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Spatial heterodyne spectroscopy (SHS) is used for quantitative analysis and classification of liquid samples. SHS is a version of a Michelson interferometer with no moving parts and with diffraction gratings in place of mirrors. The instrument converts frequency-resolved information into spatially resolved one and records it in the form of interferograms. The back-extraction of spectral information is done by the Fast Fourier transform. A SHS instrument is constructed with the resolving power 5000 and spectral range 522 - 593 nm. Two original technical solutions are used as compared to previous SHS instruments: the use of a high frequency diode pumped solid state (DPSS) laser for excitation of Raman spectra and a microscope-based collection system. Raman spectra are excited at 532 nm at the repetition rate 80 kHz. Raman shifts between 330 cm-1 and 1600 cm-1 are measured. A new application of SHS is demonstrated: for the first time it is used for quantitative Raman analysis to determine concentrations of cyclohexane in isopropanol and glycerol in water. Two calibration strategies are employed: univariate based on the construction of a calibration plot and multivariate based on partial least square regression (PLSR). The detection limits for both cyclohexane in isopropanol and glycerol in water are at a 0.5 mass% level. In addition to the Raman-SHS chemical analysis, classification of industrial oils (biodiesel, poly(1-decene), gasoline, heavy oil IFO380, polybutenes, and lubricant) is performed using their Raman-fluorescence spectra and principal component analysis (PCA). The oils are easily discriminated as they show distinct non-overlapping patterns in the space of principal components.
An improved algorithm for calibration-free laser induced breakdown spectroscopy (CF LIBS) is presented which includes several novel features in comparison with previously proposed similar algorithms. In particular, it allows using spectral lines with arbitrary optical thickness for the construction of Saha-Boltzmann plots, retrieves the absorption path length (plasma diameter) directly from a spectrum, replaces the Lorentzian line profile function by the Voigt function, and allows for self-absorption correction using pre-calculated and tabulated data rather than approximating functions. The tabulated data embody the solutions of the radiative transfer equation for numerous combinations of optical thicknesses and line widths. The algorithm is used to analyze 100 low alloy steel spectra.
Stimulated emission observed experimentally in aluminum laser induced plasma is modeled via a kinetic approach. The simulated emission at several cascade transitions is created by a pump laser guided through the plasma at several microseconds after its creation and tuned in resonance with the strong transition at 266 nm. A two-dimensional space-time collisional radiative plasma model explains the creation of the population inversion and lasing at wavelengths 2.1 μm and 396.1 nm. The population inversion for lasing at 2.1 μm is created by depopulation of the ground state and population of the upper state via absorption of resonant radiation at 266 nm. The population inversion for lasing at 396.1 nm occurs during the laser pulse via the decay of the population of the pumped upper state to the lasing state via cascade transitions driven optically and by collisions. The model predicts that the population inversion and corresponding gain may reach high values even at moderate pump energies of several μJ per pulse. The efficiency of lasing at 2.1 μm and 396.1 nm is estimated to be on the order of a percent of laser pump energy. The polarization effect that the pump radiation at 266 nm imposes on the stimulated emission at 396.1 nm is discussed. The calculated results are favorably compared to experiment.
Laser-induced plasmas are widely used in many areas of science and technology; examples include spectrochemical analysis, thin film deposition, material processing, and even jet propulsion. Several topics will be addressed. First, general phenomenology of laser-induced plasmas will be discussed. Then, a chemical model will be presented based on a coupled solution of Navier-Stokes, state, radiative transfer, material transport, and chemical (Guldberg-Waage) equations. Results of computer simulations for several chemical systems will be shown and compared to experimental observations obtained by optical imaging, spectroscopy, and tomography. The latter diagnostic tools will also be briefly discussed.
