Atomic Excitation Temperature Research Articles

Spectral characteristics of laser-induced plasma generated on porous silicon produced by metal-assisted etching

Porous silicon (Si) produced by metal-assisted etching using nanoparticles (Si nanohole array with straight pores) has potential as a sample loading substrate of laser-induced breakdown spectroscopy (LIBS). In this work, we investigated the characteristics of laser-induced plasma generated on the porous Si with different pore depths (pore depth: approximately 180, 320, and 990 nm) from spectroscopic aspects. The intensity of Si I line was enhanced by using the porous Si instead of a flat Si, it increased with increasing the pore depth of porous Si, and its enhancement factor reached 45 (pore depth: approximately 990 nm). The atomic excitation temperature and atomic number density increased with increasing the pore depth, whereas the intensity ratio of Si II line to Si I line and the intensity of nitrogen I line decreased. These results provide an insight into the plasma formation on nanomaterials.

Analysis of pulse-to-pulse fluctuation in underwater Laser-Induced Breakdown Spectroscopy on the basis of error propagation calculation

We study the pulse-to-pulse variation of the emission spectral intensity of underwater laser-induced breakdown spectroscopy. Emission spectral intensity and its dispersion were measured as a function of the fluence of the laser pulse at a metal target surface immersed in water. The coefficient of variation, which is an index of the dispersion, showed a minimum at a certain fluence. The dispersion at the low fluences was attributed to the variations of the population density and the atomic excitation temperature, according to the error propagation analysis of the theoretical spectral line intensity based on the Boltzmann distribution. The population density and the temperature showed a negative correlation, which is consistently explained by the unstable division of pulse energy into two parts, i.e., the early part of a pulse is attributed to materials ablation and hence to the population density, while the other part to plasma heating and hence to the temperature.

Optical emission spectroscopy of non-equilibrium microwave plasma torch sustained by focused radiation of gyrotron at 24 GHz

The excitation and gas temperature of a non-equilibrium argon plasma torch sustained by a focused 24 GHz microwave beam in the external atmosphere of air was measured by optical emission spectroscopy. The excitation temperature of argon atoms was found about 5000 K, while the vibrational and rotational temperatures of ambient nitrogen molecules were respectively estimated at 3000 K and 2000 K. The effective mixing of the external gas atmosphere into the plasma torch is experimentally demonstrated. The vibrational and rotational temperatures of nitrogen molecules slightly increased with the distance from the nozzle exit. Experimental results demonstrate that microwave discharge at atmospheric pressure may be relevant for the decomposition of highly stable molecules, e.g. carbon dioxide, in non-equilibrium conditions.

Non-equilibrium Atmospheric-Pressure Plasma Torch Sustained in a Quasi-optical Beam of Subterahertz Radiation

This paper studies a continuous atmospheric-pressure discharge maintained by focused subterahertz radiation of a 263 GHz gyrotron. A plasma torch propagating towards microwave radiation was sustained on an argon flow exiting a metal gas tube in the surrounding atmosphere of air. Using a high-speed camera, the spatial structure of the torch was studied and the temporal dynamics on timescales of the order of 20 ns was examined. The emission spectra of the optical wavelength range were used to estimate the excitation temperature of argon atoms and electron density. It was shown that a discharge of this type is substantially non-equilibrium and the electron temperature is many times higher than the vibrational and gas temperatures, being 1.5-1.7 eV in order of magnitude. The electron density measured by the Stark broadening hydrogen lines exceeds a critical value for the frequency of the heating field.

Local thermodynamic equilibrium in a laser-induced plasma evidenced by blackbody radiation

We show that the plasma produced by laser ablation of solid materials in specific conditions has an emission spectrum that is characterized by the saturation of the most intense spectral lines at the blackbody radiance. The blackbody temperature equals the excitation temperature of atoms and ions, proving directly and unambiguously a plasma in local thermodynamic equilibrium. The present investigations take benefit from the very rich and intense emission spectrum generated by ablation of a nickel-chromium-molybdenum alloy. This alternative and direct proof of the plasma equilibrium state re-opens the perspectives of quantitative material analyses via calibration-free laser-induced breakdown spectroscopy. Moreover, the unique properties of this laser-produced plasma promote its use as radiation standard for intensity calibration of spectroscopic instruments.

