Space and time-resolved laser-induced breakdown spectroscopy using charge-coupled device detection

Space and time-resolved studies of laser induced plasmas in air at atmospheric pressure are presented. Photovoltaic solar cells have been used as samples. The second harmonic (532 nm) of a Nd : YAG laser at an irradiance of 18 × 10¹² W/cm² has been used. The precise focus of the beam allows a microanalysis at a 0.02 mm² surface area working in single-shot mode. The use of an intensified charge-coupled device (CCD) detector has allowed time-resolved studies in both imaging or spectroscopy modes. The two-dimensional capability of the CCD has enabled the study of atomic and ionic species distribution along the plume. Most data have been recorded using single-laser shot experiments. Spectral lines have been assigned to transitions in atomic components of the material under investigation in the neutral or ionic states of the corresponding atoms. Effects of delay in improving spectral resolution and some examples of spectral characterization of species as a function of its decay are shown.     

Spectral line selection for time-resolved investigations of laser-induced plasmas by an iterative Boltzmann plot method

The aim of this work is to provide a procedure to determine time-resolved electron temperatures with minimized relative errors by the Boltzmann plot method. The applied procedure consists of two parts, a systematic theoretical spectral line selection and an iterative Boltzmann plot algorithm. After a pre-selection of an appropriate non-disturbed or overlapped set of spectral lines of a particular atomic or ionic species Boltzmann plots are generated using experimentally recorded data for every time window and laser pulse energy of interest. Spectral lines with the highest average deviations from the regression function are assumed as being not representative for the considered ensemble of spectral lines and are therefore discarded gradually until a threshold value for the coefficient of determination is exceeded. Laser-induced breakdown spectroscopy (LIBS) is applied for time-resolved and spatially integrated investigations of plasmas on 1.1750 C75 steel alloy samples with laser pulse energies ranging between 200 µJ and 2 mJ. For the specific chemical composition of these samples a selection of atomic and ionic Fe spectral lines has been carried out. In spite of the fact that only laser pulse energies in the low millijoule regime are applied the final sets of spectral lines comprise in total 61 Fe I and 12 Fe II emission lines. By applying this method electron temperatures can be determined with averaged relative errors of down to 1.8% for Fe I and 4.4% for Fe II emission lines.     

Laser-induced breakdown spectroscopy of silicate, vanadate and sulfide rocks

Laser-induced breakdown spectroscopy (LIBS) in air at atmospheric pressure has been used to study four geological samples belonging to different structural families. Atomic emission spectra of vanadinite, pyrite, garnet and a type of quartz (compostela's quartz) are shown. The 532 nm line of a Nd:YAG laser at an irradiance of 18 x 10(11) W cm(-2) was used. The precise focus of the beam allowed microanalysis of a 0.02 mm(2) surface area working in single-laser shot mode. The use of an intensified gateable charge-coupled-device (CCD) detector permitted time-resolved studies. The spectral lines have been assigned to transitions in the neutral charge state of the corresponding atom of the material under investigation. The behavior of different transitions with time delay are shown. In experiments, minor components contained in several minerals have been detected. This fact has been used to demonstrate the applicability of the technique to characterize and identify similar minerals.     

Spectroscopic diagnostics on CW-laser welding plasmas of aluminum alloys

In-process diagnostics intended to correlate spectrometric measurements to the occurrence of laser welding defects such as notches and blowholes have been carried out. The plasma light emitted during high-power CO₂ laser welding of a 6013 aluminum alloy was guided to an imaging spectrograph and the dispersed light was detected with a CCD system. A transient recorder was used to record the signal from a fast laser-power monitor and the sync signals from the CCD and a fast speed video camera. Accuracy of the measurements are discussed in relation to the low and fast acquisition rate approaches (58 spectra s⁻¹ and 4×10³ spectra s⁻¹, respectively) used in the experiments. Spectroscopic measurements at fast acquisition rates showed both an increase in the intensity of the overall spectral emission and the growth of lines corresponding to ionic aluminum species taking place right before the occurrence of weld defects.     

