Spectroscopic investigations of high-power laser-induced dielectric breakdown in gas mixtures containing carbon monoxide

Large-scale plasma was created in gas mixtures containing carbon monoxide by high-power laser-induced dielectric breakdown (LIDB). The composition of the mixtures used corresponded to a cometary and/or meteoritic impact into the Earth's early atmosphere. A multiple-centimeter-sized fireball was created by focusing a single 85 J, 450 ps near-infrared laser pulse into the center of a 15 L gas cell. The excited reaction intermediates that formed in various stages of the LIDB plasma chemical evolution were investigated by optical emission spectroscopy (OES) with temporal resolution. Special attention was paid to any OES signs of molecular ions. However, carbon monoxide cations were registered only if their production was enhanced by Penning ionization, i.e., excess He was added to the CO. The chemical consequences of laser-produced plasma generation in a CO-N 2-H 2O mixture were investigated using high resolution Fourier-transform infrared absorption spectroscopy (FTIR) and gas chromatography (GC). Several simple inorganic and organic compounds were identified in the reaction mixture exposed to ten laser sparks. H 2 (18)O was used to avoid possible contamination. The large laser spark triggered more complex reactivity originating in carbon monoxide than expected, when taking into account the strong triple bond of carbon monoxide causing typically inefficient dissociation of this molecule in electrical discharges.     

Optical and X-ray emission spectroscopy of high-power laser-induced dielectric breakdown in molecular gases and their mixtures

Large-scale plasma was created in molecular gases (CO, CO2, N2, H2O) and their mixtures by high-power laser-induced dielectric breakdown (LIDB). Compositions of the mixtures used are those suggested for the early earth's atmosphere of neutral and/or mildly reducing character. Time-integrated optical spectra emitted from the laser spark have been measured and analyzed. The spectra of the plasma generated in the CO-containing mixtures are dominated by emission of both C2 and CN radicals. A vibrational temperature of approximately 10(4) K was determined according to an intensity distribution in a vibronic structure of the CN (B2Sigma(+)u-X2Sigma(+)g) violet band. For comparison, the NH3-CH4-H2-H2O mixture has been irradiated as a model of the strongly reducing version of the early earth's atmosphere. In this mixture, excited CN seems to be significantly less abundant than C2. The LIDB experiments were in the molecular gases carried out not only in the static cell but also using a large, double stream pulse jet (gas puff target) placed in the vacuum interaction chamber. The obtained soft X-ray emission spectra indicate the presence of highly charged atomic ions in the hot core of high-power laser sparks.     

Detection of trace concentrations of helium and argon in gas mixtures by laser-induced breakdown spectroscopy

We report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary gas mixtures with nitrogen by laser-induced breakdown spectroscopy (LIBS). Although significant quenching of helium transitions due to collisional deactivation of excited species was observed, it was found that losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analyses were performed. In consequence, limits of detection of parts-per-million and tens of parts-per-million and linear dynamic ranges of several orders of magnitude in analyte concentration were obtained. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations.     

Diagnostics of water-containing gas mixtures using thermal laser-induced gratings

Laser-induced gratings (LIGs) experiments, employing vibrational-overtone excitation of H₂O molecules through rovibrational transitions of the (2 1 1) ← (0 0 0) band around 820 nm to produce gratings, have been performed in humid ambient air, in a room-temperature cell with neat water vapor or that diluted in buffer gas at pressures of 0.02–4 bar, and in the vicinity of premixed atmospheric pressure CH₄-air flame, with the aim to investigate the feasibilities of the techniques for accomplishing gas diagnostics or tracing H₂O at various temperatures and pressures.

In highly vibrationally-excited H₂O rapid multi-stage collisional thermalization processes occur, and as a result both strong oscillatory (due to standing acoustic waves) and stationary (due to stationary density modulations) contributions to LIGs have been observed in gas mixtures.