The spatial heterodyne detection principle has a great potential in spectroscopy. It has an optical setup similar to that of a Michelson interferometer, with the mirrors replaced by diffraction gratings positioned at fixed, equal distances from the beamsplitter and are slightly tilted. The resulting interference pattern is recorded by a digital camera and the spectrum is recovered by using Fourier transformation. Although SHS was initially developed for astronomical and satellite-based atmospheric measurements, but in recent years it has been started to be applied in other branches of spectroscopy too. Recently the area of laser-induced breakdown spectroscopy (LIBS) has also discovered the potential of SHS. The main appeal of SHS detection in LIBS includes the compactness and robustness of the setup (in view of field applications) and the flexibility to optimize the setup for either high sensitivity or for high resolution, which can be benficially exploited in applications like stand-off measurements, quantitative analysis with isotope resolution, etc.
In the present work, we have improved and further optimized our initial LIBS-SHS setup described in a previous conference. By using optical simulations, we have modelled the light transmission efficiency, instrumental function and imaging properties of the system. We significantly improved and automated the spectral and image data processing sequence. The optimizations carried out resulted in an improved spectral resolution and repeatability, a lower spectral background and the elimination of the central line artifact originating from the Fourier transformation procedure. A detailed characterization of the LIBS spectroscopy performance (e.g. resolution, spectral coverage, tuning range, linearity, etc.), including a comparison with that of a LIBS setup based on a conventional dispersion CCD spectrometer was also performed.
The approach, which was developed earlier for modeling chemical reactions in laser induced plasmas, is applied to radio-frequency discharge plasmas. The model is based on the assumption that all ionization processes and chemical reactions are at local thermodynamic equilibrium. A chemical composition of an argon-hydrogen plasma with an Addition of boron trichloride is studied as a function of plasma temperature and mole ratio H2∕BCl3. It is established that more than twenty simple and composite molecules and ions can be formed in the course of chemical reactions. The results are compared with those obtained earlier by means of another equilibrium model that uses ab-initio quantum chemical computations of thermochemical and kinetic data and a 0D thermochemical quilibrium solver.
A goal of this work is to extend the model, which was initially developed for laser induced plasmas, to plasmas used in chemical reactors, in particular, the inductively-coupled-RF discharge plasma. The model predicts equilibrium chemical compositions of reaction mixtures as functions of plasma temperature and stoichiometry of reactants. The mixtures investigated are BCl3/H2/Ar and BF3/H2/Ar where Ar serves as the plasma-forming gas and H2 as a binding agent which binds the active species Cl and F and Cl- and F-containing intermediates to produce gaseous B and its condensate. An additional goal is to obtain information about intermediate reaction products for different ratios of BCl3/H2 and BF3/H2 and at different temperatures and different Ar flow rates.
It is found that the desired components B and B2 appear at appreciable concentrations of >0.1% and ~0.01% respectively only at temperatures above 3000 K. It is also established that the effect of charged species on the reaction products is miniscule for temperatures below 5000 K. The expected yield of boron as a function of the original mole fraction H2/BCl3 and H2/BF3 is calculated. The mole fractions are varied in the range 0.1-1000 and the temperature in the range 1000-10000 K. It is shown that the yield of boron increases with increasing the molar ratio H2/BCl3 and H2/BF3 up to ~100 in the temperature range 2000-5000 K. At higher temperatures, T>5000 K, the boron concentration reaches its maximum and does not depend on the concentration of hydrogen; all molecules dissociate and chemical reactions proceed only between charged particles (mostly elemental ions) and electrons. The calculated plasma parameters and composition are compared with experimental data obtained by optical emission spectroscopy. The calculated plasma temperature and electron density are shown to be in good agreement with the measured ones.
High purity halides of III-VI group elements, especially chloride and fluorides, are used in gas phase technologies for obtaining high purity materials and coatings. The reduction of halides in hydrogen-halide mixtures can be achieved in various discharge plasmas, e.g. inductively coupled, ark, and even laser-induced plasmas. Existing models of such plasmas are not sufficiently accurate to predict a yield of the targeted compounds and to describe the plasma processes involved in formation of these compounds. Besides, a construction of costly plasma-chemical reactors can be alleviated by the prior modeling of plasma processes that may occur in such reactors.
