J.A. Aguilera

Influence of the optical depth on spectral line emission from laser-induced plasmas ☆

Abstract The emission from a laser-induced plasma of seven self-absorbed Fe(I) spectral lines has been studied to investigate the influence of the optical depth on the line intensity. The plasma was generated using an infrared Q-switched Nd:YAG laser in air at atmospheric pressure. The plasma emission was detected with temporal resolution, using a delay of 5 μs from the laser pulse and a gate width of 1 μs. Experimental curves of growth (COGs) were obtained by measuring the line intensity for Fe–Ni alloy samples with Fe concentrations in the range 0.2–95%. Using a simple model for self-absorption, based in a homogeneous plasma with the absence of matrix effects, theoretical COGs have been calculated that fit the experimental data with good correlation. The method used allows prediction of the COG that will be obtained for a given spectral line, starting from its transition parameters (oscillator strength, energy levels and degeneracy of the lower level), the plasma temperature and the damping constant of the line. The plasma temperature (8200±100 K) was determined using the Boltzmann plot method. The existence of local thermodynamic equilibrium was verified by estimating the plasma electron density (2.6×1016 cm−3) using the Stark broadening of an emission line. The damping constant (0.9±0.2) was estimated through the determination of the Lorentzian line width from measured line profiles. The density of Fe atoms in the plasma for the sample with 100% Fe (7.3×1015 cm−3) was estimated using all the COGs of the lines studied. The experimental results indicate that matrix effects are not present in the ablation process of the Fe–Ni samples.

Improvements in Quantitative Analysis of Steel Composition by Laser-Induced Breakdown Spectroscopy at Atmospheric Pressure Using an Infrared Nd:YAG Laser

The experimental conditions and procedure for quantitative analysis of steel by laser-induced breakdown spectroscopy in argon at atmospheric pressure using an infrared Nd:YAG laser have been studied. Satisfactory analytical results have been obtained for the determination of C, Si, Cr, and Ni contents in low-alloyed steels. The lens-to-sample distance is shown to be a relevant parameter, which can be selected at each pulse energy to enhance the line intensities and the repeatability of measurements. A higher precision has been obtained for line-intensity ratios (0.9–2.5% relative standard deviation for concentrations higher than 0.1%) than for absolute intensity measurements. The calibration curves for all the elements have correlation coefficients above 0.999. Detection limits are in the range 6–80 ppm.

Plasma shielding effect in laser ablation of metallic samples and its influence on LIBS analysis

Abstract Line emission from plasmas formed during laser ablation of steel in air at atmospheric pressure has been measured for varying pulse energies and focusing distances. By using a Nd:YAG laser with pulse energies in the range of 25–250 mJ, values of the power density up to 710 GW/cm 2 are obtained. The variation of emission intensities with the focusing distance and the pulse energy is related to shielding effects of the plasma produced, which depend on the type of absorption wave obtained at different power densities during the initial formation process. The influence of these effects on the elemental analysis by LIBS is studied by obtaining the precision of nickel content determination in steel samples. At each pulse energy, a focusing position below the sample can be found that produces maximum intensity and higher precision. A limit of detection of 64 ppm of nickel in steel was obtained by focusing the laser beam 12 mm below the sample surface for a 100-mm focal-length lens.

Curves of growth of spectral lines emitted by a laser-induced plasma: influence of the temporal evolution and spatial inhomogeneity of the plasma ☆

Abstract The curves of growth (COG) of five Fe I lines emitted from a laser-induced plasma, generated with Fe–Ni alloys in air at atmospheric pressure, have been investigated. Spectral lines with different energy levels and line widths, emitted with a broad range of optical depths, have been included in the study in order to check the validity of theoretical models proposed for COG generation, based in the radiative transfer within a plasma in local thermodynamic equilibrium. The COGs have been measured at time windows of 4–5 μs and 15–18 μs. The Stark widths of the Fe I lines have been obtained, and the line widths have been determined by measuring the plasma electron density at the time windows selected. It is shown that at a time window of 4–5 μs, the inhomogeneity of the plasma magnitudes has an important influence on the COGs of intense lines. For this time window, a two-region model of the plasma has been used to generate theoretical COGs that describe satisfactorily the experimental curves of all the lines using a single set of plasma parameters. The results reveal the existence of considerable gradients between the inner and the outer plasma regions in the temperature (9400–7800 K) and in the density of Fe atoms (4×10 16 –0.02×10 16 cm −3 for a sample with 100% Fe). On the contrary, at the time window 15–18 μs, at which the plasma has suffered most of its expansion and cooling process, the COGs of all the lines may be described by a single-region model, corresponding to a plasma with uniform temperature (6700 K) and density of Fe atoms (0.06×10 16 cm −3 for a sample with 100% Fe). It is also shown that at initial times, the plasma inhomogeneity has an important effect in the line profiles of intense spectral lines, which are described by using the two-region model of the laser-induced plasma.

