Measured Stark widths of several spectral lines of Pb III

The Stark full widths at half of the maximal line intensity (FWHM, ω) have been measured for 25 spectral lines of Pb III (15 measured for the first time) arising from the 5d¹⁰6s8s, 5d¹⁰6s7p, 5d¹⁰6s5f and 5d¹⁰6s5g electronic configurations, in a lead plasma produced by ablation with a Nd:YAG laser. The optical emission spectroscopy from a laser-induced plasma generated by a 10 640 Å radiation, with an irradiance of 2 × 10¹⁰ W cm^− 2 on a lead target (99.99% purity) in an atmosphere of argon was analysed in the wavelength interval between 2000 and 7000 Å. The broadening parameters were obtained with the target placed in argon atmosphere at 6 Torr and 400 ns after each laser light pulse, which provides appropriate measurement conditions. A Boltzmann plot was used to obtain the plasma temperature (21,400 K) and published values of the Stark widths in Pb I, Pb II and Pb III to obtain the electron number density (7 × 10¹⁶ cm^− 3); with these values, the plasma composition was determined by means of the Saha equation. Local Thermodynamic Equilibrium (LTE) conditions and plasma homogeneity has been checked. Special attention was dedicated to the possible self-absorption of the different transitions. Comparison of the new results with recent available data is also presented.     

Measurement of Stark broadening of Mn I and Mn II spectral lines in plasmas used for Laser-Induced Breakdown Spectroscopy

We present measurements of the Stark broadening of several Mn lines in the conditions of typical laser-induced plasmas. Single-and double-pulse Laser-Induced Breakdown spectroscopy (LIBS) configurations are studied on a series of Fe–Mn alloy samples with Mn concentration ranging from 6% to 30%. The effects of self-absorption on the measured line broadenings are discussed in detail. In particular, the experimental results evidence that self-absorption is much higher in laser-induced plasmas generated with double pulses, compared to the case of single pulse. After measurement of the electron density, the Stark coefficients of several neutral and ionic Mn lines are derived through the measure of the broadening in conditions of optically thin plasma. The results obtained for singly ionized Mn lines are compared with the theoretical and experimental data present in the literature. For the first time, experimental measurements of the Stark coefficient for several neutral Mn lines are also presented.     

Evaluation of self-absorption coefficients of aluminum emission lines in laser-induced breakdown spectroscopy measurements

In quantitative Laser Induced Breakdown Spectroscopy (LIBS) measurements it is essential to account for the effect of self-absorption on the emission lines intensity. In order to quantify this effect, in this paper we propose a simple method for evaluating the ratio between the actual measured line intensity and the intensity expected in absence of self-absorption and, if necessary, correcting the effect of self-absorption on line intensity. The method, based on a homogeneous plasma model, is applicable when the plasma electron density is known and in particular to lines whose Stark broadening parameter is available.     

Application of a laser produced plasma: Experimental Stark widths of single ionized lead lines

The optical emission spectroscopy from laser produced plasma generated by a 10,640 Å radiation, with an irradiance of 1.4 × 10¹⁰ W cm^− 2 on several lead targets, in vacuum and in an atmosphere of argon, was recorded and analyzed between 1900 and 7000 Å. The Local Thermodynamic Equilibrium conditions and plasma homogeneity have been checked. Stark widths for 31 lines of Pb II have been measured. These lines measured in this work correspond to the transitions 7s → 6p, n(n = 8, 9, 10)s → 7p, n(n = 7, 8)p → 7s, n(n = 7, 8)p → 6p², n(n = 7, 8)d → 7p, n(n = 5, 6)f → 6d, n(n = 5, 6)f → 6p². The population level distribution and the corresponding temperatures were obtained using Boltzmann plots. The plasma electron densities were determined using well-known Stark broadening parameters of spectral lines. Special attention was dedicated to the possible self-absorption of the different transitions. Temporal evolution of the plasma parameters was studied between 0.1 and 9 μs. Stark broadening parameters of the spectral lines were measured at 2.5 μs where the electron temperature was close to 11,300 K and the electron density to 0.8 × 10¹⁶ cm^− 3. The experimental results obtained have been compared with the experimental and theoretical values given by other authors. A systematic trend of this parameter versus temperature of 4244.9 Å Pb II line has been presented.     

