José M. Vadillo

Nanometric range depth-resolved analysis of coated-steels using laser-induced breakdown spectrometry with a 308 nm collimated beam

The spatial profile from an XeCl excimer laser was modified using a simple two-lenses telescope to generate a flat energy-profile beam that impinged a layered sample (a Zn-coated steel) without beam conditioning. The irradiance obtained (about 107 W cm–2) was high enough to vaporize the target, to cause plasma formation and to allow atomic emission spectrometry with acceptable signal-to-noise ratio. Modification in beam energy distribution resulted in flat ablated profiles and improved depth-resolution up to the few nm pulse–1 range was attained. The net intensity areas were transformed into normalized values leading to plots in excellent agreement with those provided by commercial depth-resolved analysis instruments.

Mapping of platinum group metals in automotive exhaust three-way catalysts using laser-induced breakdown spectrometry.

The use of laser-induced breakdown spectrometry for spatial distribution analysis of platinum, rhodium, and palladium in car catalytic converters is discussed. Fresh converters were extracted from the car exhaust system, cut in pieces of an appropriate size, and analyzed for mapping purposes. Spectral detection, pulse energy, and beam focal conditions were optimized according to the ablation behavior of the material. Difficulties in distribution analysis caused by the complex elemental composition of the sample were overcome by an extensive spectral analysis using appropriate internal standards. Data on the spatial distribution of the active metals in both the axial and radial directions of the catalytic structures are presented.

Effect of plasma shielding on laser ablation rate of pure metals at reduced pressure

The ablation rate expressed as the amount of removed material per laser shot was calculated for pure metal samples under different experimental conditions: laser fluence (1.3‐16.7 J cm 2 ), buffer gas (air, He and Ar) and gas pressure (10 3 ‐10 5 mbar). Fluence values covered the range between the plasma threshold (~1‐2 J cm 2 for most elements) and 16.7 J cm 2 . The 581 nm output of an excimer-pumped dye laser was used. Results pointed out a strong dependence of ablation rate on experimental parameters. At high fluence, the ablated material efficiently attenuates the incoming laser radiation (plasma shielding) and reduces the ablation rate. The extent of this shielding effect depend also on the experimental variables (buffer gas, pressure) and sample nature. These studies are useful to determine the amount of ablated material as a function of experimental parameters, to understand the extension of the shielding process and to establish the conditions under which it may be avoided. Copyright ” 1999 John Wiley & Sons, Ltd.

Depth-resolved Anaylsis of Multilayered Samples by Laser-inducedBreakdown Spectrometry

The capability of laser-induced breakdown spectrometry (LIBS) to resolve complex depth profiles is demonstrated. Electrolytically deposited brass samples were analyzed by monitoring the emission corresponding to Cr (357.8 nm), Ni (341.4 nm), Cu (327.4 nm) and Zn (334.5 nm). The nominal thickness of the layers was known, which permitted an estimate of the ablated mass in the range between 150 and 500 nm per pulse depending on the matrix and laser irradiance. Laser irradiance was varied by defocusing, and its effect on the depth-resolution of LIBS was tested. For comparison purposes, a commercial zinc-coated steel was also studied by following the Zn and Fe emission intensity depth profiles with a commercial glow-discharge optical emission spectrometry system to obtain information on the exact location of the Zn–Fe interface (12 µm). The ablation rate in terms of ablated mass per pulse was found to be at the ng per pulse level and depended on the laser pulse irradiance.

Angle-Resolved Laser-Induced Breakdown Spectrometry for Depth Profiling of Coated Materials

The combination of angle-resolved laser ablation with the use of a collimated beam is presented as a new approach to increase the depth-resolved capabilities of laser-induced breakdown spectrometry (LIBS). The effect of beam conditioning and the reduction of the effective sampling depth due to the angular dependence of laser ablation allow ablation rates lower than 2 nm/pulse for coated materials (Sn-coated steels and Cr-coated samples). Spectral information is obtained on a single-laser-shot basis. The effect of incidence angle has been examined from differentiated emission profiles, demonstrating the beneficial effect of working at incidence different from normal. A compromise between depth resolution and emission signal must be found at large angles due to the lower irradiance resulting from the increase in beam size at the interface for large angles of incidence. A comparison of the proposed approach with the analysis provided by a commercial glow-discharge device [glow discharge optical emission spectroscopy (GD-OES)] demonstrated quite satisfactory results.

