Temperature Distributions Across the Plasma Layer of Planar Low Pressure Microwave Plasmas — A Comparative Investigation by Optical Emission Spectroscopy

Nowadays low temperature non-equilibrium plasmas received considerable attention in very different fields of plasma processing. The subject of the present paper is the comparative measurement of neutral gas temperature and optical excitation temperature to analyze the temperature distributions across the plasma layer of H₂ non-equilibrium plasmas (p = 0.2 – 1.5 kPa) with small admixtures of hydrocarbons in a novel planar microwave plasma source (2.45 GHz) used for plasmachemical deposition purposes by means of optical emission spectroscopy. Typical microwave power flux densities into the plasma lie within a range of 2 W cm^?2 to 20 W cm^?2. Results of neutral gas temperature measurements derived from H_α line Doppler profiles are compared with rotational temperatures of H₂ and N₂ molecules. The neutral gas temperature (800–1700 K) corresponds to the rotational temperature of the H₂ molecules (Fulcher band, R 0–0 branch) but shows a more distinct spatial gradient. The rotational temperature of admixtured N₂ molecules (2000–3000 K) is much more higher although Boltzmann distribution was ensured. The spatially resolved measured excitation temperature (1–3 eV) determined with the help of line intensity ratios of admixtured Ar well agrees with Langmuir probe measurements. The reported measurements as a whole demonstrate the feasibility of comparative investigations of different optically determined temperatures for expressive characterization of low pressure microwave plasmas.     

Rotational and Vibrational Temperature Measurements in a High‐Pressure Cylindrical Dielectric Barrier Discharge (C‐DBD)

The rotational (T_R) and vibrational (T_v) temperatures of N₂ molecules were measured in a high‐pressure cylindrical dielectric barrier discharge (C‐DBD) source in Ne with trace amounts (0.02 %) of N₂ and dry air excited by radio‐frequency (rf) power. Both T_R and T_v of the N₂ molecules in the C ³Π_u state were determined from an emission spectroscopic analysis the 2^nd positive system (C ³Π_u → B³Π_g). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both T_R and T_v also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N₂ to the Ne in the discharge resulted in higher T_R and T_v values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characteristics of an argon rf plasma: Active discharge and laminar sonic flow region

The electron density, the excitation and the gas temperatures have been obtained from optical spectrometric, microwave interferometric and thermocouple measurements in an argon rf plasma. The investigation was carried out in an active discharge and a channel region characterized by sonic laminar flow. Optical measurements of the excitation temperature were made in the 4000–5000 Å range for discharge pressures of ∼ 63–644 torr and input rf power of 42–60 kW. Excitation temperatures in the discharge ranged between ∼ 7000–10,000 K. Electron densities measured by optical and microwave techniques showed good agreement. Thermocouple measurements in the channel and N₂ rotational spectra from traces of N₂ injected in the argon, as well as gas dynamic considerations, indicated that the gas temperature in the discharge and the channel regions were 2900–4400K and 1900–3300K, respectively. These values were substantially lower than the excitation temperatures corresponding to these regions, indicating that the plasma was in a two-temperature state in both regions. Standard tests for local thermal equilibrium (LTE) showed that the first excited level of argon constituted the bottleneck level.     

Spectroscopic diagnostics of R.F. discharges at moderate pressure and chemical applications—I. Pure nitrogen

A vibro-rotational analysis has been performed of the second positive system (SPS) of N₂ and of the first negative system (FNS) of N⁺₂ emitted by 35 MHz discharges in flowing nitrogen operated at pressures of 5–50 torr and power densities of the order of 1–10 W-cm^-3. The distributions of the vibrational and of the rotational levels follow Boltzmann's law in both the SPS and the FNS with T_v = 4000°K and T_R = 2800°K for the N₂(C³Π_u) state and T_v = 5100°K and T_R = 5800°K for the N⁺₂(B²Σ⁺_g) state and independent of pressure. Kinetic gas temperatures are between 1200 and 2000°K and increase with pressure; electron temperatures are in the range 15,000–9,500°K. The reversal of line intensities of the SPS and of the FNS observed with increasing pressure has been tentatively interpreted. Possible chemical implications of these results have been examined.     