Laser-induced fluorescence detection of lead atoms in a laser-induced plasma: An experimental analytical optimization study

The combination of the laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) techniques was investigated to improve the limit of detection (LoD) of trace elements in solid matrices. The influence of the main experimental parameters on the LIF signal, namely the ablation fluence, the excitation energy, and the inter-pulse delay, was studied experimentally and a discussion of the results was presented. For illustrative purpose we considered detection of lead in brass samples. The plasma was produced by a Q-switched Nd:YAG laser and then re-excited by a nanosecond Optical Parametric Oscillator (OPO) laser. The experiments were performed in air at atmospheric pressure. We found out that the optimal conditions were obtained for our experimental set-up using relatively weak ablation fluence of 2–3 J/cm² and an inter-pulse delay of about 5–10 μs. Also, a few tens of microjoules was typically required to maximize the LIF signal. Using the LIBS–LIFS technique, a single-shot LoD for lead of about 1.5 part per million (ppm) was obtained while a value of 0.2 ppm was obtained after accumulating over 100 shots. These values represent an improvement of about two orders of magnitude with respect to LIBS.     

Parametric study of an atmospheric pressure microwave-induced plasma of the mini MIP torch — I. Two-dimensional spatially resolved electron-number density measurements

The results of spatial distribution measurements of the electron-number density, N_e, in an atmospheric pressure microwave-induced plasma in argon are presented. Electron-number density is determined from the width of the hydrogen H_β 486.13-nm line. The influence of the microwave input power in the range 80–150 W on the spatial distribution of N_e is first determined. For the selected input power of 100 W, the influence of molecular hydrogen in wet and desolvated nebulizer and support gas, and the corresponding changes in the electron density distributions are studied. The influence of potassium as a low ionization-potential element on the spatial distribution of N_e is also studied, with the result that a considerable lowering of the electron number density is detected.     

Application of calibration-free laser-induced breakdown spectroscopy to radially resolved spectra from a copper-based alloy laser-induced plasma

In this work, the Calibration-Free approach for Laser-Induced Breakdown Spectroscopy (CF-LIBS) was applied for the first time to radially resolved spectra emitted by a laser-induced plasma. The radial profiles of plasma temperature and electron number density were used to calculate the local relative concentration of the elements of interest. We analyzed a set of profiles of the local spectral emission coefficient obtained previously by means of spatial deconvolution of the spectra from a copper-based alloy (Cu 93, Fe 5, Mn 1, Ni 1 wt.%) laser-induced plasma. A spatially integrated spectrum of the same plasma was also analyzed for comparison purpose. The relative abundance of the minor components Fe, Mn and Ni was calculated. The results obtained from the central region of the plasma were closer to the nominal concentrations than those obtained from the spatially integrated spectrum. However, an increasing deviation was observed towards the plasma edge. It is proposed that this deviation could be the result of a gradual departure from Local Thermodynamic Equilibrium (LTE) conditions due the significant decrease of the electron density at the external shells of the plasma.     

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.     

Spectroscopic diagnostics of laser-induced plasmas

An overview of spectroscopic diagnostics techniques for low temperature plasmas is presented with an emphasis to electron number density — N_e measurement. Stark broadening of non-hydrogenic atom and positive ion spectral lines is given. The attention is drawn to experimental techniques used for line intensity and line profile measurement. Self-absorption test, importance of Abel inversion, deconvolution of experimental line profiles and measurement of line asymmetry are treated in some detail in order to improve N_e measurements. Finally the sources of theoretical and experimental Stark broadening data are reviewed and some details discussed.     

An evaluation of methods for estimating the electron Stark widths of atomic spectral lines

Four methods for the calculation of electron Stark width of atomic spectral lines are evaluated. All methods provide estimates to the same order of magnitude, but no technique is best for all spectral lines. The success of a method depends on the nature and extent of knowledge of properties of the excited state.Use of the appropriate method will allow the computation of a meaningful estimate for the Stark width of any spectral line of interest in an analytical plasma.     

Analysis of glass and glass melts during the vitrification process of fly and bottom ashes by laser-induced plasma spectroscopy. Part I: Normalization and plasma diagnostics

For laser-induced plasma spectroscopy (LIPS) analysis of the main components (Si, Al, and Ca) in glasses utilized for vitrification of ashes from waste incineration, a normalization procedure for line ratios is presented. Even in homogeneous glass samples, considerable pulse-to-pulse variations of the plasma electronic excitation temperature and electron density were observed because of changes in the material–laser interaction. A normalization procedure is outlined using Saha–Boltzmann equilibrium relationships to include the electronic excitation temperature and density in the calibration model. As a result of the normalization, the variation of the line ratios is reduced and linear calibrations for LIPS intensity ratios versus concentration ratios are achieved. For samples with high aluminum concentrations, the analysis was hampered by self-reversal effects.     

