Laser-Induced Breakdown Spectroscopy in open-path configuration for the analysis of distant objects

A review of recent results on stand-off Laser-Induced Breakdown Spectroscopy (LIBS) analysis and applications is presented. Stand-off LIBS was suggested for elemental analysis of materials located in environments where any physical access was not possible but optical access could be envisaged. This review only refers to the use of the open-path LIBS configuration in which the laser beam and the returning plasma light are transmitted through the atmosphere. It does not present the results obtained with a transportation of the laser pulses to the target through an optical fiber. Open-path stand-off LIBS has mainly been used with nanosecond laser pulses for solid sample analysis at distances of tens of meters. Liquid samples have also been analyzed at distances of a few meters. The distances achievable depend on many parameters including the laser characteristics (pulse energy and power, beam divergence, spatial profile) and the optical system used to focus the pulses at a distance. A large variety of laser focusing systems have been employed for stand-off analysis comprising refracting or reflecting telescope. Efficient collection of the plasma light is also needed to obtain analytically useful signals. For stand-off LIBS analysis, a lens or a mirror is required to increase the solid angle over which the plasma light can be collected. The light collection device can be either at an angle from the laser beam path or collinear with the optical axis of the system used to focus the laser pulses on the target surface. These different configurations have been used depending on the application such as rapid sorting of metal samples, identification of material in nuclear industry, process control and monitoring in metallurgical industry, applications in future planetary missions, detection of environmental contamination or cleaning of objects of cultural heritage. Recent stand-off analyses of metal samples have been reported using femtosecond laser pulses to extend LIBS capabilities to very long distances. The high-power densities achievable with these laser pulses can also induce self-guided filaments in the atmosphere which produce LIBS excitation of a sample. The first results obtained with remote filament-induced breakdown spectroscopy predict sample analysis at kilometer ranges.     

Early stage emission spectroscopy study of metallic titanium plasma induced in air by femtosecond- and nanosecond-laser pulses

In this work the laser induced plasma obtained in air at atmospheric pressure by the interaction of a fs (femtosecond) or a ns (nanosecond) laser pulse with a metallic titanium target has been investigated by optical emission spectroscopy. The temporal evolution of plasma parameters such as electron number density and excitation temperature has been determined in order to highlight the processes involved when the emission spectra are acquired at short time delays from the ablating laser pulse. A survey of elementary processes implicated during plasma formation and expansion of ns- and fs-Laser Induced Plasma has been performed. Departures from equilibrium conditions are even discussed. The dynamic aspects corresponding to ns- and fs-LIP have been investigated by optical time of flight (TOF) and by fast emission imaging. The overall results have been used for clarifying the basic mechanisms occurring during plasma expansion due to either ns or fs laser source when experimental conditions usually used for laser-induced breakdown spectroscopy (LIBS) applications are employed.     

Effect of power and carrier gas flow rate on the tolerance to water loading in inductively coupled plasma atomic emission spectrometry

The effects of the carrier gas flow rate and the power on the amount of water that can be tolerated by the plasma have been studied by ICP-AES. Pneumatic nebulization, ultrasonic nebulization associated with desolvation and laser ablation have been used to obtain wet, partially desolvated and dry aerosols. It has been found that water is beneficial in improving the plasma electron number density and the excitation temperature when so-called robust conditions are used, i.e. high power and low carrier gas flow rate. This can be explained by the release of hydrogen. Under these conditions, desolvation had almost no effect on the plasma characteristics. When non-robust conditions were used, the plasma was highly sensitive to water loading. Desolvation led to an improvement in the plasma conditions. In this instance, the addition of hydrogen was most useful to restore the properties of the plasma and to act as a load buffer to minimize the matrix effects. The plasma characteristics have been evaluated based on simple diagnostics such as the Mg II/Mg I line intensity ratio, the Fe excitation temperature, the Ar line and the Ar continuum.     