Temporal profiles of LIG signals recorded at various conditions of gas mixtures employed have been analysed in frame of a model of a two- or three-stage R–T, V–V′, and V–T collisional relaxation of H₂O and buffer gas molecules. The informative parameters, like adiabatic sound velocity, gas temperature, H₂O mole fraction have been derived. The results of the analysis are presented, demonstrating possibilities and challenges of the technique in application to gas diagnostics.     

Influence of ambient gas pressure on laser-induced breakdown spectroscopy technique in the parallel double-pulse configuration

Single-pulse and double-pulse laser-induced breakdown spectroscopy experiments have been performed using two Nd:YAG lasers in the fundamental mode on a brass sample at different air pressures, ranging from 0.1 Torr to atmospheric conditions, in order to obtain information about the different ablation and plasma evolution processes in the different configurations. Neutral and ionized lines originated both by species deriving from the target and from the air environment were analysed. The temperature and electron density values were estimated in all the experimental conditions. A different behavior of the plasma emission versus the air pressure, in the case of lines deriving from the target, was observed in the single-pulse and double-pulse configurations, suggesting that the different environmental conditions in the first and the second laser ablation may be responsible in determining the plasma emission in the two cases. An interpretative model based on the cavity produced in air by the laser-induced shock wave, according to the Sedov theory of the blast wave expansion, was able to qualitatively describe the effects observed in single-pulse and double-pulse experiments.

Besides, the influence of the interpulse delay time between the two laser pulses was explored in the range between 0 and 20 μs. The results, according to the model proposed, provide information on the plume evolution in the single-pulse and double-pulse configurations at different air pressures. In particular, different optimum interpulse delays were found for the observation of neutral lines and ionic lines.     

Determination of the deuterium/hydrogen ratio in gas reaction products by laser-induced breakdown spectroscopy

This paper reports the first application of laser-induced breakdown spectroscopy technique (LIBS) to the determination of deuterium/hydrogen numeric ratio (β) in the headspace gases, essentially HD + H₂, that are generated by the hydrolysis of NaBD₄–NaBH₄ mixtures (molar fraction of NaBD₄, x = 50–100%) in acidic H₂O media (0 < pH < 1). The LIBS measurement of β can be easily achieved with a coefficient of variation better than 5% (over four replicates). The value of β allowed the calculation of the molar fraction of NaBD₄, x_LIBS, with a coefficient of variation better than 2.5%. The comparison of x vs. x_LIBS gives results that are in good agreement, within an average deviation of 3%, for x in the range of 50–100%. The best performances are obtained for β close to unit, which makes LIBS perfectly suited for the detection of H–D exchange taking place during aqueous hydrolysis of NaBD₄ or NaBH₄.     

Application of laser-induced breakdown spectrometry in urban health

The use of laser-induced breakdown spectrometry (LIBS) for elemental determination and detection in urban health is reviewed. It highlights the unique use of LIBS in fiber-based optics, in process control, and for field instrumentation. Selected applications confirm that LIBS could be the technique of choice for many applications in urban health.     

Femtosecond laser-induced breakdown spectroscopy of sea water

The composition of the line and band spectra of the plasma induced by a femtosecond laser pulse on the surface of sea water is determined. The temporal behaviors of the intensity of the continuum and the Ca II, Mg II and Na I lines are investigated. It is shown that the time dependence of the intensity of the Na I line is described by a monoexponential function. The characteristic decay times of the line intensities of Mg II and Na I were used to estimate the three-body recombination times. Using these values, we estimate the electron number density and the feasibility of Local Thermodynamic Equilibrium (LTE) criterion. A method involving excitation rate constants is proposed for the comparison of detection limits. For a plasma generated on a liquid surface, the following relation among detection limits will be obtained: LOD(Na) < LOD(K) < LOD(Ca) < LOD(Al) < LOD(Mg) < LOD(Zn).     