A goal of this work is to extend the model, which was initially developed for laser induced Plasmas, to plasmas used in chemical reactors, in particular, the inductively-coupled-RF discharge Plasma. The model predicts equilibrium chemical compositions of reaction mixtures as functions of plasma temperature and stoichiometry of reactants. The mixtures investigated are BCl3/H2/Ar and BF3/H2/Ar where Ar serves as the plasma-forming gas and H2 as a binding agent which binds the active species Cl and F and Cl- and F-containing intermediates to produce gaseous B and its condensate. An additional goal is to obtain information about intermediate reaction products for different ratios of BCl3/H2 and BF3/H2 and at different temperatures and different Ar flow rates.
It is found that the desired components B and B2 appear at appreciable concentrations of >0.1% and ~0.01% respectively only at temperatures above 3000 K. It is also established that the effect of charged species on the reaction products is miniscule for temperatures below 5000 K. The expected yield of boron as a function of the original mole fraction H2/BCl3 and H2/BF3 is calculated. The mole fractions are varied in the range 0.1-1000 and the temperature in the range 1000-10000 K. It is shown that the yield of boron increases with increasing the molar ratio H2/BCl3 and H2/BF3 up to ~100 in the temperature range 2000-5000 K. At higher temperatures, T>5000 K, the boron concentration reaches its maximum and does not depend on the concentration of hydrogen; all molecules dissociate and chemical reactions proceed only between charged particles (mostly elemental ions) and electrons. The calculated plasma parameters and composition are compared with experimental data obtained by optical emission spectroscopy. The calculated plasma temperature and electron density are shown to be in good agreement with the measured ones.
An improved algorithm for calibration-free laser induced breakdown spectroscopy (CF LIBS) will be presented which includes several novel features in comparison with previously proposed similar algorithms. In particular, it allows using spectral lines with arbitrary optical thickness for the construction of Saha-Boltzmann plots, retrieves the absorption path length (plasma diameter) directly from a spectrum, replaces the Lorentzian line profile function by the Voigt function, and allows for self-absorption correction using pre-calculated and tabulated data rather than approximating functions. The tabulated data embody the solutions of the radiative transfer equation for numerous combinations of optical thicknesses and line widths. The algorithm is thoroughly verified using synthetic spectra.
Expansion dynamics of laser-induced plasma is studied for different focal positions of the ablation laser in the pressure range 10-2 - 105 Pa of the ambient air. The experimental results indicate that both the parameters significantly affect the plasma size, shape, intensity, reproducibility, and distance from the target surface. At pressures above 10 Pa, the plasma plume is confined by the ambient gas; the plumes are more compact and travel shorter distances from the target as compared to the analogous plume characteristics at pressures below 10 Pa. The pulse-to-pulse reproducibility of the integral emission intensity of the plasma is also different for different focal positions and pressures. It is found that the focal positions -1 cm and -2 cm below the target surface yield the most reproducible and intense emission signals as measured at the 600 ns delay time with the 100 ns gate. The information obtained can be of importance for pulsed laser deposition, laser welding, and analytical spectroscopy at reduced pressures. In general, a correct choice of the focal position and pressure of an ambient gas is very important for obtaining the strongest plasma emission, good reproducibility, and desired plasma plume shape.
The equation of state for plasmas containing negative and positive ions of elements and molecules formed by these elements is modeled under the assumption that all ionization processes and chemical reactions are at local thermal equilibrium and the Coulomb interaction in the plasma is described by the Debye–Hückel theory. The hierarchy problem for constants of molecular reactions is resolved by using three different algorithms for high, medium, and low temperatures: the contraction principle, the Newton–Raphson method, and a scaled Newton–Raphson method, respectively. These algorithms are shown to have overlapping temperature ranges in which they are stable. The latter allows one to use the developed method for calculating the equation of state in combination with numerical solvers of Navier–Stokes equations to simulate laser-induced Plasmas initiated in an atmosphere and to study formation of molecules and their ions in such plasmas. The method is applicable to a general chemical network. It is illustrated with examples of Ca–Cl and C–Si–N laser-induced plasmas.
Laser induced plasma (LIP) is a highly dynamic, short living event which presents significant difficulty for both diagnostics and modeling. The former requires precise spatially- and time-resolved measurements on a micron-nanosecond scale while the latter needs numerous descriptive parameters; many of them can only be obtained from experiment. Diagnostics and modeling should always complement each other for obtaining a truthful picture of LIP.
