Plasma chemistry in laser ablation processes

Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10¹⁴ cm⁻³ at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C₂ molecules indicate that the dominant mechanism for production of C₂ is the atomic carbon recombination.     

激光烧蚀La~2O~3产生的等离子体的发光光谱研究

用发光光谱法研究了355nm脉冲激光在真空和O~2气氛中烧蚀La~2O~3产生的等离子体的组成和形成过程。对等离子体中La^+离子和LaO的空间和时间分辨发光光谱分析表明,在O~2气氛中LaO有两个生成通道:一是在靶附近的等离子体内直接生成的,另一是由La,La^+与O^2发生氧化反应而生成的。测定了激光能量密度,离靶表面的距离和O~2压力对产物发光的延迟时间和发光强度的影响。此外,还讨论了激光烧蚀La~2O~3诱导产生等离子体的形成和演化机理。     

Sampling modulation technique in radio-frequency helium glow discharge emission source by use of pulsed laser ablation

An emission excitation source comprising a high-frequency diode-pumped Q-switched Nd:YAG laser and a radio-frequency powered glow discharge lamp is proposed. In this system sample atoms ablated by the laser irradiation are introduced into the lamp chamber and subsequently excited by the helium glow discharge plasma. The pulsed operation of the laser can produce a cyclic variation in the emission intensities of the sample atoms whereas the plasma gas species emit the radiation continuously. The salient feature of the proposed technique is the selective detection of the laser modulation signal from the rest of the continuous background emissions, which can be achieved with the phase sensitive detection of the lock-in amplifier. The arrangement may be used to estimate the emission intensity of the laser ablated atom, free from the interference of other species present in the plasma. The experiments were conducted with a 13.56 MHz radio-frequency (rf) generator operated at 80 W power to produce plasma and the laser at a wavelength of 1064 nm (pulse duration:34 ns, repetition rate:7 kHz and average pulse energy of about 0.36 mJ) was employed for sample ablation. The measurements resulted in almost complete removal of nitrogen molecular bands (N₂⁺ 391.44 nm). Considerable reduction (about 75%) in the emission intensity of a carbon atomic line (C I 193.03 nm) was also observed.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd?:?YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented.     

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

The 581 nm output from a dye laser in a fluence range between 2.86 and 11.47 J cm⁻² was used to ablate pure Zn and Fe foils. The average ablation rate (AAR, μm per shot) was calculated for different experimental variables (buffer gas, pressure, laser fluence and focal conditions). Deposition of previously ablated material in the ablation crater results in large variation of the observed AAR values. This effect was observed in air and argon buffer 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 effect 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 defined and the background becomes flat. 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.     

Optical emission spectroscopy and modeling of plasma produced by laser ablation of titanium oxides

In the present study, the time evolution of electron number density, of electron, atom and ion temperatures, of plasma produced by KrF excimer laser ablation of titanium dioxide and monoxide targets, are investigated by temporally and spatially resolved optical emission spectroscopy over a wide range of laser fluence from 1.7 to 6 J cm⁻², oxygen pressures of 10⁻²–10⁻¹ torr and in a vacuum. A state-to-state collisional radiative model is proposed for the first time to interpret the experimental results at a distance of 0.6 mm from the target surface, in vacuum and for a time delay from 100 to 300 ns from the beginning of the laser pulse. In particular, we concentrate our attention on problems concerning the existence of the local thermodynamic conditions in the laser-induced plasma and deviation from them, as observed in our experiment. The numerical model proposed for calculating the electron number density and the population densities of atoms and ions in excited states give good quantitative agreement with the experimental results of the optical emission spectroscopy measurements.     