Atomic and molecular emissions in laser-induced breakdown spectroscopy

This article summarizes measurements and analysis of hydrogen Balmer series atomic lines following laser-induced optical breakdown. Electron number density on the order of 1 × 10²⁵ m^− 3 can be measured using H_α Stark width and shift in the analysis of breakdown plasma in 1 to 1.3 × 10⁵ Pa, gaseous hydrogen. The H_β line can be utilized for electron number density up to 7 × 10²³ m^− 3. The historic significance is elaborated of accurate H_β measurements. Electron excitation temperature is inferred utilizing Boltzmann plot techniques that include H_γ atomic lines and further members of the Balmer series. Laser ablation of aluminum is discussed in view of limits of application of the Balmer series. H_β and H_γ lines show presence of molecular carbon in a 2.7 and 6.5 × 10⁵ Pa, expanding methane flow. Diagnostic of such diatomic emission spectra is discussed as well. Laser-induced breakdown spectroscopy historically embraces elemental analysis, or atomic spectroscopy, and to a lesser extent molecular spectroscopy. Yet occurrence of superposition spectra in the plasma decay due to recombination or due to onset of chemical reactions necessitates consideration of both atomic and molecular emissions following laser-induced optical breakdown. Molecular excitation temperature is determined using so-called modified Boltzmann plots and fitting of spectra from selected molecular transitions. The primary interest is micro-plasma characterization during the first few micro-seconds following optical breakdown, including shadowgraph visualizations.     

Lead determination in glasses by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been used to determine the lead content of different types of lead silicate glasses commercially designed as sonorous glass (which contain ∼ 10 wt.% PbO); crystal glass (with at least 24 wt.% PbO) and superior crystal glass (with at least 30 wt.% PbO). Seven different types of glass samples were selected, including historic-original, model and commercially available. The selected samples were artificially weathered under neutral, acid and alkaline attack. Analysis by LIBS was carried out in vacuum under excitation at 266 nm and results were compared with those obtained by conventional techniques used for glass characterization. Composition of the bulk glasses was analyzed by XRF (X-ray fluorescence) and the corroded surfaces were characterized by SEM/EDX (scanning electron microscopy/energy dispersive X-ray microanalysis). A linear correlation was obtained between the intensity of selected Pb lines in the LIB spectra and the PbO content. The effect of corrosion could be characterized by comparing successive LIB spectra recorded on the same area; acid attack resulted in a decrease of PbO, CaO and Na₂O content in the surface with respect to the bulk of the sample, while minor changes in the composition were noticed under alkaline attack. These results show LIBS as a useful technique to classify the different types of lead glasses by their lead content and to determine and asses the degree and type of corrosion.     

Pulse chirp effects in ultrafast laser-induced breakdown spectroscopy

When compared to many other sensitive methods for material detection, such as inductively coupled mass spectroscopy and thermal ionization mass spectroscopy, laser-induced breakdown spectroscopy (LIBS) typically exhibits a lower signal-to-noise ratio (SNR), resulting in higher detection limits. Increasing the SNR of LIBS would improve the ability to characterize the sample composition with increased accuracy and speed and reduce the amount of material needed to perform analysis. We have been investigating the effect of simple ultrashort laser pulse shaping on the SNR of LIBS. Our goal is to control the dynamics of the ionization and recombination processes in the laser-produced plasma to favorably affect the SNR associated with the line emission from the plasma. Pulse shaping is performed using an acousto-optic programmable dispersive filter. An adaptive learning algorithm is being developed to automate the pulse shape optimization process for maximization of LIBS SNR in nuclear security-relevant material characterization scenarios. We report a 27 % increase of the SNR for non-gated LIBS measurements of uranium by utilizing simple pulse shaping limited exclusively to excess quadratic spectral phase of the laser pulse.     

Analysis of heavy metals in soils using laser-induced breakdown spectrometry combined with laser-induced fluorescence

