Semiempirical model for analysis of inhomogeneous optically thick laser-induced plasmas

In this paper, a more realistic approach of a non-uniform optically thick plasma in local thermodynamic equilibrium was applied to describe self-reversal of Co I 340.51 nm emission line recorded from a laser-induced plasma generated on a Co–Cr–Mo metallic alloy. This line was selected because it is one of the most absorbed of the major elements in air at atmospheric pressure.The model describes the behavior of the plasma after the breakdown, and it was semiempirical thus, some information was taken from the experiment. A cylinder-symmetrical plasma column with a parabolic temperature distribution having a maximum at the center and decreasing toward the edges was considered. The input parameters were the plasma length, the temperature in the plasma core, and the Co total density, which were estimated from measurements and previous work. Moreover, the distribution of electron density depended on the temperature, and the ionization degree was taken into account through Saha equation. Then, plasma parameters were adjusted in such a way calculations reproduced the experimentally measured line profiles.The effect of varying laser power on plasma homogeneity and its evolution in time were investigated. Moreover, preliminary results of spatial distribution of plasma parameters were obtained that confirmed the practical application of the model on plasma diagnostics.     

Stagnation layers at the collision front between two laser-induced plasmas: A study using time-resolved imaging and spectroscopy

When two dense laser-induced plasmas collide a stagnation layer may form at the interface region. The degree of stagnation depends on the so-called collisionality parameter given by the ratio of the geometric scale length of the experiment (in effect the initial plasma–plasma separation) to the ion–ion mean free path. Our experiments confirm that both target geometry and elemental composition strongly influence the collision process and degree of stagnation. In particular, we show that moving from flat to wedge-shaped targets and from aluminium to calcium results in a decrease in the value of the collisionality parameter accompanied by a concomitant reduction in the degree of stagnation. Time-resolved emission imaging was employed to track the size and shape of the stagnation layer which acts as a signature of hard versus soft stagnation. Characterization of the stagnation layer in terms of electron density and temperature has been performed using optical emission spectroscopy.     

Characterization of laser-induced plasmas by emission spectroscopy with curve-of-growth measurements. Part I: Temporal evolution of plasma parameters and self-absorption

Laser-induced plasmas have been characterized by emission spectroscopy, including the measurement of curves of growth. The plasmas have been generated in air at atmospheric pressure using an infrared Nd:YAG laser from a set of Fe–Ni alloys with varying Fe concentrations. The procedure used provides, in addition to the apparent temperature T and electron density N_e, a parameter N′l (the atom number density for 100% concentration times the length of the plasma along the line-of-sight), relevant to obtain the self-absorption and the intensity of the emission lines. The temporal evolution of the plasma parameters has been deduced from the measurement and fitting of the curves of growth. A fast temporal decrease of N′l is obtained for ions, whereas a gradual increase takes place for neutral atoms. The temporal evolution of the line intensity in the optically thin limit and the self-absorption of neutral atom and ion lines have been obtained experimentally and calculated from the evolution of the plasma parameters. The usefulness of the curve-of-growth method in measurements with time integration, in spite of the fast variation of the plasma parameters, has been demonstrated.     

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.     

Investigation of helium addition for laser-induced plasma spectroscopy of pure gas phase systems: Analyte interactions and signal enhancement

The role of helium addition on the analyte signal enhancement in laser-induced breakdown spectroscopy for analysis of pure gaseous systems was examined using carbon and hydrogen atomic emission lines. Increased analyte response, as measured by peak-to-base and signal-to-noise ratios, was observed with increasing helium addition, with maximum enhancement approaching a factor of 7. Additional measurements revealed a significant decrease in plasma electron density with increasing helium addition. To explore the mechanisms of analyte signal enhancement, the helium emission lines were also examined and found to be effectively quenched with nitrogen addition. In consideration of the data, it is concluded that the role of metastable helium is not as important as the overall changes in plasma properties, namely electron density and laser-plasma coupling. Helium addition is concluded to affect the electron density via Penning ionization, as well as to play a role in the initial plasma breakdown processes.     

Analysis of two colliding laser-produced plasmas by emission spectroscopy and fast photography

In this work two colliding laser-induced plasmas (LIP) on Cu and C were studied by means of time resolved emission spectroscopy and fast photography. The experiments were performed using two opposing parallel targets of Cu and C in vacuum, ablated with two synchronized ns lasers. The results showed an increased emission intensity from copper ions Cu II (368.65, 490.97, 493.16, 495.37 and 630.10 nm) and Cu III (374.47 and 379.08 nm) due to the ionization that occurs during collisions of Cu and C species. It was found that the optimum delay between pulses, which yields the maximum emission enhancement of Cu ions, depends on the sampling distance. On the other hand, the emission intensity of C lines, C II (426.70 nm), C III (406.99 and 464.74 nm) and C IV (465.83 nm), decreased and the formation of C₂ molecules was observed. A comparison between the temporal evolution of the individual plasmas and their collision performed by combining imaging and the time resolved emission diagnostics, revealed an increase of the electron temperature and electron density and the splitting of the plume into slow and fast components.     

