Local optical emission spectroscopy of excited species effused from an evaporation cell and a sputter source into dense plasmas – Basic studies for the deposition of thin gradient films

Space resolved optical emission spectroscopy has been applied to determine the distribution of excited species in dense plasmas which are used for the deposition of thin coatings. Typical electron densities and electron temperatures in the plasma facility PETRA ( Plasma Engineering and Technology Research Assembly) are in the range of n(e) = 10(12) cm(-3) and T(e) = 10 eV. During the deposition process material (Al) is evaporated from a vapour cell under controlled conditions. The vapour stream is guided into a dense plasma which is composed of inert gas, Ar or He, and hydrocarbon species produced from the dissociation of C(2)H(2). The evaporated Al-stream which travels with thermal velocity into a plasma of high electron density, is nearly completely ionized due to the short mean free path for electron impact ionization in the above mentioned parameter range. Optical emission spectroscopy has been applied to investigate the interaction processes between the vapour stream and the plasma as well as the transport of the ionized Al along the applied magnetic field. For the measurements space resolved optical emission spectroscopy with an in-situ translation mechanism of the optical fibre has been used to measure the local concentrations of excited Al neutrals and ions as well as the concentration of the background plasma species.     

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.     

Characterization of laser-induced plasmas by emission spectroscopy with curve-of-growth measurements. Part I: Temporal evolution of plasma parameters and self-absorption

Laser-induced plasmas have been characterized by emission spectroscopy, including the measurement of curves of growth. The plasmas have been generated in air at atmospheric pressure using an infrared Nd:YAG laser from a set of Fe–Ni alloys with varying Fe concentrations. The procedure used provides, in addition to the apparent temperature T and electron density N_e, a parameter N′l (the atom number density for 100% concentration times the length of the plasma along the line-of-sight), relevant to obtain the self-absorption and the intensity of the emission lines. The temporal evolution of the plasma parameters has been deduced from the measurement and fitting of the curves of growth. A fast temporal decrease of N′l is obtained for ions, whereas a gradual increase takes place for neutral atoms. The temporal evolution of the line intensity in the optically thin limit and the self-absorption of neutral atom and ion lines have been obtained experimentally and calculated from the evolution of the plasma parameters. The usefulness of the curve-of-growth method in measurements with time integration, in spite of the fast variation of the plasma parameters, has been demonstrated.     

Characterization of laser-induced plasmas by emission spectroscopy with curve-of-growth measurements. Part II: Effect of the focusing distance and the pulse energy

Laser-induced plasmas generated with different focusing distances and pulse energies have been characterized by a method based in emission spectroscopy that includes the measurement and calculation of curves of growth. An infrared Nd:YAG laser is used to generated the plasmas from Fe–Ni samples placed in air at atmospheric pressure. The characterization method provides a reduced set of plasma parameters (N_e, T, N′l, αA) that describe the line emission in optically thin and optically thick conditions. For a pulse energy of 100 mJ, the plasma parameters for varying focusing distances are obtained. The apparent (population averaged) temperatures for neutral atoms and ions are shown to be different in the plasmas generated with all the focusing distances. For each pulse energy (in the range 20–100 mJ), the plasmas generated with the optimum focusing distance, which corresponds to a constant value of irradiance, have been investigated. In these conditions, simple laws have been obtained for the variation of the plasma parameters with the pulse energy E: the electron density N_e and the apparent temperature T are independent of E while linear relations with E are obtained for the parameters N′l, αA. These simple laws lead to a quadratic dependence on E of the line intensities in the optically thin limit and to a variation of the intersection concentration C_int that characterizes self-absorption as E^− 1.     

Diagnostics of a thermal plasma jet by optical emission spectroscopy and enthalpy probe measurements

An accurate determination of electron density, temperature, and velocity distributions is of primary interest for the characterization of steady-state thermal plasma spray jets. Our diagnostic capabilities based on optical emission spectroscopy include measurements of absolute emission coefficients and Stark broadening. In addition, enthalpy probe diagnostics has also been used for temperature and velocity measurements. Observation of large discrepancies between temperatures derived from absolute emission coefficients, Stark broadening, and from enthalpy probe measurements indicate that severe deviations from LTE (local thermal equilibrium) exist in various regimes of plasma spray jets. Nonequilibrum characterization of such turbulent thermal plasma jets suggests that diffusion of high-energy electrons into the fringes of plasma jets and deviations from chemical equilibrium due to high velocities in the core of plasma jets and entrainment of cold gas, are the main reasons for these discrepancies. The establishment of a reliable data base, taking these nonequilibrium effects into account, is a prerequisite for meaningful modeling of real plasma jets.     