Laser-excited ionic fluorescence spectrometry of rare-earth elements in the inductively-coupled plasma

A pulsed tunable dye laser pumped with an excimer laser is used to excite ionic fluorescence of the rare earth elements in the inductively-coupled plasma. Because several fluorescence lines were observed after laser excitation, it was possible to draw partial energy-level diagrams for most of the rare earths. Non-resonance fluorescence lines were used for all measurements in order to minimize spectral interferences. Detection limits at given excitation wavelengths are reported for each element. Laser-excited ionic fluorescence eliminates the problem of spectral interferences which has been associated with the determination of the rare earths by atomic emission spectrometry in the inductively-coupled plasma.     

Spatial distribution profiles of magnesium and strontium in speleothems using laser-induced breakdown spectrometry

Laser-induced breakdown spectrometry (LIBS) has been applied to spatially locate several atomic species in speleothems taken from the Nerja’s Cave (Málaga, Spain). Spatial distribution profiles of Mg at 285.21 nm and Sr at 407.77 nm were obtained while the laser was rastered through different paths along the sample. These elements were selected due to their importance as palaeoclimatic indicators. The 532 nm output of a Nd:YAG laser was used to irradiate the samples and generate the plasma that was spectrally analyzed and detected by using an intensified CCD detector. The signals were normalized to the Ca line to minimize pulse-to-pulse fluctuations in the laser source. Several studies were carried out to check for the point-to-point heterogeneity of the natural speleothem.     

New compact Echelle spectrographs with multichannel time-resolved recording capabilities

The paper describes a new type of Echelle spectrograph system that utilises a gated interline CCD camera. The system, that contains no moving parts, records simultaneously 6000 time-resolved spectral channels in the range 200–1100 nm with a lowest gate time of 100 ns and with a constant spectral resolution. Results from time-resolved recordings made both from a Xe-flash source and from laser induced breakdown, are presented. It is concluded that the gated CCD sensors are of great interest for applications in spectroscopy. They are of special interest for the application with Echelle spectrographs. The high resolution of the gatable CCD with 6.7×6.7 μm² surpasses that of any ICCD today and in the foreseeable future. The gated CCD also opens the NIR wavelength region for time-resolved multichannel spectroscopy.     

Time-resolved measurement of copper emission spectrum excited by a low-pressure argon laser-induced plasma.

A laser-induced plasma generated with a pulsed Nd:YAG laser under evacuated conditions has complicated structures both temporally and spatially. The time-resolved spectra of copper in three different wavelength regions were observed in detail for elucidating the excitation mechanisms of many atomic/ionic copper emission lines. The emission intensities of copper emission lines, measured in a time-resolved mode, were strongly dependent on the kind of copper lines: ionic or atomic lines, and their excitation energies. Generally, copper ionic lines were rapidly decayed and dominantly emitted from the initial breakdown zone, because the copper ions requiring larger excitation energies were produced mainly in the hot breakdown zone. On the other hand, the atomic lines were emitted during prolonged periods, implying that they could also be excited in the expanded plasma zone. The excitation phenomena occurring in the laser-induced plasma could be better understood by analyzing the time-resolved copper spectra.     

Detection of trace amounts of Ni by laser-induced fluorescence in graphite furnace with intensified charge coupled device

A laser-induced fluorescence in graphite furnace (LIF-GF) set-up has been equipped with an intensified CCD detector (ICCD) that enables simultaneous multichannel detection of large wavelength regions. The main advantages of such a system in comparison with conventional photomultiplier detection are: simultaneous detection of several fluorescence wavelengths for easy characterization of excitation and fluorescence spectra and for an increase of the absolute sensitivity and spectral selectivity; simultaneous monitoring of background signals, such as those due to matrix interferences, blackbody radiation and scattered laser light; decrease of the susceptibility to radio-frequency pick-ups emitted from the pump laser due to the delayed read-out procedure; time-resolved studies of fluorescence spectra for improved elemental selectivity or for studies of atomization processes, and a possibility to perform two-dimensional imaging of height distributions of atomization and, in combination with an imaging spectrometer, diffusion processes in the furnace. The first work on LIF-GF with ICCD detection has been performed on Ni. The most sensitive and versatile excitation and detection wavelengths have been identified. Detection limits in water solutions by the LIF-GF technique have been improved by two orders of magnitude and are found to be 0.015 pg with ICCD and 0.01 pg using a photomultiplier at the most sensitive excitation and detection wavelengths. Nickel in concentrations has been detected in aqueous standard reference samples with sodium concentrations ranging from to % (riverine water and estuarine water) with good accuracy and precision. The Ni contents of standard riverine and estuarine water were determined to 1.00 ± 0.11 and 0.75 ± 0.07 ng/ml, respectively. The certified concentrations are 1.03 ± 0.10 and 0.743 ± 0.078 .     

