Characterization of laser induced breakdown plasmas used for measurements of arsenic,antimony and selenium hydrides

Studies have been performed to characterize laser induced breakdown spectroscopy (LIBS) plasmas formed in Ar/H₂ gas mixtures that are used for hydride generation (HG) LIBS measurements of arsenic (As), antimony (Sb) and selenium (Se) hydrides. The plasma electron density and plasma excitation temperature have been determined through hydrogen, argon and arsenic emission measurements. The electron density ranges from 4.5 × 10¹⁷ to 8.3 × 10¹⁵ cm^?3 over time delays of 0.2 to 15 μs. The plasma temperatures range from 8800 to 7700 K for Ar and from 8800 to 6500 K for As in the HG LIBS plasmas. Evaluation of the plasma properties leads to the conclusion that partial local thermodynamic equilibrium conditions are present in the HG LIBS plasmas. Comparison measurements in LIBS plasmas formed in Ar gas only indicate that the temperatures are similar in both plasmas. However it is also observed that the electron density is higher in the Ar only plasmas and that the emission intensities of Ar are higher and decay more slowly in the Ar only plasmas. These differences are attributed to the presence of H₂ which has a higher thermal conductivity and provides additional dissociation, excitation and ionization processes in the HG LIBS plasma environment. Based on the observed results, it is anticipated that changes to the HG conditions that change the amount of H₂ in the plasma will have a significant effect on analyte emission in the HG LIBS plasmas that is independent of changes in the HG efficiency. The HG LIBS plasmas have been evaluated for measurements of elements hydrides using a constant set of HG LIBS plasma conditions. Linear responses are observed and limits of detection of 0.7, 0.2 and 0.6 mg/L are reported for As, Sb and Se, respectively.     

Optical emission spectroscopy of electron-cyclotron-resonance-heated helium mirror plasmas

In this experiment emission spectroscopy in the 3000–5000 Å range has been utilized to determine the electron temperature (15–60 eV) and ion density (2–5 x 10¹¹ cm^–3) of helium plasmas produced by the Michigan mirror machine⁽¹⁾ (MIMI). The plasma is generated and heated by whistler-mode electron-cyclotron resonance (ECR) waves at 7.43 GHz with 400–900 W power in 80-ms-long pulses. Gas fueling is provided at the midplane region by a leak valve with a range in pressure of 3 x 10 to 2 x 10⁴ Torr. Emission line intensities are interpreted using a model of the important collisional and radiative processes occurring in the plasma. The model examines secondary processes such as radiation trapping, excitation transfer between levels of the carne principle quantum number, and excitation front metastable states for plasmas in the parameter range of MIMI (n _c = 1–6 x 10¹¹ cm^–3). Front the analysis of line intensity ratios for neutral helium, the electron temperature is measured and its dependence upon the gas pressure and microwave power is determined. These temperatures agree with those obtained by Langmuir probe measurements. Art analysis of the line intensity ratio between singly ionized helium and neutral helium yields a measurement of the ion density which is in good agreement with electron density measurements made by a microwave interferometer.     

Kinetics of ultrasound‐assisted esterification of maleic acid and butanol using heterogeneous catalyst

The present research investigates the kinetics of ultrasound‐assisted synthesis of dibutyl maleate using a heterogeneous catalyst (Amberlyst‐15) in solvent‐free system. Reaction parameters were optimized based on conversion by varying the various parameters such as n‐butanol to maleic acid mole ratio, temperature, molecular sieves, catalyst loading, power, and duty cycle. Optimization of parameters resulted in 56.2% yield at 343 K, alcohol to acid mole ratio as 4:1, catalyst loading of 4%, molecular sieves of 4% with an ultrasound power input of 100 W with 60% duty cycle and 22 kHz frequency. In the presence of ultrasound, the reaction time reduced to 120 min in comparison with 240 min of the conventional process. The experimental kinetic data were correlated using Pseudo‐Homogeneous model as well as heterogeneous models like Eley‐Rideal model and Langmuir‐Hinshelwood‐Hougen‐Watson (LHHW) model based on single as well as dual‐site mechanisms. LHHW (reactants and products) model was found to be the best fit. The results proved that the reaction follows second‐order kinetics. The activation energy of the reaction was calculated as 14.64 kJ/mol.     

