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.     

Studies of Space and Time Resolved Emission Measurements of an ArF Excimer Laser Ablated Plasma

Abstract

A laser ablated plasma created by an ArF excimer laser of wavelength of 193 nm, laser power of 150-200 mJ, repetition rate of 1 Hz, and zinc, copper, iron and nickel targets was studied. Quantitative emission studies are reported that are both space and time resolved. The space results of the laser ablated plasma suggest a confined plasma with a gaussian type distribution of atoms and ions. The time resolved studies show a maximum emission signal occurring at approximately 20 μs with a lifetime of less than 100 μs.     

Development of a laser ablation-hollow cathode glow discharge emission source and the application to the analysis of steel samples.

A novel atomic emission spectrometry comprising laser ablation as a sampling source and hollow cathode plasma for the excitation of ablated sample atoms is proposed. In this arrangement, a conventional Grimm-type discharge lamp is employed, but the polarity of the power supply is reversed so that the cylindrical hollow tube acts as a cathode and the glow discharge plasma is produced within this tube. A laser is irradiated to introduce sample atoms into the discharge plasma. Ablated atoms are excited by collisions with electrons and gas species, and emit characteristic radiation upon de-excitation. The experiments were conducted only in an atmosphere of helium gas so as to avoid a rapid erosion of the cathode hollow tube. Phase-sensitive detection with a lock-in amplifier was utilized to reject the continuous background emission of the plasma gas and emissions of sputtered atoms from the tube material. The unique feature of this technique is that the sampling and excitation processes can be controlled independently. The proposed technique was employed for the determination of Cr, Mn, and Ni in low-alloyed steel samples. The obtained concentrations are in good agreement with the reported values. The relative standard deviation (RSD), a measure of the analytical precision, was estimated to be 2-9% for Cr, 3-4% for Mn, and 4-11% for Ni determination.     

Application of primary plasma standardization to Nd-YAG laser-induced shock wave plasma spectrometry for quantitative analysis of high concentration Au–Ag–Cu alloy

A study was performed on a laser-induced shock wave plasma generated on high concentration Au–Ag–Cu alloys by a Q-switched Nd-YAG laser of 4.8 mJ under reduced air pressure of 2 torr. It was found that the total emission intensity of the secondary plasma is proportional to the intensity of the primary plasma. Assuming linear proportionality between the intensity of the primary plasma and the number of atoms vaporized from the target, it is proposed that quantitative analysis can be applied to the intensities of the analytical emission lines normalized by the total intensity of the primary plasma. This experimental result demonstrated for each metal element shows an excellent linear relationship between the normalized emission line intensity and the content of the corresponding element.     

Spatial distribution profiles of Ca, Cu and Mn atoms and ions in an inductively coupled plasma by means of laser excited fluorescence

Radial distribution profiles of ground state atoms and/or ions for calcium, manganese, and copper in an inductively coupled plasma have been measured using an excimer (XeCI) pumped dye laser as an excitation source of fluorescence. As a comparison, radial emission profiles also have been measured with an Abel-inversion procedure. In our low-flow nebulizer plasma system at low observation heights, the profiles of excited state atoms and ions resemble each other with a minimum in the center of the plasma. The profiles of ground state atoms and ions, however, possess a bell shape except for calcium ground state ions which have a double-peaked distribution.     

Basic investigations for laser microanalysis: III. Application of different buffer gases for laser-produced sample plumes

We have measured the diameters and depths of craters in a copper sample and the amount of material ablated by the 1.06-m radiation of a pulsed Nd: YAG laser in the buffer gases argon, neon, helium, air and nitrogen as well as the emission intensities of analyte atoms in dependence on laser power and buffer gas pressure. The results are correlated with corresponding data of the plasma temperatures and the relative electron densities in the plasma. Criteria for the choice of the buffer gas, the buffer gas pressure and the laser power for optical emission spectrometry of microplasmas are given.     

Characterization of laser induced plasmas by optical emission spectroscopy: A review of experiments and methods

Advances in characterization of laser induced plasmas by optical emission spectroscopy are reviewed in this article. The review is focused on the progress achieved in the determination of the physical parameters characteristic of the plasma, such as electron density, temperature and densities of atoms and ions. The experimental issues important for characterization by optical emission spectroscopy, as well as the different measurement methods are discussed. The main assumptions of the methods, namely the optical thin emission of spectral lines and the existence of local thermodynamic equilibrium in the plasma are evaluated. For dense and inhomogeneous sources of radiation such as laser induced plasmas, the characterization methods are classified in terms of the optical depth and the spatial resolution of the emission used for the measurements. The review deals firstly with optically thin spatially integrated measurements. Next, local measurements and characterization in not optically thin conditions are discussed. Two tables are included that provide reference to the works reporting measurements of electron density and temperature of laser induced plasmas generated with diverse samples.     