The investigation of a hyphenated technique combining laser-induced breakdown spectrometry (LIBS) with laser-induced fluorescence (LIF) for the analysis of heavy metals in soils is described. In order to evaluate the applicability of the technique for fast in-situ analytical purposes, measurements were performed at atmospheric pressure. The plasma radiation was detected using a Paschen–Runge spectrometer equipped with photomultipliers for the simultaneous analysis of 22 different elements. The photomultiplier signals were processed by a fast gateable multichannel integrator. Calibration curves were recorded using a set of spiked soil samples. Limits of detection were derived from these curves for As (3.3 μg/g), Cd (6 μg/g), Cr (2.5 μg/g), Cu (3.3 μg/g), Hg (84 μg/g), Ni (6.8 μg/g), Pb (17 μg/g), Tl (48 μg/g) and Zn (98 μg/g) using the LIBS signals. LIBS-LIF measurements were performed for Cd and Tl. The excitation wavelength as well as the detected fluorescence wavelength for Cd was 228.8 nm. Alternatively, Tl was excited at 276.8 nm, where the observed fluorescence wavelength was 351.9 nm. The calibration curves based on the LIF signals showed significantly improved limits of detection of 0.3 and 0.5 μg/g for Cd and Tl, respectively.     

Radiative models of laser-induced plasma and pump-probe diagnostics relevant to laser-induced breakdown spectroscopy

The paper describes past and present efforts in modeling of laser-induced plasma and overviews plasma diagnostics carried out by pump-probe techniques. Besides general information on existing plasma models, the emphasis is given to models relevant to spectrochemical analysis, i.e. models of radiating plasma. Special attention is paid to collisional-radiative (CR) and collisional-dominated (CD) plasma models where radiative processes play an important role. Also, calibration-free (CF) models are considered which may endow with the possibility for standardless spectroscopic analysis. In the diagnostic part, only methods based on the use of additional diagnostic tools (auxiliary lasers, optics, and probes) are described omitting those based on plasma own radiation. A short review is provided on image-based diagnostics (shadowgraphy, schlieren, and interferometry), absorption and fluorescence, Langmuir probe, and less frequently used cavity ringdown and Thomson scattering methods.     

Vacuum ultraviolet laser-induced breakdown spectroscopy analysis of polymers

Laser-induced breakdown spectroscopy (LIBS) in the vacuum ultraviolet range (VUV, λ < 200 nm) is employed for the detection of trace elements in polyethylene (PE) that are difficult to detect in the UV/VIS range. For effective laser ablation of PE, we use a F₂ laser (wavelength λ = 157 nm) with a laser pulse length of 20 ns, a pulse energy up to 50 mJ, and pulse repetition rate of 10 Hz. The optical radiation of the laser-induced plasma is measured by a VUV spectrometer with detection range down to λ = 115 nm. A gated photon-counting system is used to acquire time-resolved spectra. From LIBS measurements of certified polymer reference materials, we obtained a limit of detection (LOD) of 50 µg/g for sulphur and 215 µg/g for zinc, respectively.The VUV LIBS spectra of PE are dominated by strong emission lines of neutral and ionized carbon atoms. From time-resolved measurements of the carbon line intensities, we determine the temporal evolution of the electronic plasma temperature, T_e. For this, we use Saha–Boltzmann plots with the electron density in the plasma, N_e, derived from the broadening of the hydrogen H-α line. With the parameters T_e and N_e, we calculate the intensity ratio of the atomic sulphur and carbon lines at 180.7 nm and at 175.2 nm, respectively. The calculated intensity ratios are in good agreement with the experimentally measured results.     

Review: Applications of single-shot laser-induced breakdown spectroscopy

As applications for laser-induced breakdown spectroscopy (LIBS) become more varied with a greater number of field and industrial LIBS systems developed and as the technique evolves to be more quantitative that qualitative, there is a more significant need for LIBS systems capable of analysis with the use of a single laser shot. In single-shot LIBS, a single laser pulse is used to form a single plasma for spectral analysis. In typical LIBS measurements, multiple laser pulses are formed and collected and an ensemble-averaged method is applied to the spectra. For some applications there is a need for rapid chemical analysis and/or non-destructive measurements; therefore, LIBS is performed using a single laser shot. This article reviews in brief several applications that demonstrate the applicability and need for single-shot LIBS.     