Calibration Measurements in laser-induced breakdown spectroscopy using nanosecond and picosecond lasers

The influence of laser pulse duration on laser-induced breakdown spectroscopy (LIBS) calibration curves is investigated in the present work. Two Nd:YAG lasers providing pulses of 35 ps and 5 ns, respectively, both operating at 1064 nm, have been used to create plasmas on aluminium, manganese, iron, and silicon targets and on prepared stoichiometric samples of these metals in a matrix. The time-resolved, space-averaged plasma temperatures have been deduced using Boltzmann plots, while the electron number density has been determined from the broadening of spectral lines. The effect of laser pulse duration on the plasma characteristics is discussed, and comparisons are made with previously reported data measured under similar experimental conditions. The optimum experimental conditions (i.e., time delay, gate width, laser energy) have been determined for reliable use of LIBS for quantitative analysis for both pulse durations. For each of the metals of interest, calibration curves have been constructed for concentrations ranging up to 2%.     

Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation

Laser-induced breakdown thresholds for several pure metals were determined using a nanosecond laser. A Q-switched pulsed Nd:YAG laser operating at infrared (1064 nm), visible (532 nm) and ultraviolet (266 nm) wavelengths has been used. The plasma was generated by focusing the Nd:YAG laser on the target in air at atmospheric pressure. The dispersed plasma light was detected using a two-dimensional intensified charge-coupled device (CCD) detector. The studied elements were chosen according to their different thermal and physical properties, particularly boiling point, melting point and thermal conductivity. The effect of wavelength on the plasma threshold has been discussed. Laser fluence thresholds in the ultraviolet were larger than those obtained using visible and infrared radiation, while the energy threshold is larger using infrared radiation. Correlations between the plasma threshold of metal targets and the melting point and boiling point at 266, 532 and 1064 nm have been established. The results indicate that thermal effects have an important influence on the ablation behavior of metals at the three wavelengths used.     

Recognition of archeological materials underwater by laser induced breakdown spectroscopy

The detection of different materials immersed in seawater has been studied by means of Laser Induced Breakdown Spectroscopy. The plasma emission was produced by a Q-Switched Nd:YAG laser operated at 1064 nm in a dual pulse mode. Different classes of materials potentially found in the undersea archaeological parks, such as iron, copper-based alloys, precious alloys, marble and wood have been examined. Data acquisition and processing were optimized for better signal control and in order to improve the detection threshold. In all the examined cases but wood, qualitative analysis was successful and allowed for the material recognition. The spectral features necessary to clearly distinguish marble materials from calcareous rocks have been also established. It was found that these characteristic spectral intervals could be also used for the recognition of sedimentary layers deposited on the underwater findings. Quantitative chemical analysis was also performed on submerged bronze samples, after generating calibration curves with standards of similar matrix composition.     

Energy characteristics and deposition rates of metals from vacuum laser plasma on dielectric substrates

The effect of the energy flux density of a pulsed Kr-F laser on the processes of generation of the laser plasma of tin, palladium and platinum and on the deposition rates of metal films on dielectric substrates in laser ablation have been studied. The use of the differential scheme of probing a laser plasma with Faraday cups and time-of-flight mass spectrometry has made it possible to distinguish, in plasma ion signals, the ranges of singly charged and multicharged ions, which differ in kinetic energy. The film deposition rate depends on both the laser energy flux density and the degree of ionization of the laser plasma.     

Application of laser plasma confinement and bending effect for direct analysis of powder sample

A new technique involving laser plasma confinement and bending effects has been developed for direct powder spectrochemical analysis by means of a laser ablation method. In this technique, a special powder sample holder has been designed to overcome the blown-off problem suffered by the powder sample and allow the detection of plasma emission to be conducted on the bended shock wave plasma outside the holder, which is practically free from the continuous emission background. The spectra of various powder samples such as CuCl₂, NaCl and ZnS were measured in this experiment. The result shows that this technique offers a promising practical alternative for direct spectrochemical analysis of powder sample of small quantity with extremely low background at reduced surrounding air pressure.     