Absolute intensities for the Q-branch of the 3ν2-ν1 (465.161 cm) band of nitrous oxide

The absolute intensities of four lines, Q15–Q18 in the 03¹0–10⁰0 band, of N₂O have been measured using a tunable diode laser spectrometer at temperatures between 380 and 420 K and pressures between 4 and 15 torr. Even though these transitions are weak and produced only about 2% of absorption at the line center for a pathlength of 52m, they were measured with a signal to noise ratio of about 20 due to the high sensitivity of the instrument. The band strength derived is 1.03 × 10^-24cm molec^-1 at 296 K.     

Rotational structure of CN emission from active nitrogen flames

The CN violet bands from active nitrogen flames have been studied. Rotational line intensities and rotational temperatures have been measured for a number of bands (0–1, 4–6, 5–7, 11-11, 12-12, and 13-13) at pressures ranging from 3 to 30 torr. The bands with ν′ >10show an anomalous rotational intensity distribution, which fits a mixture of two temperatures (one around 1200–3000 K and another near room temperature); bands with ν′<7 show a single temperature of 300–400 K. The relaxation of the high temperature group with pressure yields a relaxation time of the order of 10^?7s for CN molecules in the B-state. Excitation mechanisms for CN(B²σ⁺) are discussed.     

Zur Asymmetrie der Reaktion139La(n,α)136Cs mit polarisierten Neutronen

Experimental and theoretical investigations have been performed to determine the thermal conductivity of hydrogen in the temperature range between 2000 and 7000 °K. For this purpose the radial temperature distributions for various currents and theE-I-characteristic of a low current wall-stabilized hydrogen arc have been measured. In the dark region of the arc outside the bright core the temperature and the thermal conductivity between 2000 and 4500 °K were found by means of the schlieren technique. The electron temperature in the core of the arc results from spectroscopic measurements. The gas temperature has been calculated with a formula, derived from the kinetic theory of gases. Assuming a constant collision integralQ _eH ¹¹ the radial distribution of electric conductivity has been calculated according to Langevin's formula. The valueQ _eH ¹¹ =30·10^?16 cm₂ results by comparing the integrated conductance with the measured one. Since now the radial distribution of power input and the temperatures are known, the thermal conductivity between 4500 and 7000 °K can be determined as well. The total course of the heat conductivity shows a strong peak at the temperature of 3740 °K characteristic for the dissociation process.     

Gas Density Distributions and Reduced Field Strenghts of the Positive Column of Low Pressure XeCl* Glow Discharges

Interferometric measurements of radial gas density distributions have been performed on the cylindrical positive column of DC low pressure glow discharges (LPGD) in pure Xe and Xe/Cl₂ gas mixtures. Absolute gas temperatures have been measured by thermocouples. In the mixtures, the gas temperature is several hundred Kelvin above the temperature in pure Xe. Additionally, the radial distribution of the gas density in the mixtures cannot be described by Bessel profiles, which would result from Schottky's diffusion theory. Combined with field strength measurements, radial profiles of E/N (electric field strength/neutral density) have been determined. Results of this work will be useful for model developments of LPGD in rare-gas/Cl₂ mixtures but also for the general understanding of the positive column in attaching gases.     

Intensity distribution in the rotational structure of the electron-vibrational bands of nitrogen

The intensity distribution was measured for the rotational lines of the N₂ molecule in the radiation of a glow discharge in the pure gas, at pressure 0.1–5 torr, or with argon, at a total pressure of 2 torr, at a current of 40 mA. The distribution found lags behind a Boltzmann distribution in the 0–3 band (corresponding to the C³-B³ transition; the second positive system) over the pressure range 0.1–2 torr and in the 1–4 band (the C³-B³ transition; second positive system) at a pressure of 0.1 torr. In the N₂ + Ar mixture there is selective amplification of the J=25, 26 rotational lines.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 17–21, September, 1970.     

Spectroscopic measurements of non-equilibrium nitrogen plasmas at moderate pressures

A local vibro-rotational analysis of the excited species, produced in a 35 MHz discharge reactor in flowing nitrogen, has been carried out by measuring radial and axial emission intensities of some vibrational sequences and selected rotational lines of the (0,2) band of the second positive system (SPS) of N₂ (C³Π_u - B³Π_g), in the pressure range 5–35 torr.Radial from lateral band or line emission intensities have been obtained by applying Abel's inversion technique to derive the corresponding vibrational and rotational temperatures with the use of Boltzmann plots. General maps of emission intensities and of vibrational and rotational temperature distributions within the reactor have been drawn.It is shown that a decrease of a factor 10³ in the emission intensities from the core to the periphery of the discharge corresponds to a variation in T_V and T_R of only a factor ?2. This result has been interpreted on the basis of the kinetic mechanisms for the excitation and deexcitation of chemical species under discharge conditions. The decrease of _V and the increase of T_R with increasing pressure are interpreted according to V-R (vibration-rotation) energy-transfer mechanisms.     

