Time-resolved investigations of laser-induced plasmas generated by nanosecond bursts in the millijoule burst energy regime

Laser-induced plasmas are investigated during laser micro structuring of a C 75 steel alloy using laser bursts that consist of nanosecond laser pulses under atmospheric pressure. The influence of the laser burst mode — single and collinear double pulses — on plasma dynamics and ablation efficiency is investigated for burst energies in the millijoule regime. Electron density and excitation temperatures measured as a function of time. The results are compared with published data looking for changes of the plasma parameters scaling with the burst energy over two orders of magnitude. For collinear double pulses at burst energies of 1–2 mJ an increase of the ablation rate by a factor of three to four compared to single pulses was observed.     

New approach to online monitoring of the Al depth profile of the hot-dip galvanised sheet steel using LIBS

In this study a new approach to the online monitoring of the Al depth profile of hot-dip galvanised sheet steel is presented, based on laser-induced breakdown spectroscopy (LIBS). The coating composition is measured by irradiating the traversing sheet steel with a series of single laser bursts, each at a different sheet steel position. An ablation depth in the same range as the coating thickness (about 10 μm) is achieved by applying a Nd:YAG laser at 1064 nm in collinear double-pulse and triple-pulse mode. The ablation depth is controlled by adjusting the burst energy with an external electro-optical attenuator. A fingerprint of the depth profile is gained by measuring the LIBS signals from zinc, aluminium and iron as a function of the burst energy, and by post-processing the data obtained. Up to three depths can be sampled simultaneously with a single laser burst by measuring the LIBS signals after each pulse within the laser burst. A concept for continuously monitoring the Al depth profile during the galvanising process is presented and applied to different hot-dip galvanised coatings. The method was tested on rotating sheet steel disks moving at a speed of up to 1 m/s. The potential and limitations of the new method are discussed.     

Systematic line selection for online coating thickness measurements of galvanised sheet steel using LIBS

LIBS can be used as an online method of characterizing galvanized coatings on sheet steel moving through a production line. The traversing sheet steel is irradiated with a series of single laser bursts, each at a different position on the sheet steel. An ablation depth in the same range as the coating thickness (about 10 μm) is achieved by using a Nd:YAG laser at 1064 nm in collinear double-pulse mode. The coating thickness is determined from the ratio of the intensities of an iron line and a zinc line measured at a burst energy high enough to penetrate the coating with a single burst. Experiments at different burst energies were carried out to optimize the thickness resolution, and a method of systematically selecting iron and zinc lines was deduced, which is based on multivariate data analysis (MVDA) of the intensity ratios calculated for a set of 6 zinc lines and 21 iron lines. A temperature correction was applied, because the parameters of the plasma change with burst energy, and the influence of this on the thickness resolution is discussed. The ambient atmosphere present (air, Ar, N₂) as well as self-absorption of spectral lines both have an influence on the thickness resolution. At optimum conditions, a thickness measurement accuracy of better than 150 nm was obtained for a set of electrolytic galvanized sheet steels with coating thicknesses in the range 4.1–11.2 μm.     

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.     

An evaluation of the analytical performance of collinear multi-pulse laser induced breakdown spectroscopy

A detailed study of the relevant analytical figures of merit for time- and spatially integrated multi-pulse laser induced breakdown spectroscopy (MP-LIBS) was performed. Laser bursts containing up to 6, ns-range duration collinear pulses, separated by 20–40 μs interpulse gaps were used in the experiments, and the effect of the number of pulses within the burst on the analytical parameters was investigated. Signal enhancement, repeatability, matrix effects, background signals, sensitivity, linear dynamic range and limits of detection were studied for 20 lines of 11 elements in different solid matrices. It was established that all analytical figures of merit significantly improved with respect to those of single- or double-pulse LIBS as a result of the use of multiple laser pulses. For example, six-pulse limits of detection values were found to be with a factor of 4.2–16.7 lower than for double-pulses and calibration plots were found to be linear up to several tens of percents concentrations in some alloys.     

Experimental study of laser-induced plasma: Influence of laser fluence and pulse duration

Influence of laser fluence and pulse duration on the morphology and the internal structure of plasma induced by infrared nanosecond laser pulse on an aluminum target placed in an argon ambient gas of one atmosphere pressure was experimentally studied. Dual-wavelength differential spectroscopic imaging was used in the experiment, which allowed observing the detailed structure inside of the ablation plume with distributions of species evaporated from the target as well as contributed by the ambient gas. Different regimes of post-ablation interaction were investigated using different laser fluences and pulse durations. We demonstrate in particular that plasma shielding due to various species localized in different zones inside of the plume leads to different morphologies and internal structures of the plasma. At moderate fluence, the plasma shielding due to the ablation vapor localized in the central part of the plume leads to its nearly spherical expansion with a layered structure of the distribution of different species. At higher fluence, the plasma shielding becomes strongly contributed by ionized ambient gas localized in the propagation front of the plume. An elongated morphology of the plume is observed with a zone of mixing between different species evaporated from the target or contributed by the ambient gas. Finally with extremely strong plasma shielding by ionized ambient gas in the case of a long duration pulse at high fluence, a delayed evaporation from the target is observed due to the ejection of melted material by splashing.     