Material ejection and redeposition following atmospheric pressure near-field laser ablation on molecular solids

Near-field laser ablation (NF-LA) coupled with mass spectrometry (MS) is very promising for highly spatially resolved chemical analyses on various substrates at atmospheric pressure, for example, in materials and life science applications. Although nanoscale sample craters can be produced routinely, no molecular mass spectra of ablated material from craters of ≤1 μm diameter have ever been acquired by NF-LA MS at atmospheric pressure. Some of the pressing questions are thus how much of the ablated material is transported into the mass spectrometer and in what form. Therefore, material redeposition on the near-field tip’s surface from laser ablation of molecular solids was characterized with scanning electron microscopy. The crater profiles were studied by scanning probe microscopy. The results shown in this study demonstrate that there could be as much as 70% of the ablated material deposited on the near-field tip’s surface. The redeposited products were found to be confined to a height of ~50 μm, thus suggesting that most components inside near-field ablation plumes propagate about the same distance for both anthracene and tris(8-hydroxyquinolinato)aluminum. Nanoparticles ablated from craters of ≤1 μm diameter are clearly observed. Furthermore, observation of tips after ablation of an anthracene surface angled at 60° with respect to a horizontal surface shows that the direction of the near-field ablation plume is neither in the direction of the surface normal nor towards the axis of incident laser beam but deflected further away from surface normal.      

Studies on laser defocusing effects on laser ablation inductively coupled plasma-atomic emission spectrometry using emission signals from a laser-induced plasma

The effect of laser defocusing on analytical performance of laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES) was studied by varying laser focus conditions with respect to the surface of a low-alloy steel and a powdered sediment pellet. Laser-induced plasma (LIP) and LA-ICP-AES emission signals and LIP excitation temperatures (LIP T_ex) were determined and compared for different laser defocus conditions. LIP Fe and LA-ICP-AES Fe emission signals and LIP T_ex decreased when the laser was defocused for the low-alloy steel. On the other hand, when the sediment pellet was ablated, LIP T_ex decreased when the laser was defocused. However, LA-ICP-AES Fe emission signals increased at first, then decreased when the laser was defocused more. It was concluded that LIP T_ex and LIP and LA-ICP-AES Fe emission signals are dependent on laser shot conditions (focus–defocus), and are also dependent on sample type (texture, mineralogy, hardness, conductivity and heat capacity).     

Investigation of polarization effects for nanosecond laser-induced breakdown spectroscopy

The spectral and temporal polarization dependencies of nanosecond laser-induced plasmas are explored for analysis of gaseous and solid samples using various experimental configurations. Plasma emission measurements were resolved into vertical and horizontal polarization components, and the ratio of the two polarization-resolved measurements was calculated for each sample and configuration. For the solid target, measurements were recorded with the sample oriented both normal to the incident laser beam as well as at oblique angles of incidence. The results for the breakdown of a pure, nitrogen gaseous sample revealed no degree of polarization in either the continuum or atomic emission, with the ratios of the horizontally-to-vertically resolved plasma emission showing values equal to unity when resolved both temporally and spectrally. The analysis of both copper and steel solid samples also showed no polarization dependency in the spectral and temporal data when the laser was incidentally normal to the sample surface. For oblique angles of incidence, some polarization (< 10%) was observed within the first tens of nanoseconds of plasma lifetime. The polarization was manifested as a slight reduction in the horizontal component of plasma emission, but significantly, the observed polarization was found to be spectrally flat, with no difference observed between continuum and atomic emission features. The small polarization effect was found to diminish with plasma residence time, effectively vanishing by about 1 μs following breakdown. The transient polarization is hypothesized to arise from reflection effects (i.e. Fresnel reflectivity) between the plasma light and the solid target surface present with oblique angles of incidence for reflected light, with temporal effects due to the dynamic nature of the plasma development and plasma–surface interactions. Overall, no evidence was found to support any inherent anisotropy or polarization specific to the plasma continuum or the atomic emission for the transitions studied.     

Experimental characterization of metallic titanium-laser induced plasma by time and space resolved optical emission spectroscopy

Time and space resolved optical emission spectroscopy has been successfully employed to investigate the evolution of the plasma produced by the interaction of UV laser beam with a metallic target of titanium at two different pressures (10⁻⁵ and 3.4×10⁻² torr) and at distances up to 3 mm from the target. By time of flight measurements and Boltzmann plots both the dynamic and the kinetic aspects have been discussed. The quasi-equilibrium state of the laser-induced plasma has been established on the basis of the failure of Saha balance equation. The effect of three-body recombination on atomic titanium temporal distribution has been explained. Temporal evolution of electron number density, as determined by Stark effect, has been used for the estimation of the three-body recombination rate constant.     