Characterization of laser induced breakdown plasmas used for measurements of arsenic,antimony and selenium hydrides

Studies have been performed to characterize laser induced breakdown spectroscopy (LIBS) plasmas formed in Ar/H₂ gas mixtures that are used for hydride generation (HG) LIBS measurements of arsenic (As), antimony (Sb) and selenium (Se) hydrides. The plasma electron density and plasma excitation temperature have been determined through hydrogen, argon and arsenic emission measurements. The electron density ranges from 4.5 × 10¹⁷ to 8.3 × 10¹⁵ cm^?3 over time delays of 0.2 to 15 μs. The plasma temperatures range from 8800 to 7700 K for Ar and from 8800 to 6500 K for As in the HG LIBS plasmas. Evaluation of the plasma properties leads to the conclusion that partial local thermodynamic equilibrium conditions are present in the HG LIBS plasmas. Comparison measurements in LIBS plasmas formed in Ar gas only indicate that the temperatures are similar in both plasmas. However it is also observed that the electron density is higher in the Ar only plasmas and that the emission intensities of Ar are higher and decay more slowly in the Ar only plasmas. These differences are attributed to the presence of H₂ which has a higher thermal conductivity and provides additional dissociation, excitation and ionization processes in the HG LIBS plasma environment. Based on the observed results, it is anticipated that changes to the HG conditions that change the amount of H₂ in the plasma will have a significant effect on analyte emission in the HG LIBS plasmas that is independent of changes in the HG efficiency. The HG LIBS plasmas have been evaluated for measurements of elements hydrides using a constant set of HG LIBS plasma conditions. Linear responses are observed and limits of detection of 0.7, 0.2 and 0.6 mg/L are reported for As, Sb and Se, respectively.     

Calculation of elemental columnar density from self-absorbed lines in laser-induced breakdown spectroscopy: A resource for quantitative analysis

The presence of self-absorption of emission lines is usually an undesired effect in laser-induced breakdown spectroscopy because it introduces non linear effects in the growth of line intensity versus the concentration of the emitting species. Several methods have been proposed in recent years for identifying and quantifying self-absorption in the emission spectra. After this diagnostic stage, the lines affected by self-absorption are usually disregarded; otherwise, appropriate corrective factors are applied to their intensity before the utilization for analytical purposes. Changing the point of view, this paper remarks as self-absorption can provide useful information for analyzing the composition of laser-induced plasmas and for their characterization. Whenever the extent of self-absorption is quantified, in fact, the optical depth of the line can be rapidly calculated; then, for plasmas in local thermodynamic equilibrium conditions, the columnar density of the emitting species can be derived. Assuming the plasma homogeneity, the concentration ratio between different elements can be obtained. Moreover, in particular cases, the columnar densities can be used to calculate the plasma temperature and the absolute number densities of plasma species. Some applications of the method are reported in the paper and potentialities and limitations are discussed.     

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.     

Temperature measurements in a decaying laser-induced plasma in air at elevated pressures

This article discusses two measurement techniques for temperature determination of laser-induced plasmas in a gas at pressures relevant for combustion engines. Plasmas induced by laser breakdown in air at initial pressures ranging from 0.3 MPa to 2.5 MPa are investigated using optical spectroscopy. Results for 0.8 MPa, 1.2 MPa and 1.6 MPa are reported here. Due to the elevated pressure, a significant contribution from continuum radiation is apparent. The first temperature measurement technique relies on the interpretation of the continuum emission. The second technique is based on the line emissions from different elements and ionization stages in the plasma and is implemented with the multi-element Saha-Boltzmann plot method. The methodology may be applicable for temperature measurements under various conditions, e.g., for plasmas in high pressure gas environments such as in industrial applications of laser-induced breakdown spectroscopy or for plasma sources for illumination purposes. We investigate optimizations of laser-induced spark ignition. The energy released in the laser-induced plasma is determined based on temperature measurements.     