Multi-element Saha–Boltzmann and Boltzmann plots in laser-induced plasmas

A multi-element Saha–Boltzmann plot method is proposed for the determination of the temperature and the relative number density in laser-induced plasmas, assuming local thermodynamic equilibrium and stoichiometry conservation. The method has been applied to the characterization of a plasma generated with a Cu–Fe–Ni–Mn alloy, using a Nd:YAG laser in air at atmospheric pressure. Spectra of the local emissivity have been obtained by spatial deconvolution of the intensity spectra, obtained with spatial resolution. Saha–Boltzmann plots obtained from the emissivities of 58 spectral lines of Fe I, Fe II, Ni I, Ni II, Mn I and Mn II have been fitted to linear behavior with high correlation, which shows the validity of the equation proposed. Radial distributions of the temperature and number densities of neutral atoms and ions have been determined. The results obtained reinforce the initial considerations of local thermodynamic equilibrium and conservation of stoichiometry. The proposed equation can also be applied to only one ionization species (multi-element Boltzmann plot). Spatially-integrated measurements of the plasma emission have also been performed to show that, in this case, the application of the method to the line intensities provides the two different apparent temperatures for neutral atoms and ions.     

Polarization and fluence dependence of the polarized emission in nanosecond laser-induced breakdown spectroscopy

Several studies have appeared in the past two years reporting that the continuum emission produced by the laser ablation of solid materials is strongly polarized. In a paper that appears to conflict with these findings, Asgill et al. report that they did not observe a significant amount of polarization produced by nanosecond laser excitation of nitrogen gas and laser ablation of copper and steel ( M.E. Asgill, H.Y. Moon, N. Omenetto, D.W. Hahn, Investigation of polarization effects for nanosecond laser-induced breakdown spectroscopy, Spectrochim. Acta Part B (2010) xxx-xxx [7]). Here we show that the apparent discrepancy is resolved when laser fluence and polarization are taken into account. Using a 532 nm Nd:YAG laser to ablate Al samples in air, we find that the degree of polarization, P, of the continuum is greater for s- vs. p-polarized excitation and that P decreases with increasing fluence. We show that P would be < 10% under the conditions of Asgill et al., whereas P > 60% is obtained at low fluences with s-polarized excitation. We also confirm that at high fluence the polarization of the discrete emission is much smaller than that of the continuum.     

Characterization of a laser-induced plasma by spatially resolved spectroscopy of neutral atom and ion emissions.: Comparison of local and spatially integrated measurements

The local values of the parameters that characterize a laser-induced plasma (temperature, electron density, relative number densities of neutral atoms and ions) have been obtained by spatially resolved emission spectroscopy, including the deconvolution of the measured intensity spectra. The plasma has been generated using a Nd:YAG laser with a Fe–Ni alloy in air at atmospheric pressure, and the emission in the time window 3.0–3.5 μs has been detected. The temperature values obtained from neutral atom and ion emissions have been compared in the cases of local and spatially-integrated measurements. Local Boltzmann and Saha–Boltzmann plots with high correlation to linear fittings have been obtained using two broad sets of optically thin neutral atom and ion lines (21 Fe I lines and 15 Fe II lines), resulting in local values of the electronic temperature that coincide within the error. These results of local measurements contrast with those of spatially integrated measurements, for which two different temperatures are obtained from the Boltzmann plots of neutral atoms (9100±150 K) and ions (13 700±300 K). This difference is explained according to the measured distributions of the electronic temperature and the neutral atom and ion number densities, that result in separated emissivity (or population) distributions of neutral atom and ion lines, leading to different neutral atom and ion apparent temperatures (population-averages of the local electronic temperature). Local values of the plasma parameters have been obtained at all the positions with significant emission, including the determination of the electronic temperature from Saha–Boltzmann or Boltzmann plots. The ionization degree is high- and low-varying at the inner part of the plasma, decaying only near the plasma front. The maximum of the ion density does not coincide with the temperature maximum; on the contrary, the axial variation of both the neutral atom and ion densities (that decrease towards the sample surface) is opposite to that of the temperature, a behaviour that is interpreted to result from the plasma expansion process.     