Determination of lead in metallic reference materials by laser ablation combined with laser excited atomic fluorescence

Concentrations of lead in the range 0.15–750 μg⁻¹ were measured in metallic matrices (copper, brass, steel, and zinc) by laser excited atomic fluorescence combined with UV laser ablation in a low-pressure (130 mbar) argon atmosphere. The mass of material ablated was determined by repetitive weighing of the target prior to and after approximately 10 000 ablating shots. The fluorescence was excited after a 100 μs delay relative to the ablation pulse. A long integration time of 200 s was used to provide a representative determination of bulk concentrations. No matrix effect was observed, providing a universal calibration curve for all samples with relative standard deviations of about 20%. The relative and absolute limits of detection were 72 ng g⁻¹ and 0.5 fg, respectively.     

On the internal standardization in optical emission spectrometry of microplasmas produced by laser ablation of solid samples

Summary The problem of internal standardization in optical emission spectrometry of a laser produced analytical microplasma has been investigated. Excellent analytical results almost independent on the plasma temperature and the state of evaporation of the ablated sample material have been found for iron and chromium using Fe/Cr binary samples. However, for elements with very different vapour pressures, internal standardization can only be carried out if the atomization process in the plasma is completed. This is demonstrated with brass samples, where the fractional evaporation of zinc and copper heavily influences the emission data in the first period of the laser produced plasma.

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Über die interne Standardisierung in der optischen Emissionsspektrometrie von Mikroplasmen, erzeugt durch Laserabtrag fester Proben

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Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday     

Spatial determination of elements in green leaves of oak trees (Quercus robur) by laser ablation-ICP-MS

Laser ablation inductively coupled plasma mass spectrometry (laser ablation-ICP-MS) has been applied to the spatially resolved determination of the elements Mg, Ca, Cu, Ni, Ba, Al, Pb, Sr and Mn in green leaves of oak trees. Instrument operating parameters such as the laser wavelength and the pulse energy have been optimized to provide the sensitivity and reproducibility required for the analysis. The method provides spatial resolution down to 300 μm with the use of the 355 nm wavelength (3rd harmonic of the 1064 nm Nd:YAG laser wavelength) and the pulse energy of 50 mJ. Plant standards and cellulose, doped with multi element solution standards, dried and pressed to pellets were used as calibration samples. To compensate for signal fluctuations caused by the variation of the ablated sample mass ¹³C was used as a “natural” internal standard. The accuracy of the calibration was verified with selected samples analyzed by ICP-MS (high pressure digestion, 170?°C, 10⁷ Pa, HNO₃, 2 h) and by laser ablation-ICP-MS. Recovery rates between 93% (Cu) and 108% (Mn) were obtained. Leaves taken from oak trees (Quercus robur) were analyzed.     

Fluorescence spectroscopy of laser vaporized species

The plasma produced by a Nd-Yag laser above a metal surface is measured by time resolved optical spectroscopy. The emission of atoms, ions and diatomics is observed for silver, copper and molybdenum. (Cu₂, ωe=263.3 cm^?1 X-state, 189.8 cm^?1 A-state). From these small species clusters are formed with a size distribution between 10 and 80 Å diameter measured by electron microscopy.     

Direct Determination of Copper in Solids and Ores by Laser Ablation-Direct Current Argon Plasma Emission Spectrometry

Abstract

The use of a laser ablation-direct current argon plasma emission spectrometric system for the direct determination of metals in solids is described. Sample preparation of solid steel samples involves machining to fit the geometry of the ablation chamber. A cellulose binder and copper ore are mixed thoroughly in a ball mill to ensure homogeneity and pelletized in a press at 20000 psi to fit the geometry of the ablation chamber. Copper, manganese, and nickel are determined using the system on standard steel samples, and copper is determined in pelletized copper ore with good agreement obtained with certified values. Precisions are typically in the 3 - 10% range with a detectable limit of 100 μg g^?1 of copper.     