Stark widths and shifts of SnI,HgII, HgIII and PbII spectral lines

Stark widths and shifts of neutral and ionized heavy atom spectral lines have been measured and calculated. The Stark parameters of three SnI (284.0, 286.3 and 303.4 nm), five HgII (226.2, 398.4, 222.5, 615.0 and 326.4 nm), two PbII (220.4 and 438.6 nm) and one HgIII (235.4 nm) spectral lines were measured for the first time except the Stark widths of one HgII (398.4 nm) and one PbII (438.6 nm) line. Stark width values for a number of corresponding transitions were calculated on the bases of semiclassical and semiempirical formulae.     

Spatial and temporal probing of a laser-induced plasma plume by cavity ringdown spectroscopy

The laser-induced plasma plume of a Ti target in vacuum is probed by the technique of cavity ringdown spectroscopy. A model is developed to perform a forward convolution of atomic absorption line profile measurements. The model accounts for laser-induced plasma characteristics such as anisotropy of the plume and velocity distributions of the ablated particles as well as of the cavity ringdown features such as geometry and time selectivity. The absorption lineshapes of atomic transitions are calculated and discussed for given sets of parameters. Calculated line profiles are fitted to experimental line profiles obtained from nanosecond-laser ablation of the target and provide data about the plume dynamics.     

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.     

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.     

Real time measurement of the electron density of a laser generated plasma using a RC circuit

A Nd:YAG laser pulse was focused, in air or on a Cu target, between the plates of a planar charged capacitor. The plasma generates a transient redistribution of the electrical charges on the plates that can be easily measured as a voltage drop across a resistor connected to the ground plate. At the same time, the Stark broadening of the H_α spectral line (656.3 nm) obtained from the optical emission spectrum of the plasma was measured. In this work, we show that the peak of electrical signal measured on the resistor is, in the energy range of our laser (30 mJ to 220 mJ) and at time delays typical of Laser-Induced Breakdown Spectroscopy applications (500–5000 ns), univocally related to the temporal evolution of the Stark broadening of the H_α line. Therefore, after a proper calibration depending on the material and the experimental geometry, the peak of the electrical signal can be used to predict the temporal evolution of the electron density of the generated plasma.     

Kinetic modeling study of the laser-induced plasma plume of cyclotrimethylenetrinitramine (RDX)

A kinetic model of a laser-induced breakdown spectroscopy (LIBS) plume of cyclotrimethylenetrinitramine (RDX) was developed for the analysis of processes responsible for the LIBS signature of explosives. Air and argon were considered as buffer gases. The model includes a set of processes involving ion chemistry, as well as excitation, ionization, and other processes affecting neutral and ion species. Modeling results show that the overall reaction process can be considered a two-stage process. The first stage corresponds to a fast approach to a quasi-stationary state, while the second stage corresponds to the change of quasi-stationary species concentrations due to the change in temperature. As a result of the two-stage process, the initial mechanism of explosive decomposition is not important in determining its signature in the LIBS measurement time window (1–30 μs). The main processes responsible for generation of excited states for the LIBS emission are electron-excitation impact processes. A mechanism for the appearance of a double peak of the C₂ species concentration in the RDX plasma plume was suggested. Double-peak behavior of the C₂ species was previously experimentally observed during laser ablation of graphite.     

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.     

Time-space distribution of laser-induced plasma parameters and its influence on emission spectra of the laser plumes

A physical model is developed accounting for dynamics and radiation of plasma plumes induced by nanosecond laser pulses on surface of solid samples. The model has been applied to simulate emission spectra of the laser erosion plasma at the elemental analysis of metals using single- and double-pulse excitation modes.     

Study of early laser-induced plasma dynamics: Transient electron density gradients via Thomson scattering and Stark Broadening,and the implications on laser-induced breakdown spectroscopy measurements

To further develop laser-induced breakdown spectroscopy (LIBS) as an analytical technique, it is necessary to better understand the fundamental processes and mechanisms taking place during the plasma evolution. This paper addresses the very early plasma dynamics (first 100 ns) using direct plasma imaging, light scattering, and transmission measurements from a synchronized 532-nm probe laser pulse. During the first 50 ns following breakdown, significant Thomson scattering was observed while the probe laser interacted with the laser-induced plasma. The Thomson scattering was observed to peak 15–25 ns following plasma initiation and then decay rapidly, thereby revealing the highly transient nature of the free electron density and plasma equilibrium immediately following breakdown. Such an intense free electron density gradient is suggestive of a non-equilibrium, free electron wave generated by the initial breakdown and growth processes. Additional probe beam transmission measurements and electron density measurements via Stark broadening of the 500.1-nm nitrogen ion line corroborate the Thomson scattering observations. In concert, the data support the finding of a highly transient plasma that deviates from local thermodynamic equilibrium (LTE) conditions during the first tens of nanoseconds of plasma lifetime. The implications of this early plasma transient behavior are discussed in the context of plasma–analyte interactions and the role on LIBS measurements.