Comparison of Different High and Low Index Materials in the Manufacture of High Laser Damage Threshold Mirrors at the 351 nm Wavelength

Sol-gel material based mirrors have been produced by forming alternate layers of high refractive index and low refractive index thin films. These mirrors have proven to have a high laser induced damage threshold [LIDT]. Using nitric acid stabilized zirconia derived from zirconium-n-propoxide and base catalyzed silica, a 16 layer mirror with a reflectivity of better than 94% at 351 nm and 45° angle of incidence was fabricated. This had an LIDT of 7.7 J/cm2 at 351 nm with a 0.7 ns pulse width. Crazing prevented further layers being deposited. Both spin and dip coating were attempted with dip coating yielding the best results.The coating structure has been analyzed using XPS depth profiling and AES. The bulk materials have been investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and TGA. High refractive index layers using Hafnia with nitric or acetic acid have also been investigated as prospective high LIDT mirrors.Alternative acidic routes to silica have been studied as a possible low index material and a route to preventing crazing.     

KrF pulsed laser ablation of thin films made from fluorinated heterocyclic poly(naphthyl-imide)s

Among the many aspects of laser ablation, development of conical structures induced by excimer laser radiation on polyimide surfaces has been thoroughly investigated. Because the mechanisms that produce these surface textures are not fully understood, two theories, photochemical bond breaking and thermal reaction, have been introduced. Here we present the first study of ultraviolet laser ablation behavior of thin films made from fluorinated poly(naphthyl-imide)s containing oxadiazole rings and the investigation of the mechanism of cone-like structure formation at two laser fluences, 57 and 240 mJ/cm(2). The morphology of thin films before and after laser ablation was studied by using various spectroscopy techniques such as Fourier transform infrared spectroscopy, time-resolved emission and X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. All of the data suggest impurities shielded at low fluence radiation (57 mJ/cm(2)) and a radiation hardening process at high value fluence (240 mJ/cm(2)), which are proposed as the main mechanisms for laser ablation of our polyimide films, and we bring evidence to support them.     

Plasma chemistry of NO in complex gas mixtures excited with a surfatron launcher

The plasma chemistry of NO has been investigated in gas mixtures with oxygen and/or hydrocarbon and Ar as carrier gas. Surface wave discharges operating at microwave frequencies have been used for this study. The different plasma reactions have been analyzed for a pressure range between 30 and 75 Torr. Differences in product concentration and/or reaction yields smaller than 10% were found as a function of this parameter. The following gas mixtures have been considered for investigation: Ar/NO, Ar/NO/O2, Ar/NO/CH4, Ar/CH4/O2, Ar/NO/CH4/O2. It is found that NO decomposes into N2 and O2, whereas other products such as CO, H2, and H2O are also formed when CH4 and O2 are present in the reaction mixture. Depending on the working conditions, other minority products such as HCN, CO2, and C2 or higher hydrocarbons have been also detected. The reaction of an Ar/NO plasma with deposits of solid carbon has also been studied. The experiments have provided useful information with respect to the possible removal of soot particles by this type of plasma. It has been shown that carbon deposits are progressively burned off by interaction with the plasma, and practically 100% decomposition of NO was found. Plasma intermediate species have been studied by optical emission spectroscopy (OES). Bands and/or peaks due to N2*, NO*, OH*, C2*, CN*, CH*, or H* were detected with different relative intensities depending on the gas mixture. From the analysis of both the reaction products and efficiency and the type of intermediate species detected by OES, different plasma reactions and processes are proposed to describe the plasma chemistry of NO in each particular mixture of gases. The results obtained provide interesting insights about the plasma removal of NO in real gas exhausts.     

Thermal-to-plasma transitions and energy thresholds in laser ablated metals monitored by atomic emission/mass spectrometry coincidence analysis

A simultaneous laser-induced plasma spectrometry/laser ionization mass spectrometry experiment has been used to follow the ion and photon intensity in laser plasmas generated over pure metallic targets as a function of fluence. The excitation conditions have been chosen to cover the range from low fluence levels, where surface desorption and thermoemission are the common processes, to the high fluence regime, characterized by plasma formation. The fluence thresholds for ion formation and plasma formation have been calculated. The dependence of both processes with melting temperature has been demonstrated.     