On the determination of plasma electron number density from Stark broadened hydrogen Balmer series lines in Laser-Induced Breakdown Spectroscopy experiments

In this work, different theories for the determination of the electron density in Laser-Induced Breakdown Spectroscopy (LIBS) utilizing the emission lines belonging to the hydrogen Balmer series have been investigated. The plasmas were generated by a Nd:Yag laser (1064 nm) pulsed irradiation of pure hydrogen gas at a pressure of 2 · 10⁴ Pa. H_α, Η_β, Η_γ, Η_δ, and H_ε Balmer lines were recorded at different delay times after the laser pulse. The plasma electron density was evaluated through the measurement of the Stark broadenings and the experimental results were compared with the predictions of three theories (the Standard Theory as developed by Kepple and Griem, the Advanced Generalized Theory by Oks et al., and the method discussed by Gigosos et al.) that are commonly employed for plasma diagnostics and that describe LIBS plasmas at different levels of approximations. A simple formula for pure hydrogen plasma in thermal equilibrium was also proposed to infer plasma electron density using the H_α line. The results obtained showed that at high hydrogen concentration, the H_α line is affected by considerable self-absorption. In this case, it is preferable to use the H_β line for a reliable calculation of the electron density.     

Measurement of electron densities by a microwave cavity method in 13.56-MHz RF plasmas of Ar,CF4, C2F6, and CHF3

Electron densit ies have been determined /or RF plasmas that were generated within a microwave resonant cavity by measuring the difference of the resonance frequencies with and without plasma. Since that method only yields a value of the electron density weighted ouer the microwave electric field distribution, to obtain real values an assumption on the spatial distribution of the electron density had to he made. Spatial profiles were taken of the emission of a 4s–5p Ar line at 419.8 not (with a small Ar admixture). The electron densities have been determined as a function of pressure and RF power in Ar, CF₄, C₂ F₆ and CHF, plasmas. The results indicate that the electron density for the last three gases decreases as a function of pressure above 50 m Torr. Typical values for the electron density for the investigated parameter range are 1–6 · 10³ cm^–3. Furthermore, the electron density is the lowest in gases with a high attachment cross .section.     

Features of the Current Sustainment in a Low-Current Discharge in Airflow

The paper relates to the investigations of a low-current discharge in a vortex airflow with the electrode configuration corresponding to classical coaxial plasmatron. The gas flow rate is varied from 0.1 to 0.3 g/s at an inner diameter of the plasmatron nozzle of 5 mm. The discharge is powered by dc voltage via a ballast resistor. Typical averaged current is changed from 0.06 to 0.15 A so that a maximum averaged power dissipated in the discharge amounts to 160 W. In these conditions, a luminous gas region at the plasmatron exit, which in most publications is associated with a plasma jet, is observed. The method for the jet diagnostics based on a usage of the additional electrodes at the plasmatron exit has been proposed. The main idea of the experiments is the elucidation of the problem whether the jet actually represents the plasma area or we have to apply the term “plasma” with care. In particular, in the case under discussion the main charged particles in the jet are electrons that are emitted from a plasma column located in the plasmatron nozzle. The model that describes the formation of electron flow in the jet has been proposed. Typical electron density in the jet estimated with a usage of the model is at a level of 10⁹ cm^?3.

     

Ultrasound-assisted enzymatic degradation of naproxen

This work describes the degradation of synthetic water containing an elevated NAP concentration using the ultrasound and commercial Laccase enzyme (LT-100) by optimizing the system's parameters, such as ultrasound power, % duty cycle, enzyme concentration, and thermal environment. The established optimum condition obtained at a temperature of 40 °C, enzyme concentration 0.15% (w/v), ultrasound power 50 W, and 50% duty cycle of 10 min resulted in extreme NAP degradation of 96% in a time of 150 min. A noticeable improvement (67%) in the degradation of NAP was observed with enzyme and ultrasound than in the ultrasound only. An ultrasound-assisted enzymatic reaction showed a significant synergy effect under US irradiation, as evidenced by synergetic index values ranging from 0.873 to 1.841 for NAP. Radical scavenging experiments and LC/MS analysis revealed that hydroxylation, demethylation, and decarboxylation are the main chemical mechanisms involved in the degradation of NAP. The kinetic study showed that the degradation of NAP follows Michaelis Menten's kinetics having Vmax and Km as 3.3 μM/min and 18.3 μM, respectively. Low Km finding indicates there is much more enzyme compatibility towards the substrate. Furthermore, a toxicity study conducted on Naproxen revealed that the solutions obtained after the process exhibited approximately 80–85% less toxicity compared to the initial naproxen solutions.     

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.     