Rotational CARS N2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen

Rotational coherent anti‐Stokes Raman spectroscopy (CARS) in fuel‐rich hydrocarbon flames, with a large content of hydrogen in the product gases (∼20%), has in previous work shown that evaluated temperatures are raised several tens of Kelvin by taking newly derived N₂ H₂ Raman line widths into account. To validate these results, in this work calibrated temperature measurements at around 300, 500 and 700 K were performed in a cell with binary gas mixtures of nitrogen and hydrogen. The temperature evaluation was made with respect to Raman line widths either from self‐broadened nitrogen only, N₂ N₂ [energy‐corrected‐sudden (ECS)], or by also taking nitrogen broadened by hydrogen, N₂ H₂ [Robert–Bonamy (RB)], Raman line widths into account. With increased amount of hydrogen in the cell at constant temperature, the evaluated CARS temperatures were clearly lowered with the use of Raman line widths from self‐broadened nitrogen only, and the case with inclusion of N₂ H₂ Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T ∼ 300 K), 43 K (at T = 500 K) and 61 K (at T = 700 K) at the highest hydrogen concentration (90%). The results from this work further emphasize the importance of using adequate Raman line widths for accurate rotational CARS thermometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Laser-fluorescence diagnostics for condensation in laser-ablated copper plasmas

We are investigating the thermodynamic conditions under which condensation occurs in laser ablated copper plasma plumes. The plasma is created by XeCl excimer laser ablation (308 nm, 300 mJ/pulse) at power densities from 500–1000 MW/cm² into backing pressures of helium in the range 0–50 torr. We use laser-induced fluorescence (LIF) to probe velocity and relative density of both atomic copper and the copper dimer molecule, Cu₂, which is formed during condensation onset. At low pressure (10 mtorr), the atomic Cu velocity peaks at approximately 2×10⁶ cm/s. Copper dimer time-of-flight data suggest that condensation onset occurs after the Cu atoms have slowed very significantly. Excitation scans of the Cu₂A-X (0,0) and (1,1) bands yield a rotational and vibrational temperature in the neighborhood of 300 K for all conditions studied. Such low temperatures support the theory that Cu₂ is formed under thermally and translationally cold conditions. Direct laser beam absorption is used to determine the number density of atomic copper. Typical densities attained with 5 torr of helium backing gas are 6–8×10¹³ cm^–3. Rayleigh scattering from particulate is easily observable under conditions favorable to particulate production.     

Radiative Transitions and Temperature Time Dependences in Pulse Excited Microwave Discharges – (N2, Ar + N2)

Quantitative spectral and microwave measurements of vibrational temperatures and electron densities were performed for 2400 MHz non-isothermic pulse excited discharges in flowing nitrogen and argon at pressures (60–2700) Pa. A detailed analysis of the N₂ vibrational states population for the N₂ C³Π_u, X¹Σ_g⁺ electronic states has been carried out. The basic difficulties encountered when comparing the spectroscopically determined values of vibrational temperatures with corresponding quantities of the ground electronics state are mentioned and the time resolved dependences of the translational gas temperature in N₂ during the microwave pulse is evaluated. The steady state in the nitrogen pulse excited microwave plasma is reached within 3 · 10^?4 s, but generally, this time depends on the gas pressure in the discharge tube. In the Ar + N₂ mixtures the excitation conditions are complicated by the metastable argon atoms (3P_2,0) creating the nonequilibrium populations of electronic, vibrational and rotational N₂ states.     

Spectroscopic study of rotational energy distribution of BH (A 1Π) and electronic excitation temperature determination

Emission spectra of BH(A ¹Π-X ¹Σ⁺) system were recorded and studied using a low pressure (3.0 torr) arc in flowing hydrogen and argon + hydrogen mixture. The rotational distributions in theA ¹Π state determined from the intensities of rotational lines for the 0–0 band of theA-X system conforms to a Maxwellian distribution with effective rotational temperature of 1000 ± 50°K. Intensities of Balmer lines of hydrogen were measured and used to determine electronic excitation temperature which was found to be around 2000°K.     