Optical emission studies of plasma induced by single and double femtosecond laser pulses

Double-pulse femtosecond laser ablation has been shown to lead to significant increase of the intensity and reproducibility of the optical emission signal compared to single-pulse ablation particularly when an appropriate interpulse delay is selected, that is typically in the range of 50–1000 ps. This effect can be especially advantageous in the context of femtosecond laser-induced breakdown spectroscopy analysis of materials. A detailed comparative study of collinear double- over single-pulse femtosecond laser-induced breakdown spectroscopy has been carried out, based on measurements of emission lifetime, temperature and electronic density of plasmas, produced during laser ablation of brass with 450 fs laser pulses at 248 nm. The results obtained show a distinct increase of plasma temperature and electronic density as well as a longer decay time in the double-pulse case. The plasma temperature increase is in agreement with the observed dependence of the emission intensity enhancement on the upper energy level of the corresponding spectral line. Namely, intensity enhancement of emission lines originating from higher lying levels is more profound compared to that of lines arising from lower energy levels. Finally, a substantial decrease of the plasma threshold fluence was observed in the double-pulse arrangement; this enables sensitive analysis with minimal damage on the sample surface.     

Optical emission spectroscopy in reactive hollow cathode arc discharge plasmas – Local distribution of active species during the deposition of hard carbon films

A low pressure arc plasma discharge from a hollow LaB(6)-cathode with up to 100 A discharge current is used to create plasmas of high density. Typical values for the electron density and temperature in PETRA ( Plasma Engineering and Technology Research Assembly) are n(e)=10(12)-10(13) cm(-3) and T(e)=5-20 eV. The ionization ratio is typically 1-10%. Optical emission spectroscopy has been applied to investigate the processes within the plasma which lead to the deposition of thin carbon films. In these experiments hydrogenated carbon films (a-C:H) have been deposited on Si-substrates by introducing hydrocarbon gases (CH(4), C(2)H(2)) into He- and Ar-plasmas. Space resolved optical emission spectroscopy using an in-situ translation mechanism of the optical fibre has been performed to measure the local concentrations of CH-radicals, carbon ions and of the excitation of He-neutrals. In addition the hydrogen liberated by the dissociation of the hydrocarbon molecules has been measured. The dissociation of the hydrocarbon molecules takes place as a localized process in the vicinity of the reactive gas inlet.     

Characterization of a laser-induced plasma by spatially resolved spectroscopy of neutral atom and ion emissions.: Comparison of local and spatially integrated measurements

The local values of the parameters that characterize a laser-induced plasma (temperature, electron density, relative number densities of neutral atoms and ions) have been obtained by spatially resolved emission spectroscopy, including the deconvolution of the measured intensity spectra. The plasma has been generated using a Nd:YAG laser with a Fe–Ni alloy in air at atmospheric pressure, and the emission in the time window 3.0–3.5 μs has been detected. The temperature values obtained from neutral atom and ion emissions have been compared in the cases of local and spatially-integrated measurements. Local Boltzmann and Saha–Boltzmann plots with high correlation to linear fittings have been obtained using two broad sets of optically thin neutral atom and ion lines (21 Fe I lines and 15 Fe II lines), resulting in local values of the electronic temperature that coincide within the error. These results of local measurements contrast with those of spatially integrated measurements, for which two different temperatures are obtained from the Boltzmann plots of neutral atoms (9100±150 K) and ions (13 700±300 K). This difference is explained according to the measured distributions of the electronic temperature and the neutral atom and ion number densities, that result in separated emissivity (or population) distributions of neutral atom and ion lines, leading to different neutral atom and ion apparent temperatures (population-averages of the local electronic temperature). Local values of the plasma parameters have been obtained at all the positions with significant emission, including the determination of the electronic temperature from Saha–Boltzmann or Boltzmann plots. The ionization degree is high- and low-varying at the inner part of the plasma, decaying only near the plasma front. The maximum of the ion density does not coincide with the temperature maximum; on the contrary, the axial variation of both the neutral atom and ion densities (that decrease towards the sample surface) is opposite to that of the temperature, a behaviour that is interpreted to result from the plasma expansion process.     