New power source from fractional quantum energy levels of atomic hydrogen that surpasses internal combustion

Extreme ultraviolet (EUV) spectroscopy was recorded on microwave discharges of helium with 2% hydrogen. Novel emission lines were observed with energies of q·13.6 eV where q=1,2,3,4,6,7,8,9, or 11 or these lines inelastically scattered by helium atoms wherein 21.2 eV was absorbed in the excitation of He (1s²) to He (1s¹2p¹). These lines were identified as hydrogen transitions to electronic energy levels below the ‘ground’ state corresponding to fractional quantum numbers. Significant line broadening corresponding to an average hydrogen atom temperature of 33–38 eV was observed for helium–hydrogen discharge plasmas; whereas pure hydrogen showed no excessive broadening corresponding to an average hydrogen atom temperature of ≈3 eV. Since a significant increase in H temperature was observed with helium–hydrogen discharge plasmas, and energetic hydrino lines were observed at short wavelengths in the corresponding microwave plasmas that required a very significant reaction rate due to low photon detection efficiency in this region, the power balance was measured on the helium–hydrogen microwave plasmas. With a microwave input power of 30 W, the thermal output power was measured to be at least 300 W corresponding to a reactor temperature rise from room temperature to 900 °C within 90 s, a power density of 30 MW/m³, and an energy balance of about −4×10⁵ kJ/mol H₂ compared to the enthalpy of combustion of hydrogen of −241.8 kJ/mol H₂.     

Spectral line selection for time-resolved investigations of laser-induced plasmas by an iterative Boltzmann plot method

The aim of this work is to provide a procedure to determine time-resolved electron temperatures with minimized relative errors by the Boltzmann plot method. The applied procedure consists of two parts, a systematic theoretical spectral line selection and an iterative Boltzmann plot algorithm. After a pre-selection of an appropriate non-disturbed or overlapped set of spectral lines of a particular atomic or ionic species Boltzmann plots are generated using experimentally recorded data for every time window and laser pulse energy of interest. Spectral lines with the highest average deviations from the regression function are assumed as being not representative for the considered ensemble of spectral lines and are therefore discarded gradually until a threshold value for the coefficient of determination is exceeded. Laser-induced breakdown spectroscopy (LIBS) is applied for time-resolved and spatially integrated investigations of plasmas on 1.1750 C75 steel alloy samples with laser pulse energies ranging between 200 µJ and 2 mJ. For the specific chemical composition of these samples a selection of atomic and ionic Fe spectral lines has been carried out. In spite of the fact that only laser pulse energies in the low millijoule regime are applied the final sets of spectral lines comprise in total 61 Fe I and 12 Fe II emission lines. By applying this method electron temperatures can be determined with averaged relative errors of down to 1.8% for Fe I and 4.4% for Fe II emission lines.     

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.     

The calculation of electron density and temperature in Ar spectroscopic plasmas from continuum and line spectra

The measurement of electron temperature and electron density from line and continuum radiation in Ar plasmas used for spectroscopic purposes is discussed. The correct equations for the calculation of these parameters from experimentally observable quantities are identified. Such expressions yield the theoretical spectral irradiance expected from line and continuum radiation. To aid calculations, grouped constants from these equations are calculated using SI units of measure. The sources of accurate values for the parameters and correction factors appearing in the irradiance equations are listed. In addition the approximations made in order to simplify electron temperature and electron density determinations are reviewed. Practical aspects of making accurate measurements of these parameters in Ar plasmas are considered. Theoretical plots of line to continuum emission ratio vs electron temperature and of continuum irradiance vs electron density are presented as an illustration using parameters appropriate for Ar emission at 430 nm and the spectrally adjacent continuum.     

Application of clear polymethylmethacrylate dosimeter Radix W to a few MeV electron in radiation processing

Characteristics of clear PMMA dosimeter (Radix W) were studied for electron irradiation and compared with the response for gamma irradiation. The dose–response curves were nearly linear up to 30 kGy and become sublinear at higher doses. The radiation-induced absorbance was reduced with 6% within 4 h after irradiation. Dose comparisons were performed for 2, 3, 4 and 5 MeV electron irradiation using cellulose triacetate dosimeter (CTA) (FTR-125) and Radix W dosimeters which were independently calibrated for 2 MeV electrons and ⁶⁰Co gamma-rays using calorimeter and ionizing chamber, respectively. The doses estimated by CTA and Radix W were different by about 20%. The magnitude of this difference was, however, independent of electron energy.     

Spectroscopic characterization of a neon surface-wave sustained (2.45 GHz) discharge at atmospheric pressure

In this paper a spectroscopic study of a microwave (2.45 GHz) neon surface-wave sustained discharge (SWD) at atmospheric pressure in a quartz tube has been carried out in order to determine the plasma characteristic parameters (e.g. electron temperature and density, gas temperature, absorbed power per electron) and also to identify possible deviations from the thermodynamic equilibrium for this kind of microwave discharge. The results have been compared to experiments in the literature for other noble gas (helium and argon) SWDs generated under similar experimental conditions. Intermediate values between those of argon and helium plasmas were obtained for characteristic neon plasma parameters (temperatures and electron density). An important departure from the Saha equilibrium was exhibited by neon SWDs.     

