Christophe Dutouquet

Laser-induced fluorescence probing during pulsed-laser ablation for three-dimensional number density mapping of plasma species

Laser-induced fluorescence spectroscopy was employed to measure ground state number densities of atoms and molecules in plasma plumes generated by pulsed-laser ablation of Al, C and Ti targets in N2 or O2 low-pressure atmospheres. A beam expander was used to transform the dye laser beam in a thin plane section of 0.2×40 mm2 dimension crossing the plasma through its symmetry axis. Using a fast intensified charged coupled device matrix for fluorescence detection, three-dimensional number density mapping of plasma species was acquired. Calibration of the measured ground state densities in an absolute scale was performed by additional absorption measurements. According to the plasma temperature, the density of atoms and molecules and their total number in the plasma plume were estimated. The species densities were compared to those obtained by intensity calibrated emission spectroscopic measurements. The time- and space-evolution of atomic and molecular densities gives information about gas-phase reactions due to the interaction of the ablated material with the surrounding low-pressure gas. The results contribute to a better understanding of thin film synthesis by reactive pulsed-laser deposition.

Analyses of gas-phase reactions during reactive laser ablation using emission spectroscopy

Time- and space-resolved emission spectroscopic measurements have been performed to investigate the formation of molecular species in the plasma plume generated by pulsed-laser ablation of Al, C and Ti targets in either N2 or O2 low-pressure atmospheres. The recorded spectra show that the interaction between the vapour plume and the ambient gas gives rise to the partial dissociation and ionization of the ambient gas and that the formation of nitride or oxide diatomics depends strongly on the binding energies of these species with respect to dissociation potential of the gas molecules. Thus, nitride formation in the gas phase occurred only during carbon ablation while oxide molecules were detected in the plasma plume originating from every target material. The influence of the atomic mass of the target material on the gas-phase reactions is also discussed. The results contribute to a better understanding of the mechanisms involved in the thin-film synthesis by reactive pulsed laser deposition.

Particle Sampling by TEM Grid Filtration

Transmission electron microscopy (TEM) coupled with energy-dispersive X-ray (EDX) offers a very comprehensive tool for individual particle analysis allowing the determination of size, morphology, specific surface, and elemental composition. This information is needed in aerosol studies, especially in the field of nanomaterials. However, observations with TEM require a controlled sampling on an adapted analysis support, namely TEM grid. Techniques allowing sampling on TEM grids are of great interest to aerosol analysis. Indeed, sample preparation is not required, thereby gaining time and avoiding a risk for the sample to be altered. The present study evaluates the efficiency of a new particle collection technique based on filtration through one class of TEM-dedicated supports, namely TEM porous grids. Two types of porous grids, considered as the best on the market for this application, have been put to the test: the “Quantifoil” type porous grid, which has a regular structure, and the “Holey” type (Agar Scientific, Stansted, Essex, England). A filter holder has been developed specifically for this application, the MPS® (Mini-Particle Sampler®, Ecomesure, Janvry, France). Experimental tests have been carried out with a flow rate of 0.3 L·min−1. They show that the collection is operational in the 5-nm to 150-nm size range, with a minimum efficiency of 15–18% around 30 nm. Simulation confirms these results and shows an increased efficiency even below 5 nm and beyond 150 nm. The filter holder MPS® designed in this study is a low-cost, portable, versatile, and easy-to-use tool. Copyright 2013 American Association for Aerosol Research

Aerosols Analysis by LIBS for Monitoring of Air Pollution by Industrial Sources

In the context of the air quality improvement, there is an increasing need to monitor gas and particle emissions originating from exhaust stacks (incinerators, foundries, etc.) for regulation enforcement purposes. Lots of pollutants are targeted; among them, heavy metals are mostly found in particulate forms. Hence, there is a need to promote the development of suitable on line analytical techniques. To that end, laser-induced breakdown spectroscopy (LIBS) appears to be a good technique. Indeed, it is quantitative, fast (<1 min), requires no sample preparation, and can be performed at remote distance. The instrumentation is compact and offers the possibility to be used for continuous and in-situ monitoring. Two different approaches have been tested by several authors to analyze aerosols by LIBS, by focusing the laser either on particles collected on a filter or directly into the aerosol. In this work, these two approaches, aiming at measuring the mass concentration of micrometer metallic particles in air, are investigated and compared. The experimental setup includes an aerosol source (an ultrasonic nebulizer producing a diluted aerosol of CuSO4 particles); two sampling lines for particle sizes and, for reference concentration measurements, a line for direct LIBS analysis; and a fourth one devoted to filter sampling for subsequent LIBS measurements. Calibration curves were obtained with those two experimental approaches and the results are compared. In terms of sampling particles number, indirect analysis appears to be more efficient than direct analysis for our experimental conditions. Better detection limits were found with direct analysis when comparing the two approaches under similar sampling conditions (analysis time and sampling flow).

Simulation of emission spectra from nonuniform reactive laser-induced plasmas.