Optical emission spectroscopy diagnostics of inductively-driven plasmas in argon gas at low pressures

An optical emission spectroscopy method for determination of electron temperature, electron density and gas temperature is developed and applied for diagnostics of inductively-driven argon discharges in a cylindrical geometry. The discharges are maintained at frequency 27 MHz, applied power varied in the limits P = (90 – 160) W and gas pressure in the range p = (1.1 – 117.3) Pa. The method combines measurements of emission spectral line intensities and profile broadenings with a collisional-radiative model of argon plasma at low pressure. The model is employed for investigation of the plasma kinetics governing the population densities of 3p⁵4s and 3p⁵4p argon configuration levels, treated separately. In the numerical calculations the electron density and electron temperature are varied whereas the values of the third plasma parameter — the gas temperature — are involved as obtained data from the experiments. Comparison of the experimental results of the line-intensity ratios with those calculated by the model yields the values of the electron density and temperature. The dependence of the electron temperature, electron density and gas temperature on the discharge conditions is obtained and discussed in the study.     

Preliminary studies of laser-induced breakdown spectrometry for the determination of Ba,Cd, Cr and Pb in toys

The performance of laser-induced breakdown spectrometry (LIBS) for the determination of Ba, Cd, Cr and Pb in toys has been evaluated by using a Nd:YAG laser operating at 1064 nm and an Echelle spectrometer with intensified charge-coupled device detector. Samples were purchased in different cities of São Paulo State market and analyzed directly without sample preparation. Laser-induced breakdown spectrometry experimental conditions (number of pulses, delay time, integration time gate and pulse energy) were optimized by using a Doehlert design. Laser-induced breakdown spectrometry signals correlated reasonably well with inductively coupled plasma optical emission spectrometry (ICP OES) concentrations after microwave-assisted acid digestion of selected samples. Thermal analysis was used for polymer identification and scanning electron microscopy to visualize differences in crater geometry of different polymers employed for toy fabrication. Results indicate that laser-induced breakdown spectrometry can be proposed as a rapid screening method for investigation of potentially toxic elements in toys. The unique application of laser-induced breakdown spectrometry for identification of contaminants in successive layers of ink and polymer is also demonstrated.     

Chlorine detection in cement with laser-induced breakdown spectroscopy in the infrared and ultraviolet spectral range

A significant parameter to monitor the status of concrete buildings like bridges or parking garages is the determination of the depth profile of the chlorine concentration below the exposed concrete surface. This information is required to define the needed volume of restoration for a construction. Conventional methods like wet chemical analysis are time- and cost-intensive so an alternative method is developed using laser-induced breakdown spectroscopy (LIBS). The idea is to deploy LIBS to analyze drill cores by scanning the sample surface with laser pulses. Chlorine spectral lines in the infrared (IR) and ultraviolet (UV)-range were studied for chlorine detection in hydrated cement samples. The excitation energies of these spectral lines are above 9.2 eV. Hence high plasma temperatures and pulse energies in the range of some hundred millijoules are needed to induce sufficient line intensity levels at the required working distance. To further increase the line intensity and to lower the detection limit (LOD) of chlorine a measuring chamber is used where different ambient pressures and gases can be chosen for the measurements. The influences on the line intensity for pressures between 5 mbar and 400 mbar using helium as process gas and the influence of different laser burst modi like single and collinear double pulses are investigated. For the first time a LOD according to DIN 32 645 of 0.1 mass% was achieved for chlorine in hydrated cement using the UV line 134.72 nm.     

Quantitative elemental analysis of nutritional,hazardous and pharmacologically active elements in medicinal Rhatany root using laser induced breakdown spectroscopy

Rhatany roots (RRs) have been used in indigenous systems of medicines to treat many common illnesses due to the presence of highly active astringent and antiviral biochemical constituents that possess strong therapeutic and pharmacological properties. Due to its widespread use, the accurate knowledge on the elemental composition of this medicinal plant can set a pharmacological research platform to investigate the effect of certain elements, and their ions in mediating the human metabolism and therapy. In this work calibration-free laser-induced breakdown spectroscopy (CF-LIBS) is used to detect the elements present in RRs sample, by analyzing the characteristic emission wavelengths and their respective intensities in the laser induced plasma, without the need for using any calibration standards or methods. Many nutritional elements, which are of human health significance and instrumental in mediating the established biological activities of RRs, were identified in a relative abundance. In addition to this, our analysis identified the trace level of a few toxic elements, whose overdose due to reckless intake wreaks havoc to human health and wellbeing. The reliability of qualitative and quantitative detection of the elements in RR by LIBS were validated by the standard inductively coupled plasma optical emission spectroscopy (ICP OES), the results of which are in good agreement with LIBS data with better relative accuracy. Also, in order to discriminate, and single out any two elements with the overlapping emission wavelength in LIBS, X-ray photoelectron spectroscopy was also carried out, which in its own right is in good agreement with the elemental analysis of LIBS in general.     

