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The LIBS signal depends both on the ablated mass and on the plasma excitation temperature. These fundamental parameters depend in a complex manner on laser ablation and on laser–plasma coupling. As several works in the literature suggest that laser ablation processes play a predominant role compared to plasma heating phenomena in the LIBS signal variations, this paper focuses on the study of laser ablation. The objective was to determine an interaction regime enabling to maximally control the laser ablation. Nanosecond laser ablation of copper at 266 nm was characterized by scanning electron microscopy and optical profilometry analysis, in air at 1 bar and in the vacuum. The laser beam spatial profile at the sample surface was characterized in order to give realistic values of the irradiance. The effect of the number of accumulated laser shots on the crater volume was studied. Then, the ablation crater morphology, volume, depth and diameter were measured as a function of irradiance between 0.35 and 96 GW/cm². Results show that in the vacuum, a regular trend is observed over the whole irradiance range. In air at 1 bar, below a certain irradiance, laser ablation is very similar to the vacuum case, and the ablation efficiency of copper was estimated at 0.15 ± 0.03 atom/photon. Beyond this irradiance, the laser beam propagation is strongly disrupted by the expansion of the dense plasma, and plasma shielding appears. The fraction of laser energy used for laser ablation and for plasma heating is estimated in the different irradiance regimes.

Laser-Induced Breakdown Spectroscopy (LIBS) experiments are performed on standard metallic samples, in air at atmospheric pressure, using a Nd:YAG laser at 1064 nm and a fiber located close to the plasma to collect its emission. This configuration is chosen because it is representative of many LIBS setups. The influence of several experimental parameters is studied in order to optimize the analytical performances: signal-to-background ratio (SBR), line intensity and repeatability. Temporal parameters of the detector are adjusted for each measurement to maximize the SBR. The signal is found to linearly depend on the pulse energy over our range of investigation. This behavior is related to the increase of the number of vaporized atoms when the pulse energy increases. Complementary measurements of plasma dimensions support our conclusions. We show the existence of an optimum fluence on the sample that gives the highest signal and the lowest relative standard deviation (RSD), and which does not depend on the pulse energy. Finally we demonstrate that ablation is much more efficient using a laser beam with a high numerical aperture, other experimental parameters being unchanged, because of a less pronounced laser shielding by the plasma. Analytical consequences of this result are discussed.

In order to set up the precision limit that can be reached with laser-induced breakdown spectrometry microprobe, a steel sample was scanned by using a 6-µm diameter spot. Besides being close to the limit of the technique, such a spot diameter resulted in a small plasma size that minimized self-absorption effects. To minimize shot noise, Cr and Fe were used as test elements because of their high contents. Scan consisted of 25 successive matrices formed by 5 × 6 shots, i.e. a total of 750 shots. Results were studied as a whole, as well as between matrices and within matrices, to search for inhomogeneity, outliers and drift. Except a few outliers, the main contribution in the experimental RSD was the drift either within a matrix or between matrices. Drift attributed to laser warm up could be compensated for either by using a polynomial fitting or by using the other element. %RSD significantly below 2 were then obtained demonstrating that there is no penalty in terms of precision to perform laser microprobe using a series of single shots.

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.

International audience; ChemCam is a remote sensing instrument suite on board the "Curiosity" rover (NASA) that uses Laser-Induced Breakdown Spectroscopy (LIBS) to provide the elemental composition of soils and rocks at the surface of Mars from a distance of 1.3 to 7 m, and a telescopic imager to return high resolution context and micro-images at distances greater than 1.16 m. We describe five analytical capabilities: rock classification, quantitative composition, depth profiling, context imaging, and passive spectroscopy. They serve as a toolbox to address most of the science questions at Gale crater. ChemCam consists of a Mast-Unit (laser, telescope, camera, and electronics) and a Body-Unit (spectrometers, digital processing unit, and optical demultiplexer), which are connected by an optical fiber and an electrical interface. We then report on the development, integration, and testing of the Mast-Unit, and summarize some key characteristics of ChemCam. This confirmed that nominal or better than nominal performances were achieved for critical parameters, in particular power density (>1 GW/cm2). The analysis spot diameter varies from 350 μm at 2 m to 550 μm at 7 m distance. For remote imaging, the camera field of view is 20 mrad for 1024×1024 pixels. Field tests demonstrated that the resolution (˜90 μrad) made it possible to identify laser shots on a wide variety of images. This is sufficient for visualizing laser shot pits and textures of rocks and soils. An auto-exposure capability optimizes the dynamical range of the images. Dedicated hardware and software focus the telescope, with precision that is appropriate for the LIBS and imaging depths-of-field. The light emitted by the plasma is collected and sent to the Body-Unit via a 6 m optical fiber. The companion to this paper (Wiens et al. this issue) reports on the development of the Body-Unit, on the analysis of the emitted light, and on the good match between instrument performance and science specifications.

An experimental arrangement for elemental analysis of solid samples in air at atmospheric pressure is presented. The arrangement couples optical emission spectroscopy with XeCl ultraviolet laser ablation. The experimental conditions to bring about optimum reproducibility of the analytical measurement are given. It is shown that the plasma emission signal is strongly dependent on the number of shots delivered at a given place. Hence, a good control of the ablation process requires a knowledge of the exact number of shots delivered before and during signals acquisition. Time resolved measurements where performed with a time delay in the range of 1–4 μs in order to eliminate background emission and to avoid data recording during plasma collapse. Analytical performances have been evaluated for detection of magnesium in aluminum alloys using the internal standard technique. Reproducibility was investigated for several aluminum matrices, by comparing measurements performed over several days, and was found to be 2%. A single straight calibration curve was obtained for four aluminum alloys, thus indicating the absence of second order matrix effects.