Spatial characterization of laser-induced plasmas by deconvolution of spatially resolved spectra

The spatial characterization of laser-induced plasmas, including their temperature, electron density, and relative atom density, has been carried out by emission spectroscopy. The plasmas were generated with iron samples in air and argon at atmospheric pressure. An imaging spectrometer equipped with an intensified CCD detector procured spectra with spatial resolution. The plasma characterization was made at three temporal gates (2-3, 5-6, and 9-11 micros) to permit the plasmas evolution to be studied. A deconvolution procedure was developed to transform the measured intensity, integrated along the line of sight, into the radial distribution of emissivity. Temperature and electron density distributions were obtained under the assumption of local thermodynamic equilibrium and Stark broadening of the emission lines. The relative atom density distributions in the plasma of the Fe atoms arising from the sample and of the Ar atoms arising from the ambient gas were determined and evidenced an important interaction between the plasma and the surrounding atmosphere.

Two-Dimensional Spatial Distribution of the Time-Integrated Emission from Laser-Produced Plasmas in Air at Atmospheric Pressure:

Emission from Nd:YAG laser-produced plasmas generated in air at atmospheric pressure has been studied with spatial resolution. With the use of a charge-coupled device (CCD), two-dimensional spatial distributions of the time-integrated line emission from plasmas of metallic (copper and stainless steel) and ceramic (alumina) samples were measured for laser power densities in the range 80–900 GW/cm2. Geometrical parameters (distance of the maximum, width in two directions) and the total space- and time-integrated emission were obtained as a function of power density. At the range 100–700 GW/cm2, the formation of a plasma in air for the metallic samples was shown to be responsible for a decrease in the total intensity of lines emitted from the sample plasma, accompanied by effects in the spatial profiles along the laser direction. Beyond 700 GW/cm2, a sharp increase of the line intensities was observed, related to the absence of the shielding air plasma.

Reproducible and controllable light induction of in vitro fruiting of the white-rot basidiomycete Pleurotus ostreatus.

Fruiting is a crucial developmental process in basidiomycetes yet the genetic and molecular factors that control it are not yet fully understood. The search for fruiting inducers is of major relevance for both basic research and for their use in industrial applications. In this paper, an efficient and reproducible protocol for controlled fruiting induction of Pleurotus ostreatus growing on synthetic medium is described. The protocol is based on the control of light intensity and photoperiod and permits the life cycle for this fungus to be completed in less than two weeks. The fruiting bodies produced by this method release fertile spores after 4-5 d of culture. Our results indicate that fruiting induction is solely dependent on the illumination regime and that it occurs long before the available nutrients are depleted in the culture. This protocol will greatly facilitate molecular and developmental biology research in this fungus as it avoids the need for complex culture media based on lignocellulosic materials or the use of chemical inducers.

Apparent excitation temperature in laser-induced plasmas

This work has shown the existence, for the purpose of experiments of laser-induced plasma spectroscopy, of an apparent (population-averaged) excitation temperature that determines with good approximation the Boltzmann population distribution for each ionisation species. In experiments in which the line intensity (integrated along the line-of-sight) is measured, the values of the apparent temperature obtained for neutral atoms and ions are different. To investigate the population-averaging process, a plasma generated with a Ni-Fe-Al alloy in air at atmospheric pressure has been characterized with complete spatial resolution, determining the local values of the electronic temperature and the relative number densities of Fe neutral atoms and ions. From the distributions of plasma parameters, synthesized distributions of intensities integrated along the line of sight have been obtained for Fe I and Fe II lines, showing good agreement with the experimental distributions. Synthesized Boltzmann plots of neutral atoms and ions, constructed with the spatially-integrated synthesized intensities, have shown linear behaviors, providing the apparent temperatures for neutral atoms (9890 K) and ions (11400 K). The synthesized Boltmann plots are also in good agreement with experimental Boltzmann plots obtained in independent spatiallyintegrated measurements of the plasma emission, carried out with a fiber optics cable.

Spatial and temporal scaling and common apparent excitation temperature of laser-induced plasmas generated at constant irradiance with different pulse energies

The characteristics of laser-induced plasmas generated with different pulse energies E in the range of 50–200mJ at constant irradiance (40GW∕cm2) have been investigated. The plasmas have been generated in air at atmospheric pressure from Fe–Ni samples using an infrared Nd:YAG laser. Plasma characterization has been carried out by emission spectroscopy with three types of measurements as follows. (1) Time-resolved, space-integrated measurements have shown that a temporal scaling exists between the curves of temporal evolution of the apparent (population-averaged) temperature and electron density of the plasmas, with scaling factors proportional to E0.4±0.1. (2) Time- and space-integrated measurements have led to a common value of the plasma apparent temperature (7200±200K) for all the pulse energies and to a scaling of the line intensities as E1.6±0.2. The coincidence of apparent temperatures is explained by the hypothesis that laser-induced plasmas generated with different pulse energies at constant irrad...

Stark width measurements of Fe II lines with wavelengths in the range 260–300 nm

The Stark widths of 21 Fe II lines with wavelengths in the range 260–300 nm have been measured using laser-induced plasmas as spectroscopic sources. A set of Fe–Cu samples has been employed to generate the plasmas. To reduce self-absorption, each line has been measured using a different sample, with an iron concentration determined by means of the curve-of-growth methodology. The remaining error due to self-absorption has been estimated to be lower than 10%. Different instants of the plasma evolution, from 0.84 to 2.5 µs, are included in the measurements. The electron density, in the range (1.6–7.3) × 1017 cm−3, is determined by the Stark broadening of the Hα line. Within this range, the Stark widths are found to be proportional to the electron density. The Boltzmann plot method is used to obtain the plasma temperature, which is in the range 12 900–15 200 K. The Stark widths obtained have been compared with available experimental and theoretical data.