Measurements of spatially resolved electron number densities and modes temperatures using optical emission spectroscopy of atmospheric pressure microplasma jet

The electrical gas discharge parameters of direct-current non-thermal microplasma jet in Ar-2%H₂ flow at open atmospheric air was investigated by using spatially resolved optical emission spectroscopy (OES). The jet was confined from microhollow of tungsten-carbide (～500 μm inner diameter) to a molybdenum foil. Despite its small volume, the atmospheric pressure microplasma jet provides a range of power densities, from low to ～10¹² W m^?3 generated either in rare gases or in molecular gases. A high resolution spectrometer (Jobin-Yvon, Czerny-Turner model THR1000, resolution of 0.001 nm, with focal length of 1.0 m and numerical aperture of 0.13 ? f/7.5) was used to allow registration of OH (A ²Σ⁺, ν = 0 → X ²Π, ν′ = 0) rotational bands at 306.357 nm, Ar I 603.213 nm line and N₂ (C ³Π _u , ν = 0 → B ³Π _g , ν′ = 0) second positive system with the band head at 337.13 nm in order to estimate the rotational temperature from the cathode sheath of the plasma jet to the anode. For currents ranging from 20 to 100 mA and for a particular excited levels, the excitation temperature was measured in the negative glow region either from a Boltzmann plot of Ar I 4p–4s and 5p–4s transitions of excited argon or using the Mo I (from 440 to 450 nm) two-lines method of excited Mo atoms sputtered from the cathode surface, giving 24 000 K (100 mA at 100 μm) and 7000 K (20 mA at 500 μm from the cathode). From the N₂ (C ³Π _u , ν = 0 → B ³Π _g , ν′ = 0) rotational transition the rotational temperature along the positive column was estimated. The vibrational temperature of the bulk plasma (1400 to 4500 K) was estimated for a current varying from 20 to 120 mA using the N₂ second positive system with Δν = ?2. Using the broadening of H _β Balmer line it was possible to estimate the electron number density of the negative glow (10¹⁴ to 10¹⁵ cm^?3) as a function of the current.     

Vibrational Excitation of N2(B,C) and N+2(B) States in N2 and N2 – H2 Flowing Microwave Discharges

The ro‐vibrational spectra of N₂ microwave discharges have been analysed by emission spectroscopy. It is deduced the rotational and vibrational temperatures of N₂ states. The characteristic of vibrational temperature Θ₁ of the N₂ (X, v) ground state has been specifically determined.It has been found that the N₂ (C, B, v') and N⁺₂ (B, v') radiative states are directly excited by electron collisions on the N₂ (X,v) ground state at a N₂ gas pressure of 0.1 Torr (discharge tube of 5 mm I.D, microwave power 100 Watt) with a Θ₁ value near 10⁴ K. At higher gas pressure up to 5 Torr, the N₂ (C, v') states remain alone to be mainly excited by electron collisions on N₂ (X, v). It is considered the excitations of the N₂ (B, v') states by collisions of electrons and N₂ (X,v > 4) vibrational molecules on the N₂ (A) metastable states.With x < 9% H₂ into N₂, it is observed an increase of N₂, 2^nd pos intensity, resulting of an increase of high energy electrons. Inversely, the N₂, 1^st pos intensity decreased, partly following the decrease of low energy electrons (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

N2第二正带系发射光谱的理论计算及实验研究

本文利用分子光谱理论系统的计算和分析N₂第二正带系(C³∏_u→B³∏_g)的发射光谱, 以研究光谱强度的分布规律与不同温度条件和气体条件的关系. 基于N₂的三重态能级结构特性, 重点计算和讨论了发射光谱的概个重要参数: 通过求解高、低电子态的哈密顿矩阵得到了振转能级特性; 利用r质心近似法求取了能级间跃迁的电偶极矩函数和爱因斯坦跃迁概率; 进而计算了不同振动和转动温度条件下谱线的强度分布. 进行了N₂和Ar的混合放电实验, 利用实验光谱数据同理论结果进行拟合分析, 确定了N₂分子的振动温度和转动温度分别约为4300 K和800 K. 另外由于潘宁离化效应, N₂浓度减小时谱线强度呈现先增强后减弱的趋势. 实验结果很好的验证了N₂第二正带系光谱理论计算的正确性.     