We demonstrate that chemical reactions leading to the formation of AlO radicals in plasmas produced by ablation of aluminum or Ti-sapphire with ultraviolet nanosecond laser pulses can be predicted by the model of local thermodynamic equilibrium. Therefore, emission spectra recorded with an echelle spectrometer and a gated detector were compared to the spectral radiance computed for uniform and nonuniform equilibrium plasmas. The calculations are based on analytical solutions of the radiation transfer equation. The simulations show that the plasmas produced in argon background gas are almost uniform, whereas temperature and density gradients are evidenced in air. Furthermore, chemical reactions exclusively occur in the cold plume periphery for ablation in air. The formation of AlO is negligible in argon as the plasma temperature is too large in the time interval of interest up to several microseconds. Finally, the validity of local thermodynamic equilibrium is shown to depend on time, space, and on the elemental composition. The presented conclusions are of interest for material analysis via laser-induced breakdown spectroscopy and for laser materials processing.

Analyses of the TiO-γ system for temperature measurements in a laser-induced plasma

Emission spectra of TiO molecules in the spectral range from 700 to 720 nm have been recorded during pulsed-laser ablation of Ti in a low-pressure O2 atmosphere. The observed emission bands belong to the Δv = 0 and Δv = + 1 vibrational sequences of the γ-system (A 3Φ-X 3Δ) of TiO. According to the relative high temperatures in the laser-induced plasma of the order of 104 K, vibrational and rotational states with large quantum number are populated. Thus, the recorded spectra result from the superposition of more than 104 rotational lines belonging to 30 different vibrational bands. For the determination of the rotational and vibrational temperatures, the complicated structure of the TiO-γ emission spectrum has been reproduced using computer simulations. As a result, the gas kinetic temperature of the laser-induced plasma has been measured as a function of time and space for different O2 pressures. The temperature measurements are useful for a better understanding of the chemical processes during thin-film deposition by reactive laser ablation. In addition, simple empirical relations have been derived for temperature determination from the TiO-γ emission spectrum in the range from 3×102 to 2×104 K.

Monitoring of heavy metal particle emission in the exhaust duct of a foundry using LIBS

Heavy metals have long been known to be detrimental to human health and the environment. Their emission is mainly considered to occur via the atmospheric route. Most of airborne heavy metals are of anthropogenic origin and produced through combustion processes at industrial sites such as incinerators and foundries. Current regulations impose threshold limits on heavy metal emissions. The reference method currently implemented for quantitative measurements at exhaust stacks consists of on-site sampling of heavy metals on filters for the particulate phase (the most prominent and only fraction considered in this study) prior to subsequent laboratory analysis. Results are therefore known only a few days after sampling. Stiffer regulations require the development of adapted tools allowing automatic, on-site or even in-situ measurements with temporal resolutions. The Laser-Induced Breakdown Spectroscopy (LIBS) technique was deemed as a potential candidate to meet these requirements. On site experiments were run by melting copper bars and monitoring emission of this element in an exhaust duct at a pilot-scale furnace in a French research center dedicated to metal casting. Two approaches designated as indirect and direct analysis were broached in these experiments. The former corresponds to filter enrichment prior to subsequent LIBS interrogation whereas the latter entails laser focusing right through the aerosol for detection. On-site calibration curves were built and compared with those obtained at laboratory scale in order to investigate possible matrix and analyte effects. Eventually, the obtained results in terms of detection limits and quantitative temporal monitoring of copper emission clearly emphasize the potentialities of the direct LIBS measurements.

Compositional Analysis of Aerosols Using Calibration-Free Laser-Induced Breakdown Spectroscopy

We demonstrate that the elemental composition of aerosols can be measured using laser-induced breakdown spectroscopy (LIBS) without any preliminary calibration with standard samples. Therefore, a nanosecond Nd:YAG laser beam was focused into a flux of helium charged with alumina aerosols of a few micrometers diameter. The emission spectrum of the laser-generated breakdown plasma was recorded with an echelle spectrometer coupled to a gated detector. The spectral features including emission from both the helium carrier gas and the Al2O3 aerosols were analyzed on the base of a partial local thermodynamic equilibrium. Thus, Boltzmann equilibrium distributions of population number densities were assumed for all plasma species except of helium atoms and ions. By analyzing spectra recorded for different delays between the laser pulse and the detector gate, it is shown that accurate composition measurements are only possible for delays ≤1 μs, when the electron density is large enough to ensure collisional equilibrium for the aerosol vapor species. The results are consistent with previous studies of calibration-free LIBS measurements of solid alumina and glass and promote compositional analysis of aerosols via laser-induced breakdown in helium.

Detection of boron nitride radicals by emission spectroscopy in a laser-induced plasma☆

Abstract Several vibrational bands of boron nitride radicals have been observed in a plasma produced by pulsed-laser ablation of a boron nitride target in low-pressure nitrogen or argon atmospheres. Using time- and space-resolved emission spectroscopic measurements with a high dynamic range, the most abundant isotopic species B11N have been detected. The emission bands in the spectral range from 340 to 380 nm belong to the Δυ =−1, 0, +1 sequences of the triplet system (transition A3Π–X3Π). For positive identification, the molecular emission bands have been compared with synthetic spectra obtained by computer simulations. Furthermore, B10N emission bands have been reproduced by computer simulation using molecular constants which have been deduced from the B11N constants. Nevertheless, the presence of the lower abundant isotopic radical B10N was not proved due the noise level which masked the low emission intensity of the B10N band heads.