Tomographic optical emission spectroscopy of a high enthalpy air plasma flow.

A method is presented allowing for locally resolved emission spectroscopy using a tomographic setup. The approach presented in this work is applied to a high enthalpy air plasma flow. The resulting data sets allow for a three-dimensional (3D) representation of the non-symmetric flow field using photographs of the test section and 2D representation of the spectrally resolved radiance of the flow field. An analysis of different exposure times shows that transient fluctuations of the plasma can result in substantial asymmetry that approaches symmetry only for longer exposure times when the temporal averaging of the emission is significant. The spectral data allows the analysis of species selective excitation and emission. A non-equilibrium between atomic and molecular excitation temperatures is concluded for the investigated air plasma flow field. The spatial distribution of atomic electronic excitation temperatures are close to rotational symmetry while molecular rotational and vibrational temperatures exhibit asymmetric behavior.

Comparison of excitation mechanisms in the analytical regions of a high-power two-jet plasma

Excitation mechanisms in the analytical regions of a high-power two-jet plasma were investigated. A new plasmatron recently developed was applied in this work. The Boltzmann population of excited levels of Fe atoms and ions was observed in both analytical regions, before and after the jet confluence, as well as in the jet confluence, which proves excitation of atoms and ions by electron impact. The disturbance of local thermodynamic equilibrium in all regions of the plasma flow was deduced on the basis of considerable difference in Fe atomic and ionic excitation temperatures. Such a difference is most likely to be caused by contribution of metastable argon to atom ionization. The region before the jet confluence has the greatest difference in Fe atomic and ionic excitation temperatures and is more non-equilibrium than the region after the confluence due to comparatively low electron and high metastable argon concentrations. Low electron concentration in this region provides lower background emission than in the region after the jet confluence, which leads to better detection limits for the majority of elements.

Spectroscopic measurements of plasma temperatures and electron number density in a uranium hollow cathode discharge lamp

The plasma parameters including neutral species temperature, atomic excitation temperature and electron number density in a see-through type, homemade uranium hollow cathode discharge lamp with neon as a buffer gas have been investigated using optical emission spectroscopic techniques. The neutral species temperature has been measured using the Doppler broadening of a neon atomic spectral line. The atomic excitation temperature has been measured using the Boltzmann plot method utilizing uranium atomic spectral lines. The electron number density has been determined from the Saha-Boltzmann equation utilizing uranium atomic and ionic spectral lines. To the best of our knowledge, all these three plasma parameters are simultaneously measured for the first time in a uranium hollow cathode discharge lamp.

Investigation of local thermodynamic equilibrium in laser-induced plasmas: Measurements of rotational and excitation temperatures at long time scales

We studied the rotational temperature of diatomic molecules in the context of laser induced plasma from a solid target. In particular, its temporal evolution is investigated at long time scales (≥30μs). The measured values are compared to ionic and atomic excitation temperatures and the issue of local thermodynamic equilibrium is discussed. The investigation was carried out using an aluminium oxide (Al2O3) target doped with titanium (Ti) and iron (Fe). The ionic and the atomic excitation temperatures are deduced from the Ti II lines and the Fe I lines respectively. For the molecular temperature, a temporally resolved study of the aluminium monoxide (AlO) blue-green spectrum was carried out. We show that underthese experimental conditions, a complete thermodynamic equilibrium is not reached for up to 50μs after the laser pulse. The plasma is identified as cold plasma, with two different temperatures: the electron kinetic temperature and the heavy species kinetic temperature.