In this presentation, a newly developed collisional-dominated model will be presented. The model is based on the coupled Navier-Stokes, state, radiative transfer, material transport, and chemical equations. The model incorporates plasma chemistry through the equilibrium approach that relies on atomic and molecular partition functions. Several chemical systems are modeled including Si-C-Cl-N and B-H-Cl systems.
The model is used to study the equilibrium states of the systems as functions of the concentrations of plasma species and plasma temperature. The model also predicts the evolution of number densities of atomic and molecular species in the expanding plasma plume.
This course will provide an introduction to plasma diagnostic techniques. The major focus of the course will be on the discussions of the practical procedures as well as the underlying physical principles for the measurements of plasma fundamental characteristics (e.g., temperatures, thermodynamic properties, and electron number density). Particular emphasis will be placed on inductively coupled plasma–atomic emission spectrometry, but other analytical plasmas will also be used as examples when appropriate. Selected examples on how one can manipulate the operating conditions of the plasma source, based on the results of plasma diagnostic measurements, to improve its performance used for spectrochemical analysis will also be covered. Topics to be covered include thermal equilibrium, line profiles, temperatures, electron densities, excitation processes, microreactions, pump and probe diagnostics, tomography, temporal and spatial resolution. Basics of plasma computer modeling will be presented.
An improved algorithm for calibration-free laser induced breakdown spectroscopy (CF LIBS) is presented which includes several novel features in comparison with previously proposed similar algorithms. In particular, it allows using spectral lines with arbitrary optical thickness for the construction of Saha-Boltzmann plots, retrieves the absorption path length (plasma diameter) directly from a spectrum, replaces the Lorentzian line profile function by the Voigt function, and allows for self-absorption correction using pre-calculated and tabulated data rather than approximating functions. The tabulated data embody the solutions of the radiative transfer equation for numerous combinations of optical thicknesses and line widths. The algorithm is thoroughly verified with synthetic spectra.
Laser-induced plasmas are widely used in many areas of science and technology; examples include spectrochemical analysis, thin film deposition, material processing, and even jet propulsion. Several topics will be addressed. First, general phenomenology of laser-induced plasmas will be discussed. Then, a chemical model will be presented based on a coupled solution of Navier-Stokes, state, radiative transfer, material transport, and chemical (Guldberg-Waage) equations. Results of computer simulations for several chemical systems will be shown and compared to experimental observations obtained by optical imaging, spectroscopy, and tomography. The latter diagnostic tools will also be briefly discussed. Finally, a prospective application of laser-induced plasma and plasma modeling will be illustrated on the example of calibration-free MC LIBS (Monte Carlo Laser Induced Breakdown Spectroscopy), in which concentrations of elements in materials are found by fitting model-generated and experimental spectra.
Accuracy of calibration-free (CF) methods in laser-induced breakdown spectroscopy (LIBS) depends on experimental conditions and instrumental parameters that must match a CF LIBS model. Here, the numerical study is performed to investigate effects of various factors, such as the optical density, plasma uniformity, line overlap, noise, spectral resolution, electron density and path length on the results of CF-LIBS analyses. The effects are examined one-by-one using synthetic spectra of steel slag samples that fully comply with the mathematical model of the method. Also, the algorithm includes several new features in comparison with previously proposed CF algorithms. In particular, it removes limits on the optical thickness of spectral lines that are used for the construction of the Saha-Boltzmann plot; it retrieves the absorption path length (Plasma diameter) directly from spectral lines; it uses the more realistic Voigt line profile function instead of the Lorentzian function; and it employs the pre-calculated and tabulated thin-to-thick line ratios instead of approximating functions for selfabsorption correction.