Characterization of laser-induced plasma in a vacuum using laser ablation mass spectrometry and laser-induced breakdown spectrometry

An analytical system for simultaneously monitoring laser-ablation mass spectra and laser-induced breakdown spectra for solid sample has been developed. The performance of the developed system is evaluated by measuring characteristics of laser-induced plasma such as lifetime of ions inside the plasma and laser power dependence of mass resolution for solid samples. Adopted samples are gadolinium plate, gadolinium coated on stainless steel plate, and one of the NIST standard samples, C-1248 (Ni–Cu alloy). The threshold laser energy in obtaining mass spectrum was dependent on the type of sample characteristics in the order of a few MW/cm², while a few hundred MW/cm² was necessary in order to observe emission signal. When laser energy was increased enough to produce emission signal, mass resolution of the time-of-flight mass spectrum was severely deteriorated. The lifetime of the continuum ion signal was estimated 200 and 250 ns for Gd plate and C-1248, respectively, by monitoring emission signals, while the lifetime of ions near sample surface was estimated as 400 ns and 430 ns for Gd plate and C-1248, respectively. The deterioration of mass resolution can be understood as originating from the space charge effect in high plasma density in a given space and different velocity distribution of ions inside the plasma, while longer lifetime of ions near sample surface can be understood as originating from speed of ion ejection near the sample surface. The details of the characteristics of laser-induced plasma are discussed and optimum experimental conditions for simultaneous monitoring are suggested.     

Resonantly laser induced plasmas in gases: The role of energy pooling and exothermic collisions in plasma breakdown and heating

The present work is a systematic experimental study of the plasma formation in cesium vapor induced by a continuous laser tuned to the resonance transition 6S_1/2–6P_3/2. Taking into account the measured absolute population densities of Cs ground and excited state atoms as well as the electron densities derived from Stark broadening of the Cs lines, complete local thermodynamic equilibrium in the laser-produced plasma was found for laser power densities ≈ 10 Wcm^− 2 at cesium ground state number densities of about 10¹⁷ cm^− 3. Direct conversion of the excitation energy or parts of the excitation energy in exothermic collisions of laser-excited atoms is concluded to be the major process for atomic vapor heating and subsequent formation of LTE plasmas.     

Plume segregation observed in hydrogen and deuterium containing plasmas produced by laser ablation of carbon fiber tiles from a fusion reactor

The plasma produced by the irradiation of a hydrogen and deuterium containing carbon fiber composite with infrared laser pulses of 4-ns pulse duration has been investigated. The experiments were carried out under argon at reduced pressure. Microscopic analyses of the irradiated sample surface were performed to measure the ablation depth. Time- and space-resolved optical emission spectroscopy was applied to characterize the evolution of spectral line emission as a function of time and distance from the surface. Particular attention was paid to the time-of-flight characteristics of the hydrogen and deuterium Balmer α spectral lines. According to the different atomic masses of both isotopes, the expansion of hydrogen into the low pressure argon atmosphere was found to be slightly faster than that of deuterium. The effect of plume segregation is pressure dependent and tends to increase the analytical signal of heavy atoms with respect to lighter ones during laser-induced breakdown spectroscopy.     

Spatial distributions of the number densities of neutral atoms and ions for the different elements in a laser induced plasma generated with a Ni-Fe-Al alloy

Spatially-resolved emission spectroscopy, including spatial devonvolution of the spectra, has been used to determine the three-dimensional distributions of the relative number densities of neutral atoms and ions of the elements present in a laser-induced plasma generated with a Ni-Fe-Al alloy. The method is based on the precise measurement of the local electronic temperature from Saha–Boltzmann plots constructed with Fe I and Fe II lines. The plasma was generated in air at atmospheric pressure using a 1064-nm Nd:YAG laser, and the emission was detected in the time window 3.0–3.5 μs. The ionization fraction was very high (above 0.9) for the three elements in the sample, only decreasing behind the expanding plasma front. The relative number densities were obtained from the emissivities of selected elemental lines as well as the temperature. The error in this procedure was estimated, and it was found that it is largely due to the uncertainties in the transition probability values used. The spatial distributions of the total relative number densities of the three elements were shown to coincide within the error, a result which is relevant to the development of models of plasma emission used in analytical applications. The ratios of the total number densities of the elements in the plasma were compared to their concentration ratios in the sample; however, the relatively high errors in the relative number densities did not permit any definitive conclusions to be drawn about the stoichiometry of the laser ablation process.