Investigation of Thermal Nonequilibrium in a Plasma Arc Welding Process: Modeling and Diagnostics

In this paper, a plasma arc welding (PAW) process operating in pure Ar with constant current at 40 A has been characterized by means of a thermofluid-dynamic modeling and optical emission spectroscopy (OES). Excitation temperature of Ar atoms in the fringes of the arc measured using the OES has been found higher than temperature calculated from local thermodynamic equilibrium modeling, even if radiation reabsorption is taken into account using a discrete ordinate model; on the contrary, excitation temperature of Ar atoms is in a good agreement with electron temperature calculated using a two-temperature model for thermal nonequilibrium neglecting radiation reabsorption. Results reported suggest that for the investigated PAW process, thermal nonequilibrium due to steep radial temperature gradients induces a Boltzmann distribution of excited states at the electron temperature.

Liquid sampling-atmospheric pressure glow discharge as a secondary excitation source: Assessment of plasma characteristics

The liquid sampling-atmospheric pressure glow discharge (LS-APGD) has been assessed as a secondary excitation source with a parametric evaluation regarding carrier gas flow rate, applied current, and electrode distance. With this parametric evaluation, plasma optical emission was monitored in order to obtain a fundamental understanding with regards to rotational temperature (Trot), excitation temperature (Texc), electron number density (ne), and plasma robustness. Incentive for these studies is not only for a greater overall fundamental knowledge of the APGD, but also in instrumenting a secondary excitation/ionization source following laser ablation (LA). Rotational temperatures were determined through experimentally fitting of the N2 and OH molecular emission bands while atomic excitation temperatures were calculated using a Boltzmann distribution of He and Mg atomic lines. The rotational and excitation temperatures were determined to be ~1000K and ~2700K respectively. Electron number density was calculated to be on the order of ~3×1015cm−3 utilizing Stark broadening effects of the Hα line of the Balmer series and a He I transition. In addition, those diagnostics were performed introducing magnesium (by solution feed and laser ablation) into the plasma in order to determine any perturbation under heavy matrix sampling. The so-called plasma robustness factor, derived by monitoring Mg II/Mg I emission ratios, is also employed as a reflection of potential perturbations in microplasma energetics across the various operation conditions and sample loadings. While truly a miniaturized source (<1mm3 volume), the LS-APGD is shown to be quite robust with plasma characteristics and temperatures being unaffected upon introduction of metal species, whether by liquid or laser ablation sample introduction.

Consideration on excitation mechanisms in a high-power two-jet plasma

The study of excitation mechanisms in the region before the jet confluence of a high-power two-jet plasma used for analysis of different powders has been undertaken. Distribution of excited levels of Fe atoms and ions according to the Boltzmann population was found. Measuring Fe atomic and ionic excitation temperatures showed their considerable difference (≈2000–2500K). The effect of argon on line intensities of a wide range of elements was investigated by the experiment with argon covering. A negligible effect of argon covering on line intensities of atoms with ionization energy of <8eV allows one to assume their predominant excitation by electron impact. The argon participation in excitation of atoms having ionization energy of >8eV was revealed. This is likely to be due to Penning ionization by metastable argon followed by ion recombination with an electron and stepwise de-excitations. A more pronounced effect of argon covering was observed for ionic lines of investigated elements with total excitation energy ranging from 11 to 21eV. Penning ionization followed by electron impact is believed to be a probable mechanism for ion excitation. The contribution of metastable argon to excitation processes results in departure from local thermodynamic equilibrium and different atomic and ionic excitation temperatures.

Two-temperature modelling and optical emission spectroscopy of a constant current plasma arc welding process

In this work, a plasma arc welding process with constant current in the range 25–70 A operating in pure Ar is characterized by means of both thermo-fluid-dynamic modelling under the assumption of local thermodynamic equilibrium (LTE) and two-temperature thermal non-equilibrium modelling (2T), allowing a comparison of the LTE temperature fields with electron and heavy particle temperature fields: thermal non-equilibrium is strongest in the fringes of the arc and upstream the plasma flow even though a temperature difference between electrons and heavy particles is also found in the arc core in the nozzle orifice, due to the high velocity of the gas. Also, excitation temperature of Ar atoms is obtained from optical emission spectroscopy measurements using a new method (called hybrid method) that extends the usability of the Boltzmann plot method to spatial regions where the signal-to-noise ratio of the spectral lines adopted in the calculation is poor. Good agreement is obtained between the modelling predicted electron temperature and the measured excitation temperature in the whole investigated spatial region.