A novel technique based on laser induced plasma imaging is proposed to measure residual pressure in sealed containers with transparent walls, e.g. high voltage vacuum interrupter in this paper. The images of plasma plumes induced on a copper target at pressure of ambient air between 10−2Pa and 105Pa were acquired at delay times of 200ns, 400ns, 600ns and 800ns. All the plasma images at specific pressures and delay times showed a good repeatability. It was found that ambient gas pressure significantly affects plasma shape, plasma integral intensities and expansion dynamics. A subsection characteristic method was proposed to extract pressure values from plasma images. The method employed three metrics for identification of high, intermediate and low pressures: the distance between the target and plume center, the integral intensity of the plume, and the lateral size of the plume, correspondingly. The accuracy of the method was estimated to be within 15% of nominal values in the entire pressure range between 10−2Pa and 105Pa. The pressure values can be easily extracted from plasma images in the whole pressure range, thus making laser induced plasma imaging a promising technique for gauge-free pressure detection.
Laser induced plasma (LIP) is a dynamic, short living event which presents significant difficulty for modeling. In this report, a collisional-dominated chemical model developed earlier* is expanded by the inclusion of a new method for calculation of chemical reactions. The model consists of the coupled Navier-Stokes, state, radiative transfer, material transport, and chemical equations. The latter are written in terms of atomic and molecular partition functions rather than reaction rates. Typically, a solution of such the system of chemical equations is difficult for the entire range of plasma temperatures and densities because reaction constants may vary by hundreds orders of magnitude owing to extreme plasma conditions. No numerical solver of non-linear systems of equations handles this situation with ease. We resolve the problem by using a hierarchical approach. First, we rank the reactions according to their ascendancy. Second, we exploit either the contraction or Newton-Raphson algorithms to solve the system of chemical equations. We illustrate the approach by performing a series of calculations for reacting species Si, C, N, Ca, Cl and their molecules in laser induced plasmas.
This course will provide an introduction to plasma diagnostic techniques. The major focus of the course will be on the discussions of the practical procedures as well as the underlying physical principles for the measurements of plasma fundamental characteristics (e.g., temperatures, thermodynamic properties, and electron number density). Particular emphasis will be placed on inductively coupled plasma–atomic emission spectrometry, but other analytical plasmas will also be used as examples when appropriate. Selected examples on how one can manipulate the operating conditions of the plasma source, based on the results of plasma diagnostic measurements, to improve its performance used for spectrochemical analysis will also be covered. Topics to be covered include thermal equilibrium, line profiles, temperatures, electron densities, excitation processes, microreactions, pump and probe diagnostics, tomography, temporal and spatial resolution. Basis of plasma computer modeling will be presented.
We found cascade IR generation in Al laser induced plasma. This generation includes doublet transitions 3s25s 2S1∕2→ 3s24p 2P1∕2,3∕2 → 3s24s 2S1∕2; corresponding to strong lines at 2110 and 2117 nm, and much weaker lines at 1312–1315 nm. The 3s25s2S 1∕2 starting IR generation level is directly pumped from the 3s23p 2P3∕2 ground level. The starting level for UV generation at 396.2 nm (transitions 3s24s 2S1∕2 → 4p 2P3∕2) is populated due to the fast collisional processes in the plasma plume. These differences led to different time and special dependences on the lasing in the IR and UV spectral range within the aluminum laser induced plasma.
Data processing in the calibration-free laser-induced breakdown spectroscopy (LIBS) is usually based on the solution of the radiative transfer equation along a particular line of sight through a plasma plume. The LIBS data processing is generalized to the case when the spectral data are collected from large portions of the plume. It is shown that by adjusting the optical depth and width of the lines the spectra obtained by collecting light from an entire spherical homogeneous plasma plume can be least-square fitted to a spectrum obtained by collecting the radiation just along a plume diameter with a relative error of 10 −11 or smaller (for the optical depth not exceeding 0.3) so that a mismatch of geometries of data processing and data collection cannot be detected by fitting. Despite the existence of such a perfect least-square fit, the errors in the line optical depth and width found by a data processing with an inappropriate geometry can be large. It is shown with analytic and numerical examples that the corresponding relative errors in the found elemental number densities and concentrations may be as high as 50% and 20%, respectively. Safe for a few found exceptions, these errors are impossible to eliminate from LIBS data processing unless a proper solution of the radiative transfer equation corresponding to the ray tracing in the spectral data collection is used.