A time-resolved quadrupole mass spectrometric study on 355 nm laser ablated species ejected from LiMn2O4

Mass, velocity and angle distributions of the ablated species generated from 355 nm pulsed laser ablation of a LiMn2O4 target were investigated with an angle- and time-resolved mass spectrometric technique. Both neutral and ionic species of Li, O, LiO, LiO2, Mn, Li2, Li4, Li6, LiMn, MnO and MnO2 were observed at the laser fluence of 0.8 J · cm-2. The yield and variety of the ablated species increase with increasing the laser fluence. The time-of-flight spectra of ablated species can be fitted by a Maxwell-Boltzmann distribution with a center-of-mass velocity. There exist laser fluence thresholds for the ablated LiMn, Li2O and LiO2 species, and the fluence threshold of ionic species is higher than that of neutral species. The angular distributions of the ionic and neutral ablated species can be simulated by a cosnθor a bicosine function αcosθ+(1- α)cos"θ. In addition, the ablation mechanism of LiMn2O4 by a 355 nm pulsed laser is discussed.     

Spatial distribution profiles of magnesium and strontium in speleothems using laser-induced breakdown spectrometry

Laser-induced breakdown spectrometry (LIBS) has been applied to spatially locate several atomic species in speleothems taken from the Nerja’s Cave (Málaga, Spain). Spatial distribution profiles of Mg at 285.21 nm and Sr at 407.77 nm were obtained while the laser was rastered through different paths along the sample. These elements were selected due to their importance as palaeoclimatic indicators. The 532 nm output of a Nd:YAG laser was used to irradiate the samples and generate the plasma that was spectrally analyzed and detected by using an intensified CCD detector. The signals were normalized to the Ca line to minimize pulse-to-pulse fluctuations in the laser source. Several studies were carried out to check for the point-to-point heterogeneity of the natural speleothem. Received: 1 August 1997 / Revised: 23 October 1997 / Accepted: 30 October 1997     

Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys

This paper reports studies on time-resolved space-integrated laser induced breakdown spectroscopy (LIBS) of plasmas produced by ultrashort laser pulses at atmospheric pressure, on aluminum alloy targets. The temporal behavior of specific ion and neutral emission lines of Al, Mg and Fe has been characterized. The results show a faster decay of continuum and spectral lines, and a shorter plasma lifetime than in the case of longer laser pulses. Spectroscopic diagnostics were used to determine the time-resolved electron density, as well as the excitation and ionization temperatures. In comparison with plasmas produced by ns laser pulses, the plasma generated by ultrashort pulses exhibits a faster thermalization. Analytical performances of fs-LIBS were also evaluated. Linear calibration curves for minor elements (Mg, Fe, Si, Mn, Cu) presented in aluminum alloys were obtained. The limits of detection are in the parts per million (ppm) range and are element-dependent.     

Time-resolved ultraviolet laser-induced breakdown spectroscopy for organic material analysis

Ultraviolet pulses (266 nm) delivered by a quadrupled Nd:YAG laser were used to analyze organic samples with laser-induced breakdown spectroscopy (LIBS). We present characteristics of the spectra obtained from organic samples with special attentions on the emissions of organic elements, O and N, and molecular bonds CN. The choice of these atomic or molecular species is justified on one hand, by the importance of these species to specify organic or biological materials; and on the other hand by the possible interferences with ambient air when laser ablation takes place in the atmosphere. Time-resolved LIBS was used to determine the time-evolution of line intensity emitted from these species. We demonstrate different kinetic behaviors corresponding to different origins of emitters: native atomic or molecular species directly vaporized from the sample or those generated through dissociation or recombination due to interaction between laser-induced plasma and air molecules. Our results show the ability of time-resolved UV-LIBS for detection and identification of native atomic or molecular species from an organic sample.     

Diagnostics of silicon plasmas produced by visible nanosecond laser ablation

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 Saha–Boltzmann 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 6000–9000 K and ionic temperatures of 12 000–17 000 K with electron number densities of the order of 10¹⁸ cm⁻³ were observed. The breakdown threshold fluence has been also measured. Silicon plasmas were also characterized in terms of their morphology (shape and size) as a function of laser energy and delay time.     