Hydrogen Balmer α line behavior in Laser-Induced Breakdown Spectroscopy depth scans of Au,Cu, Mn,Pb targets in air

The behavior of hydrogen spectral emission of the plasmas obtained by Laser-Induced Breakdown Spectroscopy (LIBS) measurement of four metal targets (Au, Cu, Mn, Pb) in air was investigated. The plasma was produced by a pulsed Nd:YAG laser emitting in the fundamental wavelength. A systematic study of the spatial-integrated plasma emission obtained by an in-depth scanning of the target was performed for each metal, both in single pulse and collinear double-pulse configurations. Further, a spatial-resolved analysis of the emission of plasma produced on the Al target by a single laser pulse was performed, in order to describe the spatial distribution of emitters deriving from the target and air elements. The line intensities of the main plasma components (target metal, nitrogen, oxygen and hydrogen) were measured in both experimental conditions. Results show that the hydrogen line intensity varies greatly as a function of the metal considered, and exhibits a marked decrease after the first laser shots. However, differently from emission lines due to surface impurities, the hydrogen line intensity reaches a constant level deep inside the target. The spatial-resolved measurements indicate that hydrogen atoms in the plasma mainly derive from the target surface and, only at a minor extent, from the dissociation of molecular hydrogen present in the surrounding air. These findings show that the calculation of plasma electron number density through the measurement of the Stark broadening of hydrogen Balmer α line is possible also in depth scanning measurements.     

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.     

Hydrogen analysis in solid samples by utilizing He metastable atoms induced by TEA CO2 laser plasma in He gas at 1 atm

A TEA CO₂ laser (350 mJ–1.5 J, 10.6 μm, 200 ns, 10 Hz) was focused onto a metal sub-target under He as host gas at 1 atmospheric pressure with a small amount of impurity gas, such as water and ethanol vapors. It was found that the TEA CO₂ laser with the help of the metal sub-target is favorable for generating a strong, large volume helium gas breakdown plasma at 1 atmospheric pressure, in which the helium metastable-excited state was then produced overwhelmingly. While the metal sub-target itself was never ablated. The helium metastable-excited state produced after the strong helium gas breakdown plasma was considered to play an important role in exciting the atoms. This was confirmed by the specific characteristics of the detected H emission, namely the strong intensity with low background, narrow spectral width, and the long lifetime. This technique can be used for gas and solid samples analysis. For nonmetal solid analysis, a metal mesh was introduced in front of the nonmetal sample surface to help initiation of the helium gas breakdown plasma. For metal sample, analysis can be carried out by combining the TEA CO₂ laser and an Nd–YAG laser where the Nd–YAG laser is used to ablate the metal sample. The ablated atoms from the metal sample are then sent into the region of helium gas breakdown plasma induced by the TEA CO₂ laser to be excited through the helium metastable-excited state. This technique can be extended to the analysis of other elements, not limited only to hydrogen, such as halogens.     

Spatially resolved concentration studies of ground state atoms and ions in an ICP: saturated absorption spectroscopic method

The saturated absorption spectroscopic method allows the direct determination of spatial distributions of absorbing species m a plasma to be made. Unlike most other emission and absorption methods which assume radial symmetry and require tedious Abel inversions, this method can be used regardless of the plasma symmetry because of its high spatial resolution. These characteristics are demonstrated with a 27.2 MHz ICP, utilizing short and long torches at power levels of 1.25 kW and 500 W for emission and fluorescence configurations, respectively. Distributions of Sr ground state atoms as well as Ba ground state ions were obtained under various conditions, using a single pulsed dye laser output as the spectroscopic probe in this diagnostic method.     

Curve of growth methodology applied to laser-induced plasma emission spectroscopy

The curve-of-growth (COG) method was applied to a laser-induced plasma. The plasma was produced by a Nd:YAG laser on the surface of steel samples containing 0.007–1.3% of Cr. The emission was collected from the top of the plasma by means of a 45° pierced mirror and aligned onto an intensified charge-coupled device (ICCD) with a gate width of 1 μs and a variable delay time. The resonance 425.4 nm Cr line was used for construction of the COG. The temperature of the plasma (∼8000 K at 5-μs delay) was determined from a Boltzmann plot. The damping constant a, proportional to the ratio of the Lorentzian to the Doppler line widths, was found from the best fit of a series of calculated COG to the experimental data points and was 0.20±0.05. The number density of neutral Cr atoms which corresponded to the transition between low and high optical densities, was ≈6.5·10¹² cm⁻³. The cross-section for broadening collisions of Cr atoms with atmospheric species (presumably N₂) was calculated to be (66±16) Å. The shape of the 425.4-nm Cr line was additionally checked by scanning an ultra-narrow cw Ti:Sapphire laser across the atomic transition and found to be in agreement with preliminary estimates. The potential of the COG method for laser breakdown spectroscopy is discussed.     