Temporal behavior of neutral and ionic lines emitted from a laser induced plasma on an aqueous surface

The temporal behavior of spectral lines emitted from a laser induced plasma has been studied. The plasma was created by using a Nd:YAG pulsed laser in air at atmospheric pressure focused on the surface of an aqueous solution. This work is an extension of previous published work [J. Ben Ahmed, Z. Ben Lakhdar, G. Taieb, Kinetics of laser induced plasma on an aqueous surface, Laser chem. 20 (2002) 123–134.]. The time evolution of lines intensities emitted from Ca, Ca⁺, Mg and Mg⁺ has been experimentally observed and simulated using a simple theoretical approach based on electron–ion recombination. It was shown that the plasma temperature and electron density are correlated to the dynamics of plasma emission. Finally, the time evolution of the optical depth of Ca⁺ resonance line at 393.4 nm was also studied.     

Elemental analysis of steel scrap metals and minerals by laser-induced breakdown spectroscopy

The atomic emission of laser-induced plasma on steel samples has been studied for quantitative elemental analysis. The plasma has been created with 8 ns wide pulses using the second-harmonic from a Q-switched Nd:YAG laser, in air at atmospheric pressure. The plasma emission is detected with temporal resolution, using an Echelle spectrometer of wide spectral range (300–900 nm) combined with an intensified charge coupled device camera. A plasma temperature of 7800 ± 400 K is determined using the Boltzmann plot method, from spectra obtained under optimized experimental conditions.As an example of an industrial application the concentration of copper in scrap metals is studied, which is an important factor to determine the quality of the samples to recycle. Cu concentrations down to 200 ppm can be detected. Another application of the laser-induced plasma spectroscopy method is the measurement of the nickel and copper concentrations in an iron-containing sample of reduced magma from the 1870s expedition to western Greenland by Adolf Erik Nordenskiöld. Different spectral lines of nickel are used for calibration, and their results are compared.     

Evaluation of femtosecond LIBS for spectrochemical microanalysis of aluminium alloys

The analytical performance of femtosecond laser-induced breakdown spectroscopy (LIBS) for elemental microanalysis of aluminium alloys and for mapping precipitate distribution on the sample surface has been studied in detail. A Ti–sapphire laser system producing pulses of 130 fs at 800 nm was used to generate the laser-induced plasma. Multi-element microanalysis of commercially available aluminium alloys was performed in air at atmospheric pressure. Crater characteristics such as diameter and crater morphology were characterized by optical and scanning-electron microscopy. Scaling of plasma emission and limit of detection as a function of laser pulse energy was also investigated. Current experimental results are presented and are compared with previous nanosecond microLIBS measurements.     

Laser induced surface modification of polydimethylsiloxane as a super-hydrophobic material

In order to render the surface of polydimethylsiloxane (PDMS) super-hydrophobic without changing its bulk properties, a PDMS film without photosensitizer was exposed to CO₂ pulsed laser, at room temperature, as the excitation source. The modified surfaces have been studied by performing scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDXA) and attenuated total reflectance infrared (ATR-IR) spectroscopy. To evaluate the surface property, the water drop contact angle was measured. The dependence of ---Si---O---Si infrared peak intensity, O/Si ratio and water drop contact angle of the treated PDMS as a function of the number of laser pulses were studied. SEM micrographs and water drop contact angle variations show the uniform porosity and super-hydrophobic nature on the surface of PDMS. ATR-FTIR spectra show that the modified PDMS surface contains carbonate groups which enriched the oxygen content of the surface. EDXA analysis shows a higher percentage of oxygen on the surface of the modified PDMS. The hydrophobicity of the samples was found to depend upon the number of laser pulses, but with significant variation between the treated samples. The bulk mechanical properties of PDMS after being laser-treated did not change as shown by dynamic mechanical thermal analysis (DMTA).     