喹喔啉衍生物与缺电子烯烃的激光闪光光解研究

利用时间分辨激光诱导瞬态吸收光谱装置,以一台Nd：YAG激光器四倍频后的266nm激光作为激发光源,详细研究了两种喹喔啉衍生物激发三重态在乙腈体系中的动力学过程,得到激光脉冲作用后不同时间的瞬态吸收光谱及激发三重态自猝灭反应动力学常数．并以五种缺电子烯烃作为猝灭剂,得到了基态缺电子烯烃对喹喔啉衍生物激发三重态的猝灭反应动力学常数,阐明了猝灭反应机理．     

时间分辨激光光散射测量系统

自选设计，装配了一套时间分辨的激光光散射测量系统，该系统摄像机，录像机，计算机和自编的软件实现了一体化的数据采集，显示和分析过程，本文给出了该系统的应用实例，球晶的光散射和环氧树脂的固化反应诱导的相分离过程。     

Computer simulation of emission from analyte atoms and ions excited in an inductively coupled plasma

A computer program has been used to simulate analyte emission spectra. These synthetic spectra are generated by combining information on fundamental properties of the ICP with basic physical data for atomic and ionic species. The program has been used to simulate both single element spectra (Fe) and the spectra of complex mixtures of elements. For the latter, the utility of computer simulations for the study of spectral line overlaps which occur when silver is analyzed in a geochemical sample has been investigated.     

Optimization of micro-Laser Induced Breakdown Spectroscopy analysis and signal processing

As a result of continuing instrumental development (Echelle spectrometer and ICCD detectors), micro-Laser Induced Breakdown Spectroscopy analysis may become an increasingly recognized analytical technique for determining elemental compositions of geologic materials. Best conditions of time resolution conditions (delay and time acquisition window) are estimated with respect to the collection geometry of optical plasma emission of our system. It turns out that the level of the Bremsstrahlung continuum emission is weak in the first tens of nanoseconds after the laser excitation pulse. The enlargement of the emission lines is identified in the first 100 ns but remains comparable to the spectral resolution of our system. Thus, results show that time-resolved conditions are not necessarily required to perform elemental analysis at the micrometric scale using LIBS, contrary to macro-LIBS. This suggests potential improvements of micro-LIBS analysis (sensitivity and spectral resolution) using non-intensified CCD connected with the laser pulse.     

Sample illumination configurations for spatially resolved Raman spectrometry using a charge-coupled device detector

In this paper two simple configurations for sample illumination using a charge-coupled device (CCD) detector have been shown. The choice of an appropriate sample illumination can be crucial to obtain spatial and spectral information of complex samples. It is demonstrated that simultaneous Raman spectra of a heterogeneous sample of three compounds can be obtained using a vertical sample illumination. Spatially resolved resonant Raman and surface-enhanced resonant Raman spectra of a complex (Ni-PAPH) have been observed with a low integration time. Dividing the CCD in two region; and with horizontal multiline sample illumination (argon-ion laser at 488 nm and HeNe laser at 632.8 nm) spatially resolved fluorescence spectra of a homogeneous mixture of dyes have been obtained. The total image was acquired in only 1 s.     

Investigation of plasmas produced by laser ablation using single and double pulses for food analysis demonstrated by probing potato skins

We report on investigations of plasmas produced by laser ablation of fresh potatoes using infrared nanosecond laser radiation. A twin laser system consisting of two Nd:YAG oscillators was used to generate single or double pulses of adjustable interpulse delay. The potatoes were irradiated under ambient air with moderate pulse energies of about 10 mJ. The expansion dynamics of the ablation plume was characterized using fast imaging with a gated camera. In addition, time-resolved optical emission spectroscopy was applied to study the spectral line emission of the various plasma species. The electron density was deduced from Stark broadening, and the plasma temperature was inferred from the relative emission intensities of spectral lines. The relative concentrations of metals were estimated from the comparison of the measured emission spectra to the spectral radiance computed for a plasma in local thermal equilibrium. It is shown that the plasma produced by double pulses has a larger volume and a lower density. These properties lead to an increase of the signal-to-noise ratio by a factor of 2 and thus to an improved measurement sensitivity.