Rotational and vibrational temperatures of electronically excited BiN radicals in a chemiluminescent flame

Rotational and vibrational temperatures of electronically excited BiN radicals in a low-pressure Bi_x+N/N₂*/N₂+Ar chemiluminescent flame have been deduced from high-resolution Fourier-transform emission spectra. Bands of three electronic transitions, a³Σ⁺(a₁1)→X¹Σ⁺(X0⁺), b⁵Σ⁺(b₁0⁺)→X¹Σ⁺(X0⁺), and b⁵Σ⁺(b₁0⁺)→a ³Σ⁺(a₁1), were analysed to determine the optical temperatures in the a³Σ⁺(a₁1) and b⁵Σ⁺(b₁0⁺) states. The rotational temperatures characterising the rotational populations in the a₁1, v=0 and 1 states were determined from the a₁→X, 0-2, 0-3, 0-4, 1-1, and 1-2 bands. The b₁→X, 0-8 and 0-11 bands, and the b₁→a₁, 0-0 bands served to determine the rotational temperature of the radicals in the b₁0⁺, v=0 state. The temperatures derived from the various bands and transitions were well consistent and the mean rotational temperature was determined to be 353±18 K, which is close to the translational temperature of the gas.Vibrational temperatures of the radicals in the a₁1 and b₁0⁺ states were derived from band intensities of the a₁→X and from the b₁→X as well as b₁→a₁ systems, respectively. The Franck-Condon factors needed were calculated with RKR potentials deduced from literature values of the rotational and vibrational constants in the three states involved. The a₁1 vibrational temperature (336±21 K) was close to the rotational temperature, while the b₁0⁺ vibrational temperature (438±36 K) differed, likely due to the previously observed perturbation of the b₁0⁺ state.     

On the Rotational Distribution of N2 in the Ar?N2-Discharge. The O?O-Band of the Second Positive System

The rotational distribution of the O? O-band of the second positive system of N₂ was determined by the Ornstein method in the glow discharge of an argon-nitrogen mixture at medium pressure. Evidently, for the rotational R- and P-lines some straight lines can be fitted for rotational quantum numbers between 2 and 60 according to usual representation. The slopes of the curves indicate that the Boltzmann distribution does not exist for all the levels. For each of these straight lines can be declared a value of rotational temperature. The dependence of the several rotational temperatures on the discharge current is essential lower than the corresponding dependence of the gas temperature, which is calculated by means of the energy balance. The influence of the energy transfer from metastable argon atoms to ground-state nitrogen molecules on the rotational distribution seems to be insignificant. Comparisons with rotational distributions in pure nitrogen plasmas point at the influence of interactions between nitrogen species.     

Effect of atmospheric pressure pulsed discharge spatial inhomogeneity on Ar I and H I lines: Electron number density study

The spatial inhomogeneity of pulsed atmospheric pressure discharge in argon is investigated using the electron number density N_e diagnostics procedure applied to asymmetrically broadened Ar I lines. A dedicated fitting procedure is used for describing Ar I 703.0 nm line shape recorded from argon gas discharge and H I (at 486.13 and 656.28 nm) lines recorded from Ar-H₂ gas mixture discharge. The results revealed the change in N_e in both axial and radial directions. The additional Ar I lines at 614.5, 710.7, 731.2, and 731.6 nm, recorded from integral spatial radiation, are analysed as well to confirm the results from the plasma column region. The possibility of using AlO (B²∑⁺–X²∑⁺) and CN (B²∑⁺–X²∑⁺) molecular bands for gas temperature T_g measurements in this type of gas discharge source is demonstrated and T_g used as an input parameter for the N_e diagnostics procedure. For the proper identification of molecular band spectral lines, the Fortrat parabolas are constructed. The results obtained from Ar I 703.0 nm line indicate three different N_e values, with N_e1 ≈ 0.6 × 10¹⁶ cm⁻³, N_e2 ≈ 3.6 × 10¹⁶ cm⁻³, and N_e3 ≈ 19 × 10¹⁶ cm⁻³ measured from the plasma column. These N_e values increase in the cathode and anode region.