激光等离子体中超金属氧化物离子LiZnO+的产生

作为一般规律 ,在一些具有热力学稳定性的氧化物分子中 ,氧原子周围共价电子数为 8.自从发现 Li3O和 Li4 O分子以来 ,这类超过 9个或更多共价电子的含金属团簇小分子呈现出来的特殊化学计量比和热力学稳定性引起人们的关注 ,人们称其为超共价分子 (Hypervalent Molecules)或超金属分子(Hypermetallic Molecules) [1～ 4 ] .目前 ,在实验上已发现了 Mn O,Mn S及 Mn C(M=Li,Na,K,Mg;n≥ 3 )等一系列超金属分子 ;在理论上 ,从头计算法已可计算出超金属分子的结构及其稳定性 ,使人们对这些超金属分子的产生与结构有一定的认识 .然而 ,由…     

Emission intensity modulation of radio-frequency helium glow-discharge emission source by laser ablation.

A novel emission excitation source comprising a high repetition rate diode-pumped Q-switched Nd:YAG laser and a Grimm-style glow-discharge lamp is described. Laser-ablated atoms are introduced into the He glow discharge plasma, which then give emission signals. By using phase-sensitive detection with a lock-in amplifier, the emission signal modulated by the pulsed laser can be detected selectively. It is possible to estimate only the emission intensity of sample atoms ablated by laser irradiation with little interference from the other species in the plasma.     

Evaluation of the continuous optical discharge for spectrochemical analysis

The continuous optical discharge (COD) has been studied as a spectrochemical excitation source for atomic emission spectroscopy. The COD was generated by focusing a 45-W cw-CO₂ laser beam in Xe gas at pressures between 1150 and 3200 torr. The high temperature ( 10 000 K) and electron density (~10 ¹⁷ cm ^?3) of the plasma should provide good excitation for elements difficult to excite by more conventional sources. Some characteristics of the plasma were examined as a function of laser power and gas pressure. The design of a gas cell for analytical measurements which increases plasma stability is presented. Linear calibration curves for O₂; and Cl₂ introduced into the plasma were obtained and detection limits established. Detection limits were also determined for solid materials laser ablated into the COD. Because the COD operates at pressures above atmospheric, gas samples are most easily introduced for analysis. To prevent contamination of optical components by analyte dissociation products, the COD should be operated as a plasmatron.     

Assessment and application of diode laser induced fluorescence spectrometry in an inductively coupled plasma to the determination of lithium

A diode laser based system for the detection of Li in an inductively coupled plasma (ICP) by diode laser induced fluorescence (DLIF-ICP) has been developed and successfully applied to the determination of lithium in several mineral waters and a thermal salt. The experimental setup is based on an unmodulated, continuous wave diode laser, emitting light at around 670 nm and exciting neutral Li atoms on their 2s ²S–2p ²P° transition, which was coupled to a commercial ICP atomic emission spectrometer. The spectrometer's monochromator, photomultiplier detector and built-in data acquisition software were utilized to collect background corrected fluorescence and emission signals. A simple, three-step measurement procedure was devised that corrected for the contribution of lithium thermal emission and scattered laser light in the analytical signals. Despite the facts that lithium was detected on its neutral atom, which accounts for less than 1% of the total concentration of Li in the ICP, and that only about 1–2% of all atoms could be excited by the laser light at any given time, the limit of detection (LOD) was still found to be as good as 8 μg/L. The LODs of the DLIF-ICP technique are therefore expected to be in the low ng/L range for elements that can be detected under more advantageous conditions. The linear dynamic range was found to be around three orders of magnitude.     

Quantitative analysis of alloys and glasses by a calibration-free method using laser-induced breakdown spectroscopy

Space-, time- and spectrally resolved optical diagnostics of laser ablation plasma has provided the opportunity to realize calibration-free analyses of solid materials. In general, this variant of optical emission spectroscopy of pulsed plasma allows the plasma matrix effects to be overcome, yielding satisfactorily precise and accurate quantitative results on elemental composition of materials without using calibration curves, certified reference materials, and internal standards. Such analysis is very close to be nondestructive due to the minimum possible ablated mass, a feature which is very important in many applications, especially for unique museum exhibits and jeweler samples. In this paper, the use of the method for the analysis of elements in bronze, brass and gold alloys, glass samples, and archaeological findings is demonstrated. The results presented confirm the suitability of the approach for routine applications of our instrumentation, while at the same time simplifying the overall analytical procedure.