Spatial distributions of the number densities of neutral atoms and ions for the different elements in a laser induced plasma generated with a Ni-Fe-Al alloy

Spatially-resolved emission spectroscopy, including spatial devonvolution of the spectra, has been used to determine the three-dimensional distributions of the relative number densities of neutral atoms and ions of the elements present in a laser-induced plasma generated with a Ni-Fe-Al alloy. The method is based on the precise measurement of the local electronic temperature from Saha–Boltzmann plots constructed with Fe I and Fe II lines. The plasma was generated in air at atmospheric pressure using a 1064-nm Nd:YAG laser, and the emission was detected in the time window 3.0–3.5 μs. The ionization fraction was very high (above 0.9) for the three elements in the sample, only decreasing behind the expanding plasma front. The relative number densities were obtained from the emissivities of selected elemental lines as well as the temperature. The error in this procedure was estimated, and it was found that it is largely due to the uncertainties in the transition probability values used. The spatial distributions of the total relative number densities of the three elements were shown to coincide within the error, a result which is relevant to the development of models of plasma emission used in analytical applications. The ratios of the total number densities of the elements in the plasma were compared to their concentration ratios in the sample; however, the relatively high errors in the relative number densities did not permit any definitive conclusions to be drawn about the stoichiometry of the laser ablation process.     

Investigation on the role of air in the dynamical evolution and thermodynamic state of a laser-induced aluminium plasma by spatial- and time-resolved spectroscopy

The amount and the spatial distribution of air atoms and ions in a laser-induced plasma in ambient air provide important information about the formation of the plasma and its successive evolution history. For this reason, in the present work, the air mixing in a laser-induced plasma in air at atmospheric pressure and its influence on its thermodynamic evolution were studied. Information about spatial distributions of atoms and ions from Al, N and O were achieved by Abel-inverted spectra in the plume. The occurrence of LTE in the plume was also assessed by the utilization of theoretical criteria, and by the analysis of experimental spectra. Aluminium atoms and ions were found to be in LTE, while nitrogen and oxygen were not because of their longer times of relaxation toward equilibrium. Nitrogen was found to be over-ionized with respect to Saha–Eggert equilibrium, indicating that the plasma is recombining. Experimental observations suggest that the concentration of air species in the plasma is larger than that of aluminium, even in the region closer to the target, where the aluminium lines are stronger. In the front part of the plume only emission lines from air species were observed. The results suggest that a Laser-Supported Detonation (LSD) regime occurs during the trailing part of the laser pulse, resulting in the strong inclusion into the plasma of air elements. In this scenario, also the thermodynamic history of the plume is affected by the predominance of air species.     

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.     

Analytical optimization of some parameters of a Laser-Induced Breakdown Spectroscopy experiment

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.     

The influence of laser-particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry

Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.     

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.     

Early stage emission spectroscopy study of metallic titanium plasma induced in air by femtosecond- and nanosecond-laser pulses

In this work the laser induced plasma obtained in air at atmospheric pressure by the interaction of a fs (femtosecond) or a ns (nanosecond) laser pulse with a metallic titanium target has been investigated by optical emission spectroscopy. The temporal evolution of plasma parameters such as electron number density and excitation temperature has been determined in order to highlight the processes involved when the emission spectra are acquired at short time delays from the ablating laser pulse. A survey of elementary processes implicated during plasma formation and expansion of ns- and fs-Laser Induced Plasma has been performed. Departures from equilibrium conditions are even discussed. The dynamic aspects corresponding to ns- and fs-LIP have been investigated by optical time of flight (TOF) and by fast emission imaging. The overall results have been used for clarifying the basic mechanisms occurring during plasma expansion due to either ns or fs laser source when experimental conditions usually used for laser-induced breakdown spectroscopy (LIBS) applications are employed.     

Plume segregation observed in hydrogen and deuterium containing plasmas produced by laser ablation of carbon fiber tiles from a fusion reactor

The plasma produced by the irradiation of a hydrogen and deuterium containing carbon fiber composite with infrared laser pulses of 4-ns pulse duration has been investigated. The experiments were carried out under argon at reduced pressure. Microscopic analyses of the irradiated sample surface were performed to measure the ablation depth. Time- and space-resolved optical emission spectroscopy was applied to characterize the evolution of spectral line emission as a function of time and distance from the surface. Particular attention was paid to the time-of-flight characteristics of the hydrogen and deuterium Balmer α spectral lines. According to the different atomic masses of both isotopes, the expansion of hydrogen into the low pressure argon atmosphere was found to be slightly faster than that of deuterium. The effect of plume segregation is pressure dependent and tends to increase the analytical signal of heavy atoms with respect to lighter ones during laser-induced breakdown spectroscopy.