Diagnostics of nonuniform plasmas for elemental analysis via laser-induced breakdown spectroscopy: demonstration on carbon-based materials

We investigate the plasma produced by the interaction of ultraviolet nanosecond laser pulses with a carbon fiber composite tile from the inner wall of a fusion reactor. The experiments are carried out in argon at a pressure of 5 × 104 Pa. Fast imaging is used to characterize the plume expansion dynamics and excited plasma species are followed by time- and space-resolved emission spectroscopy. The measurements show that ionized and highly excited plasma species are located in the front of the expanding plume, whereas neutral atoms and lower excited species dominate in the plasma core. By measuring the excitation temperature of metallic ions and neutral atoms, we evidence the existence of a temperature gradient that appears during the early expansion stage and remains up to delays typically used in material analysis via laser-induced breakdown spectroscopy. The knowledge of the spatial distribution of the plasma properties is then used to model the plasma emission spectrum. Assuming two plasma zones in local thermodynamic equilibrium of different temperatures and electron densities, we calculate the spectral radiance and compare it to the spatially integrated spectrum recorded with an Echelle spectrometer. From the best agreement between measured and computed spectra we deduce the elemental concentrations of carbon, hydrogen and metal impurities. The validity of the model is critically discussed and the measurement uncertainties are evaluated. The present approach is foreseen to improve the accuracy of analysis via laser-induced breakdown spectroscopy in many applications, in particular when materials of elements with significantly different ionization potentials are investigated.

Atomic gas temperature in a nonequilibrium high-intensity discharge lamp determined from the red wing of the resonance mercury line 254 nm

For developing low-wattage high intensity discharge (HID) lamps, a better understanding of the relatively unexplored nonequilibrium phenomena is essential. This needs interpretation of diagnostic results by methods free from equilibrium assumptions. In this paper, the atomic temperature is determined from the simulation of a quasistatic broadened resonance line by distinguishing between atomic temperature and excitation temperature in the equation of radiative transfer. The proposed method is applied to the red wing of the resonance mercury line 254 nm emitted from a HID lamp working on ac. The experimental results show severe deviation from local thermodynamic equilibrium. More than one thousand degrees difference was obtained between atomic and electron temperatures at the maximum current phase.

Inner Wall Treatment of a Plastic Microfluidic Chip using Pulse Microdischarge

A pulsed corona μ plasma was used to modify the surfaces of microchannel walls of commercial cyclic olefin copolymer microfluidic chips. Pt wire electrodes were inserted into the reservoirs of a chip in order to develop surface discharge along the microchannel inner wall. He, Ar and room air μ plasmas were generated throughout a 60-mm-long microchannel at a crude vacuum of 0.8-30 kPa using a discharge pulse spacing of 5 ms, a pulse duration of 0.7 ms and Vpp of 8-9 kV. He and Ar atomic excitation temperatures estimated using Boltzmann plots were approximately 9,600 K and 23,700 K, respectively, whereas the gas remained at room temperature. The plasma-treated microchannels exhibited highly hydrophilic properties. X-ray photoelectron spectroscopy revealed that the oxygen-containing polar groups such as -C-O-, C=O and -COOH were introduced into the plasma-treated inner walls. Furthermore, microchannels treated by such energetic plasma showed no damage and had smooth surfaces.