Characterization of jewellery products by laser-induced breakdown spectroscopy

The suitability of laser-induced breakdown spectroscopy (LIBS) for the characterization of jewellery products is demonstrated by the development of a method based on the use of an Nd-YAG laser (operating at 532 nm) which induces ablation of the material and the production of a plasma whose emission reaches 1/8 m spectrograph (connected to a coupled charge detector (CCD)) through an optic fiber. The treatment of the instrumental signal provides enough analytical information, both for identifying and quantifying the major metals present in this type of material. The method proposed has been developed both by multivariate optimization and calibration procedures with application of the appropriate quality criteria. The chemometric analysis of the data and the use of PLS regression for calibration guarantee the ruggedness of the proposed method. The study of the emission spectra allows characterization of the most common noble metals (gold and silver) as well as other metals present in jewellery pieces.     

Calibration free laser-induced breakdown spectroscopy of oxide materials

The quantitative determination of oxide concentration by laser-induced breakdown spectroscopy is relevant in various fields of applications (e.g.: analysis of ores, concrete, slag). Calibration free laser-induced breakdown spectroscopy and the multivariate calibration are among the methods employed for quantitative concentration analysis of complex materials. We measured the intensity of neutral and ionized atomic emission lines of oxide materials by laser-induced breakdown spectroscopy and we modified the calibration free laser-induced breakdown spectroscopy method to increase the accuracy. The concentration of oxides was obtained by using stoichiometric relations. Sample materials were prepared from oxide powder (Fe₂O₃, MgO, CaO) by mixing and pressing. The concentration was 9.8–33.3 wt.% Fe₂O₃, 7.6–33.3 wt.% MgO and 33.3–81.2 wt.% CaO for different samples. Nd:YAG laser (wavelength 1064 nm, pulse duration ≈ 6 ns) ablation was performed in air. The laser-induced plasma emission was measured by an Echelle spectrometer equipped with a sensitivity calibrated ICCD camera. The numerical calibration free laser-induced breakdown spectroscopy algorithm included the fast deconvolution of instrumental function, and the correction of self-absorption effects. The oxide concentration C_CF calculated from calibration free laser-induced breakdown spectroscopy results and the nominal concentration C_N were very close for all samples investigated. The relative error in concentration, |C_CF–C_N|/C_N, was < 10%, < 20%, and < 5% for Fe₂O₃, MgO, and CaO, respectively. The results indicate that this method can be employed for the analysis of major elements in multi-component technical materials.     

Absolute characterization of laser-induced breakdown spectroscopy detection systems

A quantitative comparison of the performance of four different laser-induced breakdown spectroscopy detection systems is presented. The systems studied are an intensified photodiode array coupled with a Czerny–Turner spectrometer, an intensified CCD coupled with a Czerny–Turner spectrometer, an intensified CCD coupled to an Echelle spectrometer, and a prototype multichannel compact CCD spectrometer system. A simple theory of LIBS detection systems is introduced, and used to define noise-equivalent spectral radiance and noise-equivalent integrated spectral radiance for spectral detectors. A detailed characterization of cathode noise sources in the intensified systems is presented.     

On chemical reactions in the laser-induced breakdown of a liquid

It is shown experimentally that a laser-induced breakdown of a liquid is accompanied by chemical reactions initiated by radicals and excited species formed in the spark. It is found that, in water, the laser-induced breakdown is accompanied by the dissociation of water and dissolved nitrogen molecules with the formation of HNO₂ and HNO₃, while, in a FeSO₄ aqueous solution, by the Fe²⁺ → Fe³⁺ oxidation reaction. It is assumed that the mechanism of the process is analogous to that of the action of ionizing radiations and the chemical action of ultrasonically induced cavitation (it is proposed that this mechanism of chemical action of a laser-induced spark proposed be termed indirect). Energy yields of these reactions are found to be of the same order of magnitude as for sonochemical redox reactions. It is shown that the laser-induced breakdown of an aqueous solution of maleic acid is accompanied by its stereoisomerization into fumaric acid, a process catalyzed by small amounts of an alkyl bromide. It is established that, for the formation of fumaric acid in a laser-induced spark, the energy yield is about five orders of magnitude higher than that typical of the above-mentioned redox reactions in the laser-induced spark.