Irradiance-dependent depth profiling of layered materials using laser-induced plasma spectrometry

Laser-induced plasma spectrometry (LIPS) is an appealing technique for depth profiling purposes due to its capabilities for performing fast analysis in air at atmospheric pressure without limitations of sample size or nature. At a fixed laser wavelength, pulse width, experiment geometry and sample type, the irradiance is the factor that will affect both the averaged ablation rate and depth resolution. In the present work, a detailed description of the effect of laser irradiance on averaged ablation rate and depth resolution of Ni–Cu-coated brass samples is presented. The results demonstrate that the best depth resolution does not correspond with the minimum ablation rate. Several facts concerning the redeposition of material around the rim of the craters and energy gradients in the laser beam are proposed to explain the experimental results.

Depth-resolved analysis by laser-induced breakdown spectrometry at reduced pressure

The 581 nm output from a dye laser in a —uence range between 2.86 and 11.47 J cmo2 was used to ablate pure Zn and Fe foils. The average ablation rate (AAR, lm per shot) was calculated for diUerent experimental variables (buUer gas, pressure, laser —uence and focal conditions). Deposition of previously ablated material in the ablation crater results in large variation of the observed AAR values. This eUect was observed in air and argon buUer gases at atmospheric pressure. The situation is largely alleviated at reduced pressure due to free expansion of the ablated material. Under these circumstances the capability of laser-induced plasmas to resolve interfacial structures is improved. The eUect on depth-resolved studies was checked with a commercial Zn-coated steel sample. Due to the Gaussian-like energy distribution of the incident laser beam, the material is ablated to produce a conical crater. This fact ensures that the Zn signal remains for a longer time because the ablated region spreads over the edge gradually. At low pressure the emission peaks are better de—ned and the background becomes —at. However, these conditions produce also the lowest net intensities and some peaks are not detected. An Ar atmosphere produces more intense spectral lines at both pressure levels. Best analytical results were obtained at reduced pressure, with a slight improvement in depth resolution in the presence of Ar. 1998 John Wiley & Sons, Ltd.

Compositional Mapping of Poisoning Elements in Automobile Three-Way Catalytic Converters by Using Laser-Induced Breakdown Spectrometry

The use of laser-induced breakdown spectrometry (LIBS) for generating two-dimensional (2D) multielemental distribution maps of poisoning agents in automobile three-way catalysts (TWCs) is discussed. A complete in-depth analysis of P, Zn, and Pb at different locations along the central channel of a used TWC in the gas flow direction was performed. The distribution maps for the metals in charge of the catalytic action and for the stabilizer of these metals—i.e., Pt, Pd, and Ce—are also presented. The 2D maps provide a fast, useful, and intuitive tool to visualize the selective distribution of poisoning elements in a used TWC. This study may help explain the deterioration processes of a catalyst with a lifetime of about 30 000 km.

Comparative analysis of layered materials using laser-induced plasma spectrometry and laser-ionization time-of-flight mass spectrometry ☆

Abstract Laser-induced plasma spectrometry (LIPS) and laser ionization time-of-flight mass spectrometry (LI-TOFMS) have been evaluated for the in-depth analysis of layered materials. LI-TOFMS shares with LIPS important advantages in terms of speed of analysis and negligible sample handling. However, additional features such as real multielemental capabilities and the absence of background contribution must be added to the former. In order to have a useful estimation of the potential of each technique, an in-depth characterized Zn-coated steel has been analyzed. Without complete optimization of the system, the averaged ablation rate has been measured to be in the range 20–30 nm/pulse without beam conditioning or optical modifications.

Removal of Air Interference in Laser- induced Breakdown Spectrometry Monitored by Spatially and Temporally Resolved Charge-coupled Device Measurements

Laser-induced breakdown spectrometry is a suitable method for the direct in-process measurement of materials composition. The emission spectrum from the plasma includes information not only on the analysis area but also on the surrounding atmosphere, mainly lines corresponding to O, N and C if the experiments are being carried out in air at atmospheric pressure. These emission lines could interfere with the sample spectrum. Although working under vacuum conditions or the use of controlled atmospheres can be considered to be the best choice, in most practical applications working in air at atmospheric pressure is the common way of analysis. A study was undertaken to evaluate the removal of air interferences in poly(methyl methacrylate) samples by using the temporal and spatial resolution of gated charge-coupled devices, without any sample treatment or alteration of the experimental set-up.