Uranium measurements taken in a hostile environment are required in the nuclear fuel cycle. Current trends are leading toward development of better sensitivity and rapidity as well as remote measurements. Time-Resolved Laser-Induced Fluorescence (TRLIF) is a very selective and sensitive method that has been widely used for uranium determinations in complex matrices. This technique evolved from analysis in a cell located in a glove box to remote measurements via fiber optics in a shielded cell. The setup of Remote Time-Resolved Laser-Induced Fluorescence (RTRLIF) includes an excitation laser focused in the excitation fiber, with the fluorescence collection fibers concentrically positioned around the excitation fiber at the optrode tip, which dips into the solution; the signal is analyzed by a pulsed intensified multichannel spectrofluorimeter. This analyzer allows for the elimination of the unwanted parasital fluorescence (organic, Raman) by the placement of a gate a few microseconds after the laser pulse. Special care is taken in terms of laser focalization, fiber-optic parameters (numerical aperture, diameter, and arrangement), parasital fluorescence, time delay, and width of the gate. Results obtained with RTRLIF in terms of limits of detection (LOD ∼ 10−10M) are presented and compared with the results obtained by TRLIF.

Laser-induced breakdown spectroscopy (LIBS) has been applied mainly to bulk analysis of solids, liquids, and gases and less frequently for elemental microanalysis of solid surfaces. A micro-LIBS device devoted to analysis of the distribution of elements on surfaces is described. This device offers rapid access with a 3-µm spatial resolution to the microchemical structures of both conductive and nonconductive samples. Quantitative microchemical results of applications to ceramics are reported. By the use of a time-resolved acquisition spectrum, cerium in a uranium matrix was characterized with a cerium detection limit of 1.14%. Calibration curves obtained with manipulations during 1 year facilitated evaluations of reproducibility and repeatability. A 2% single-shot repeatability with a calibration reproducibility of ∼7% is reported.

Plasma plume expansion and crater formation in the interaction of Nd -YAG laser beam (TEM oo - mode, 532 nm, 6 ns pulse duration and 10 μm waist at FWHM) with various metal targets in air were investigated. The craters formed at the surfaces were measured with 0,1 μm longitudinal and 0,5 μm transversal resolutions. Laser plasma expansion during laser pulse / surface interaction was measured by ICCD camera with 3 μm spatial and I ns temporal resolutions. These measurements were performed with different optical filters in the following spectral ranges (250< λ

Laser ablation of pure metals by femtosecond, picosecond and nanosecond pulses is studied experimentally in air at atmospheric pressure. Craters created by interaction of visible and UV laser pulses with the targets are investigated. The dependence of the ablation efficiency in terms of ablated volume per unit of energy on the pulse duration and wavelength is discussed.

We have investigated the craters created after high intensity visible and UV-laser pulse interaction with the metalsurfaces in air. The experiments were performed on different pure metal samples in the intensity range typical for laserplasma analytical application experiments with nanosecond laser pulses. The craters were characterised by their depth,diameter and volume. The analysis and discussion ofthe obtained results are presented.Keywords: laser ablation, metal samples, spectrochemical analysis, ablation efficiency.I. INTRODUCTION.High intensity laser pulse interaction with a solid surface leads to crater formation resulting from matter ejection. Asthe ablation process depends on physical properties of solid matter, environmental conditions and laser parameters , the description of crater characteristics is seen as an interesting method to understand the physics of the ablation process.In this work, the crater was characterised by its depth, diameter and volume (Fig. 1).The experiments were performed on different metal samples with a large variety of matter properties. Laser intensitieswere chosen from 0.5

Laser ablation of pure metals by femtosecond, picosecond and nanosecond pulses is studied in air at atmospheric pressure. Craters created after interaction of visible and UV laser pulses with the targets are investigated. The dependence of the ablation efficiency in terms of ablated volume per unit of energy on the pulse duration is discussed.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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Direct analysis of solid materials without sample digestion are requirement in analytical plasma emission spectrometry. Laser ablation permits the production of fine particles to be atomized, excited and ionized in a secondary source such as an inductively coupled plasma. Laser characteristics such as the pulse duration, the beam shape and the wavelength seem to play a significant role in the shape of the crater and the amount of ablated material. A Nd:YAG laser at 1064, 532 and 355 nm has been compared with a XeCl laser at 308 nm. Using the various lasers, crater shapes are described and amounts of ablated material are compared.

The ChemCam instrument that will equip the Mars Science Laboratory (MSL) rover uses laser-induced breakdown spectroscopy (LIBS) to remotely identify Martian rocks in the proximity of the rover, and to quantitatively assess their composition. Sample identification is then the first step of the chemical analysis, as a decision aid for monitoring the rover and/or before a subsequent composition measurement. In this paper we analyze our experimental spectra obtained in the laboratory by three chemometric methods—principal components analysis (PCA), soft independent modeling of class analogy (SIMCA) and partial least-squares discriminant analysis (PLS-DA)—to investigate the feasibility of rocks classification by remote LIBS. If PCA is very interesting for data visualization, SIMCA and PLS-DA enable the making of automatic predictions. We show that SIMCA is less sensitive than PLS-DA, but also more robust when it encounters spectra of an unknown rock. The instrument accuracy during MSL operations will benefit from a combination of the two approaches.

Intensite et signe des signaux de lentilles thermiques variant avec la taille du faisceau et les localisations de la cellule de mesure