Doubly ionized ion emission in laser-induced breakdown spectroscopy in air

The emission from doubly ionized species in laser-induced plasmas has not been properly investigated before since most analytical measurements were made at relatively long delays. This work proves that doubly ionized species, such as boron (B) III and iron (Fe) III, can exist during the first 150–200 ns of the plasma lifetime in plasmas produced in air by typical lasers with irradiances of 10⁹–10¹¹ W/cm². The emission from these ions was detected using both the double- and single-pulse excitations. The sum of the second ionization potential and the energy of corresponding excited states is approximately 30 eV. The presence of doubly charged ions in the early plasma was additionally confirmed by computer simulations using a collision-dominated plasma model. The emission from doubly ionized species may be used for analytical purpose. For example, in the spectrum from a B–Fe ore, the B III analytical line at 206.6 nm is free from Fe spectral interference thus enabling the online laser-induced breakdown spectroscopy sorting of ores into three products with high, medium, and low B₂O₃ contents.     

Process intensification for synthesis of triglycerides of capric acid using green approaches

The present research focuses on intensified synthesis of tricaprin by esterification reaction between capric acid and glycerol catalysed by dry amberlyst-15 using ultrasonication approach. Effect of several reaction conditions like molar ratio, reaction temperature, and amberlyst-15 loading on the rate of conversion has been studied. Effect of ultrasonic conditions like duty cycle and irradiation time on the intensified synthesis is investigated. Recyclability of amberlyst-15 is studied to make process more economical. It is investigated that the optimum reaction conditions which gave maximum conversion of 95% were molar ratio of capric acid: glycerol as 3:5, reaction temperature 90 ​°C, 4% amberlyst −15 loading. It was further investigated that ultrasonic conditions which gave intensified synthesis were 70% duty cycle and irradiation time of 120 ​min. The ultrasonic assisted process was compared with conventional synthesis. Conventional synthesis gave 30% yield in 120 ​min and 82% in 18 ​h. Amberlyst-15 was successfully reused for 13 cycle without any change in the conversion (%) of reaction.     

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.

     

Measurement of electron density utilizing the Hα-line from laser produced plasma in air

The electron density in a laser produced plasma experiment was measured utilizing the Stark broadening of the H_α-line at 656.27 nm. This line results from the interaction of the Nd:YAG laser at the fundamental wavelength of 1.06 μm with a plane solid aluminum target in a humid air. The measurements were repeated at several delay times (0–10 μs) and at a fixed gate time of 1 μs. The electron density from the optically thin Al II-line at 281.62 nm was measured in parallel from the same spectra. The electron density was found in the range from 10¹⁸ cm^− 3 down to 6 × 10¹⁶ cm^− 3 at longer delay time. The electron density from the H_α-line using the Griem's standard theory was compared with the predictions of other model due to Gigosos et al. The agreement between the measured electron density from both the H_α-line and the Al II-line would confirm the reliability of utilizing the H_α-line as an electron density standard reference line in LIBS experiments. Several important features characterize the H_α-line: it is a well isolated line, it gives large signal to background ratio, it lasts a long time after the termination of the laser (up to 10 μs), its Stark width is relatively large and does not exhibit self-absorption.     

Effect of RF and microwave oxygen plasma on the performance of Pd gate MOS sensor for hydrogen

The combined effect of microwave and RF oxygen plasma treatment of SiO₂ surface on the hydrogen sensitivity of Pd gate MOS sensor has been studied. Nine different samples of thermally grown SiO₂ surface have been taken and treated with oxygen plasma of different microwave power (100 W, 150 W and 200 W respectively) while keeping RF power fixed (20 W) for different durations (5 min, 10 min and 15 min). Pd gate MOS sensors with these plasma treated SiO₂ surface as dielectric have been fabricated and tested for different concentrations (500–3500 ppm) of hydrogen at room temperature. It is observed that the sensitivity of the sensor increases for higher duration of plasma exposure and also with microwave power but decreases when the sensor is treated with 200 W microwave power for 10 min and 15 min durations. The sensor treated with oxygen plasma of 200 W microwave power for 5 min duration exhibited the highest hydrogen sensitivity (74.4%). Fixed oxide charge density has also been evaluated as a function of exposure time for varying microwave power. Surface morphology of plasma treated SiO₂ surfaces was studied by AFM to have the estimation of porosity. The high sensitivity can be attributed to the fact that oxygen plasma treatment provides the availability of higher number of adsorption sites and modification in the surface state density i.e. surface state density increases for plasma treated sensors.