Laser Induced Breakdown Spectroscopy methodology for the analysis of copper-based-alloys used in ancient artworks

In this paper Laser Induced Breakdown Spectroscopy has been applied for determining the elemental composition of a set of ancient bronze artworks coming from archaeological site of Minervino Murge — Southern of Italy (dated around VII b.C.). Before carrying on the analysis of the archaeological samples, the characterization of the analytical technique has been accomplished by investigating the trueness of the typical assumptions adopted in LIBS, such as Local Thermodynamic Equilibrium, congruent ablation and plasma homogeneity. With this purpose, two different laser pulse durations, 7 ns and 350 fs, have been used. We have focused our attention on LIBS analysis of bronze standards by considering and discussing the bases of both methodology and analytical approach to be followed for the analysis of ancient copper-based-alloy samples. Unexpectedly, regardless from the laser pulse duration, the LIBS technique has shown, by considering an adequate approach on the emitting plasma features, that its peculiarities are anyway preserved so that a fast analysis of ancient copper-based-alloys can be achieved. After verifying the suitability of the methodology, it has been possible to fulfill the typical assumptions considered for the LIBS calibration curves method and use it for ancient bronze artworks analysis.     

Ion optics of the LAMAS-10 laser time-of-flight mass spectrometer

In laser time-of-flight mass spectrometers, satisfactory resolution by weight is achieved for a fixed initial kinetic energy acquired by the ions in the plasma. The integration of mass spectra in a wide energy range intended for obtaining satisfactory results of analysis leads to a 3–5-fold reduction of the resolving capacity. In the case of the LAMAS-10 device, the resolution for W = 50–300 eV energy dispersion of the ions varies from R ∼ 1000 (for W = 300 eV) to R ∼ 150 (for W = 50 eV), such values being insufficient for quantitative elemental analysis. It has been demonstrated that the main reason for the differences in the resolution values for LAMAS-10 are time aberrations by energies in the laser plasma drift space. The modification of the ion optics theory for LAMAS-10 allows the determination of the principles of the influence of the initial ion energies generated in the laser plasma on the resolution. The dependence of the conditions of the time focusing of ions by energies has been determined in a wide range of the initial values. This dependence provides software-controlled readjustment of the time focusing before the registration of the mass spectra for each initial ion energy. When time focusing of the ions was performed in a wide range of the initial energies, the resolution increased to R = 1300 at W = 50 eV, such parameters providing the quantitative analysis of solid bodies and powders.     

Time-resolved laser-induced plasma spectrometry for determination of minor elements in steelmaking process samples

A pulsed Nd:YAG laser operating on the fourth (266 nm) and second (532 nm) harmonics has been used to generate plasmas on the target surface in air at atmospheric pressure. The influence of wavelength on quantitative analysis of 4 minor elements in stainless steel samples (Si, Ti, Nb and Mo) was investigated. Stainless steel samples with different elemental concentrations were prepared and analyzed by laser-induced plasma spectrometry (LIPS). The effect of laser wavelength on analytical figures of merit (calibration curves, correlation coefficients, linear dynamic ranges, analytical precision, and accuracy values) was found to be negligible when internal standardization (an Fe line) and time-resolved laser-induced plasma are employed. For both wavelengths, the calibration curves presented a good linearity and an acceptable linear dynamic range in the concentration interval investigated. For the four elements studied, limits of detection lower than 150 microg g(-1) were achieved. To evaluate the influence of wavelength on precision and accuracy, a set of fifteen high-alloyed steel samples from different stages of steelmaking process have been analyzed. Finally, the long-term stability of the analytical measurements for Mo with 532 nm wavelength has been discussed. RSD values were lower than 5.3% for the elements studied.     

Elemental chemical analysis of submerged targets by double-pulse laser-induced breakdown spectroscopy

Double-pulse laser-induced plasma spectroscopy (DP-LIPS) is applied to submerged targets to investigate its feasibility for elemental analysis. The role of experimental parameters, such as inter-pulse delay and detection time, has been discussed in terms of the dynamics of the laser-induced bubble produced by the first pulse and its confinement effect on the plasma produced by the second laser pulse. The analytical performance of this technique applied to targets in a water environment are discussed. The elemental analysis of submerged copper alloys by DP-LIPS has been compared with conventional (single-pulse) LIBS in air. Theoretical investigation of the plasma dynamics in water bubbles and open air has been performed.