TFE were different from those for ethylene. The shape of its diagram indicates that a considerable number of negative ions exist in TFE plasma. The levels of n_p for TFE were also nearly six times greater than those for ethylene at the same discharge current. The dependence of TFE polymer deposition and the chemical structure of the polymer, based on ESCA data on discharge current, was related to T_e and n_p measured by the probe method. The values of T_e and n_p may not be directly related to the polymer formation in a plasma; the method provides a direct measure of plasma energy density where plasma polymerization takes place, whereas it cannot be accurately estimated by the input energy of a discharge. It was found that plasma energy density based on (n_pT_e) for TFE plasma and that for ethylene differ significantly at the same level of input parameter (W/FM), where W is the discharge wattage, F is the volume flow rate, and M is the molecular weight of the monomer.     

Internal rotation and equilibrium structure of the 2-methyl-2-nitropropane molecule from joint processing of gas phase electron diffraction data,vibrational and microwave spectroscopy data,and quantum chemical calculation results

The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO₂ group. The equilibrium eclipsed configuration of the C _s symmetry molecule has the following experimental bond lengths and valence angles: r _e(N=O) = 1.226//1.226(8) Å, r _e(C–N)//r _e(C–C) = 1.520//1.515/1,521(4) Å, ∠_еC–C–N = = 109.1/106,1(8)°, ∠_еO=N=O = 124.2(6)°, ∠_eC–C–H_avg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C–C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.     

Internal Rotation and Equilibrium Structure of the Bromonitromethane Molecule According to Gas Electron Diffraction Data and Quantum Chemical Calculations

The structure and internal rotation of the bromonitromethane molecule are studied using electron diffraction analysis and quantum chemical calculations. The electron diffraction data are analyzed within the models of a general intramolecular anharmonic force field and quantum chemical pseudoconformers to account for the adiabatic separation of a large amplitude motion associated with the internal rotation of the NO₂ group. The following experimental bond lengths and valence angles are obtained for the equilibrium orthogonal configuration of the molecule with C_s symmetry: r_e(N=O) = 1.217(5) Å, r_e(C–N) = 1.48(2) Å, r_e(C–Br) = 1.919(5) Å, ∠_еBr–C–N = 109.6(9)°, ∠_еO=N=O = 125.9(9)°. The equilibrium geometry parameters are in good agreement with CCSD(T)/cc-pVTZ calculations. Thermally averaged parameters are calculated using the equilibrium geometry and quadratic and cubic quantum chemical force constants. The barrier to internal rotation cannot be determined reliably based on the electron diffraction data used in this work. There is a 82% probability that the equilibrium configuration with orthogonal C–Br and N=O bonds is most preferable, and internal rotation barrier does not exceed 280 cm^-1, which agrees with CCSD(T)/cc-pVTZ calculations.