Capacitively Coupled Capillary Plasma Sources

Capacitively coupled capillary Ar and He plasmas have been generated in a quartz tube with the inner/outer diameters of 1.0/3.0 mm at the RF (13.56 MHz) powers of 3-7 W using the externally attached parallel-plate electrodes. Plasma expansion along a tube axis was a maximum at a gas pressure of 10-20 kPa, in which Vpp was a minimum and so Paschen's minimum condition was satisfied. The capillary plasma was magnetized by a strong magnetic field of 0.4 T to sustain the plasma at a low pressure, since the plasma did not ignite at the gas pressure below 2-4 kPa. The optical emission spectrum analysis revealed that atomic excitation temperature increased with decreasing gas pressure. Furthermore, it was found that the metastable atoms play an important role in the plasma sustenance at a high gas pressure.

Magnetized microplasmas generated in a narrow quartz tube

Magnetized He and Ar microplasmas in a narrow quartz tube having an inner diameter of 1 mm (referred to as magnetized capillary microplasmas) are reported. A capillary microplasma can be magnetized by the occurrence of a radio frequency (rf) oscillating E×B drift motion along the tube axis, provided that the external magnetic field is perpendicular to both the rf electric field and the tube axis and that the Larmor radius of an electron is sufficiently smaller than both the electron mean free path and the tube radius. When a magnetic flux density of 0.4 T was applied, a magnetized capillary microplasma could be generated at gas pressures lower than 1.5 kPa because the electron cyclotron frequency exceeds the electron-neutral collision frequency. However, plasma ignition at low gas pressure below 4 kPa was hardly taken place without a strong magnetic field. The Ar atomic excitation temperature was estimated by optical emission spectroscopy and found to increase dramatically from 5000 to 15 000 K when the gas pressure was reduced from 4 to 0.2 kPa. This implies an increase in the electron temperature. Furthermore, Ar ionic emission (Ar II) lines were clearly observed under the magnetized plasma conditions.

Spectral analysis of the light emitted from streamers in chlorinated alkane and alkene liquids

We have studied the time-averaged optical emission from fast positive and negative non-breakdown streamers under pulsed divergent field conditions in five chlorocarbon liquids, namely, dichloromethane, 1,2-dichloroethane, tetrachloromethane, trichloroethene and tetrachloroethene. We have accumulated light emitted from the first 10–15 µm trail of a few thousand streamers. We have also briefly studied single breakdown arcs in tetrachloromethane. Atomic lines of hydrogen, chlorine and carbon as well as excited states of C2 radicals (Swan bands) have been observed, with sufficient resolution for evaluating line and band-shapes. The characteristic broadening, shift and asymmetry of atomic lines varied significantly between the liquids. Differences between the two streamer polarities were comparatively small. Densities of electrons and neutrals in the illuminated phase have been deduced from broadening of atomic lines, atomic excitation temperatures from absolute line intensities and rotational and vibrational temperatures from the Swan bands. The gas densities of the propagating streamers were generally very high (∼10% of critical) and with a high degree of ionization (∼1‰). Dichloromethane and 1,2-dichloroethane produced re-illuminating streamers with densities close to atmospheric conditions, in agreement with a rapid pressure relaxation. Rotational temperatures were high and in the range 2 × 103–6 × 103 K for the different liquids. Results can be interpreted to suggest a partial local thermodynamic equilibrium in the streamer plasmas.

Characteristics of atmospheric pressure air discharges with a liquid cathode and a metal anode

Electrical and optical emission properties of a burning plasma between a liquid cathode and a metal anode are presented in this paper. The plasma has constricted contact points at the liquid cathode and is clearly filamentary in nature near the water surface.The cathode voltage drop depends on conductivity rather than pH and is significantly different for distilled water and electrolyte solutions. An acidification of the liquid due to the plasma is always observed.The rotational temperature of OH and N2 in the bulk of the plasma is, respectively, in the range 3200–3750 K and 2500–2750 K. The rotational temperature of nitrogen near the metal anode is typically two times smaller. Electron densities near the cathode measured by Stark broadening of Hβ are in the range (5.5–8.0) × 1014 cm−3, the atomic excitation temperatures in the range 5750–7250 K. Differences in electrical and optical emission properties between the cases when distilled water and electrolyte solutions are used as cathode are discussed in detail.