J.J. Laserna

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.

Diagnostics of silicon plasmas produced by visible nanosecond laser ablation

Ž. The second harmonic of a pulsed Nd:YAG laser 532 nm has been used for the ablation of silicon samples in air at atmospheric pressure. In order to study the interaction for silicon targets, the laser-induced plasma characteristics were examined in detail with the use of a space- and time-resolved technique. Electron temperatures, ionic temperatures and electron number densities were determined. A discussion of thermodynamic equilibrium status of the silicon-microplasma is presented. Electron number densities are deduced from the Stark broadening of the line profiles of atomic silicon. Plasma ionization and excitation temperatures were determined from the Boltzmann plot and the SahaBoltzmann equation, respectively. A limited number of suitable silicon lines for the studies of temperatures were found and the effect of these lines on the temperature measurements is discussed. Electron temperatures in the range of 60009000 K and ionic temperatures of 12 00017 000 K with electron number densities of the order of 10 18 cm 3 were observed. The breakdown threshold fluence has been also measured. Silicon plasmas Ž. were also characterized in terms of their morphology shape and size as a function of laser energy and delay time. 2001 Elsevier Science B.V. All rights reserved.

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.

Open-path laser-induced plasma spectrometry for remote analytical measurements on solid surfaces

a ´´ Abstract Open-path laser-induced plasma spectrometry has been studied for elemental analysis at a distance of 45 m from the target. The 230-mJ pulsed radiation of a Q-switched Nd:YAG laser at 1064 nm has been used to produce a plasma on the sample and light emission has been collected under an off-axis open-path scheme. Under such conditions, the main variables influencing the signal response such as beam focal conditions, laser incidence angle and laser penetration depth have been identified and diagnosed on the basis of spectral signal-to-noise ratio considerations. The incidence angle is critical beyond 608. Crater morphology and ablation rates have been studied also. A semi-quantitative analysis of several stainless steel grades has been implemented using a pattern recognition algorithm, which allowed to discriminate successfully the samples on the basis of their variable content in alloying elements. 2002 Elsevier Science B.V. All rights reserved.

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.

Combining fingerprinting capability with trace analytical detection: surface-enhanced raman spectrometry

Abstract The demand to measure unique spectral information on trace analytes in complex samples continues to increase. This paper discusses surface-enhanced Raman spectrometric (SERS) methods which are currently being explored for molecular recognition at trace concentration levels. SERS is amenable to the analysis of solids, liquids and gases in a variety of experimental configurations for both qualitative and quantitative tasks. Batch and flow systems are also easily implemented in colloid SERS experiments. Typical instrumental systems for each of these analytical approaches are described and compared. The shape of the calibration graphs and the factors limiting the linear range of response are discussed. Other figures of merit, including limits of detection, precision and selectivity of measurement, are given. Applications to compounds of technical, environmental, biomedical and pharmaceutical interest are presented.

Quantitative analysis of low-alloy steel by microchip laser induced breakdown spectroscopy

The development of a compact laser induced breakdown spectroscopy (LIBS) system increases the possibilities of applying the technique in industrial arenas, field applications and process monitoring. Significant progress has been achieved in miniaturization of optical detectors and lasers, allowing portable, low-cost LIBS equipment to be devised. Conventional lasers for LIBS, like actively Q-switched Nd:YAG lasers are limited by their bulkiness, the need for a cooling system and high power consumption. The use of a miniature solid state microchip laser overcomes these drawbacks and offers further advantages of good beam quality, high pulse repetition frequency and less damage to target. In this work we studied the quantification of elemental composition of low alloy steel samples using a higher power microchip (“powerchip”) laser. The possibility of real time, in situ quantification of such materials by powerchip LIBS enhances the applicability of the technique to process monitoring in the steelmaking industry. The performance of the LIBS technique based on a powerchip laser and a portable non-intensified, non-gated detector for elemental quantification is evaluated and compared to that obtained using an intensified detector. Calibrations were achieved for Cr, Mo, Ni, Mn and Si with linear regression coefficients between 0.98–0.99 and limits of detection below 100 ppm in most cases.

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.

Depth Profiling of Phosphorus in Photonic-Grade Silicon Using Laser-Induced Breakdown Spectrometry

Laser-induced breakdown spectrometry (LIBS) has been evaluated for depth profiling of phosphorus doping in silicon. Laser plasmas were formed by focusing a Nd:YAG laser (operating in the second harmonic, 532 nm) on the sample surface. Plasma emission was collected, dispersed, and detected with the use of a charge-coupled device (CCD). Experimental parameters, such as delay time and sample position relative to the laser focal point, were optimized to improve the signal-to-background ratio of phosphorus line emission. Diffusion profiles by LIBS of samples with different phosphorus diffusion steps are shown. Crater depth per pulse and ablated mass per pulse were measured to be 1.2 μm pulse−1 and 50 ng pulse−1, respectively. The knowledge of depth per pulse permitted the estimation of thickness of the P diffusion layer.

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.