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.     

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.     

Modeling an inhomogeneous optically thick laser induced plasma: a simplified theoretical approach

A simplified theoretical approach is developed for an optically thick inhomogeneous laser induced plasma. The model describes the time evolution of the plasma continuum and specific atomic emission after the laser pulse has terminated and interaction with a target material has ended. Local thermodynamic equilibrium is assumed allowing the application of the collision-dominated plasma model and standard statistical distributions. Calculations are performed for a two-component Si/N system. The model input parameters are the number of plasma species (or plasma pressure) and the ratio of atomic constituents. Functions are introduced which describe the evolution of temperature and size of the plasma. All model inputs are experimentally measurable. The model outputs are spatial and temporal distributions of atom, ion and electron number densities, evolution of an atomic line profile and optical thickness and the resulting absolute intensity of plasma emission in the vicinity of a strong non-resonance atomic transition. Practical applications of the model include prediction of temperature, electron density and the dominating broadening mechanism. The model can also be used to choose the optimal line for quantitative analysis.     

Comparison of active and passive spectroscopic methods to investigate atmospheric inductively coupled plasmas

A comparison of Thomson and Rayleigh scattering, diode laser absorption and line emission measurements is performed on a 100 MHz atmospheric argon-flowing inductively coupled plasma. The parameters, which are measured in two or more ways, are the electron density, the electron temperature and the heavy particle temperature. The optimized diagnostics show the same behavior for the electron density and temperature. Nevertheless, the Thomson scattering diagnostic is the best at retrieving the radial profile. The heavy particle temperature, as measured by using both Rayleigh scattering and diode laser absorption, is identical within the estimated errors. The technique of measuring the temperature during power interruption, with both Thomson scattering and emission spectroscopy, shows that the electron and heavy particle temperatures are not equal during the period of power interruption.     

On the usefulness of a duplicating mirror to evaluate self-absorption effects in laser induced breakdown spectroscopy

This paper illustrates the application of the well-known approach of duplicating the emission from a plasma by placing a spherical mirror behind it in order to characterize the degree of self-absorption of atomic transitions. It is shown that this simple expedient provides a quick check for the existence of optically thick plasma conditions, and allows one to follow the temporal evolution of the plasma optical depth from the early decay of the continuum emission to the end of the plasma lifetime. The method is applied to a plasma induced at atmospheric pressure by focusing an Nd:YAG laser on different Al-alloy targets. Moreover, if the resolution of the monochromator allows one to obtain the true physical profiles of the lines investigated, a self-absorption correction factor can be calculated, following a methodology described in the plasma diagnostic literature. It is shown that this correction can be used to improve the linearity of calibration curves and to identify outliers in the Saha–Boltzmann plot for temperature evaluation. The data obtained are still the result of line-of-sight measurements, and therefore can only be interpreted in terms of some space-averaged values of the parameters evaluated. Despite this limitation, it is argued that the simple addition of a mirror to a laser induced plasma emission experiment has many advantageous features and should find a more widespread use when performing laser induced breakdown spectroscopy experiments.     

L.D.A. Simultaneous Measurements of Local Density and Velocity Distribution of Particles in Plasma Fluidized Bed at Atmospheric Pressure

Laser Doppler anemometry (L.D.A.) is an efficient and nonintrusive technique. Today, improved in its configuration, the L.D.A. has been applied even in flowing plasmas. ^(1,2) In-flight simultaneous measurements were performed for local density and velocity of particle distribution. The measurements provide an insight into thermal and mass transfer, chemical reactivity, and the distribution of residence times of particles in a plasma fluidized bed. The difficulties of L.D.A. in a plasma fludized bed such as high emission intensity of the plasma torch, high temperature, high particle density, and large distribution of particle granulometry were overcomed in the present investigation. The aims achieved were the characterization of the plasma fluidized bed distribution together with accurate measurements of local particle density and velocity as measured by L.D.A.