An influence of pretreatment conditions on surface structure and reactivity of Pt(100) towards CO oxidation reaction

We present a combined electrochemical and in situ STM study of the surface structure of Pt(100) single crystal electrodes in dependence on the cooling atmosphere after flame annealing. The following cooling conditions were applied: Ar/H₂ and Ar/CO mixtures (reductive atmosphere), argon (inert gas) and air (oxidative atmosphere). Surface characterization by in-situ STM allows deriving direct correlations between surface structure and macroscopic electrochemical behavior of the respective platinum electrodes. We investigated the influence of defect type and density as well as long range surface order on the kinetics of the CO electro-oxidation reaction. The defect-rich Pt(100) electrodes as cooled in air or Ar, and followed by immersion in the hydrogen adsorption region display higher activities as compared to the rather smooth Pt(100)-(1 × 1) electrode cooled in an Ar/H₂-atmosphere.     

Characterization of laser ablation and ionization in helium and argon: A comparative study by time-of-flight mass spectrometry

Laser ablation and ionization in ambient helium and argon gases were studied by multiple-stage time-of-flight mass spectrometry. Measurements made at different gas pressures indicated that there exists an optimal pressure for adequately cooling energetic ions and reducing multiply charged ions that are higher for He than for Ar. The temporal distributions of ions were compared at various laser fluences and gas pressures, and the broad distributions for He could be ascribed to elastic scattering and thermodynamic processes. The diffusion of ions in He resulted in a longer delay before the instrument registered its maximal signal. Ions with different masses were observed to have the same kinetic energies in He, which was confirmed using the SIMION software, while ion movement was hydrodynamically controlled in Ar. The velocities of singly and doubly charged ions were also studied, and doubly charged ions showed much higher kinetic energy because of their frontal location in the plasma expansion.     

Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement

A review of recent results of the studies of double laser pulse plasma and ablation for laser induced breakdown spectroscopy applications is presented. The double pulse laser induced breakdown spectroscopy configuration was suggested with the aim of overcoming the sensitivity shortcomings of the conventional single pulse laser induced breakdown spectroscopy technique. Several configurations have been suggested for the realization of the double pulse laser induced breakdown spectroscopy technique: collinear, orthogonal pre-spark, orthogonal pre-heating and dual pulse crossed beam modes. In addition, combinations of laser pulses with different wavelengths, different energies and durations were studied, thus providing flexibility in the choice of wavelength, pulse width, energy and pulse sequence. The double pulse laser induced breakdown spectroscopy approach provides a significant enhancement in the intensity of laser induced breakdown spectroscopy emission lines up to two orders of magnitude greater than a conventional single pulse laser induced breakdown spectroscopy. The double pulse technique leads to a better coupling of the laser beam with the plasma plume and target material, thus providing a more temporally effective energy delivery to the plasma and target. The experimental results demonstrate that the maximum effect is obtained at some optimum separation delay time between pulses. The optimum value of the interpulse delay depends on several factors, such as the target material, the energy level of excited states responsible for the emission, and the type of enhancement process considered. Depending on the specified parameter, the enhancement effects were observed on different time scales ranging from the picosecond time level (e.g., ion yield, ablation mass) up to the hundred microsecond level (e.g., increased emission intensity for laser induced breakdown spectroscopy of submerged metal target in water). Several suggestions have been proposed to explain the mechanism of double pulse enhancement.     

Effect of laser pulse energies in laser induced breakdown spectroscopy in double-pulse configuration

In this paper, the effect of laser pulse energy on double-pulse laser induced breakdown spectroscopy signal is studied. In particular, the energy of the first pulse has been changed, while the second pulse energy is held fixed. A systematic study of the laser induced breakdown spectroscopy signal dependence on the interpulse delay is performed, and the results are compared with the ones obtained with a single laser pulse of energy corresponding to the sum of the two pulses. At the same time, the crater formed at the target surface is studied by video-confocal microscopy, and the variation in crater dimensions is correlated to the enhancement of the laser induced breakdown spectroscopy signal. The results obtained are consistent with the interpretation of the double-pulse laser induced breakdown spectroscopy signal enhancement in terms of the changes in ambient gas pressure produced by the shock wave induced by the first laser pulse.     

Multi-element model for the simulation of inductively coupled plasmas: Effects of helium addition to the central gas stream

A model for an atmospheric pressure inductively coupled plasma (ICP) is developed which allows rather easy extension to a variable number of species and ionisation degrees. This encompasses an easy calculation of transport parameters for mixtures, ionisation and heat capacity. The ICP is modeled in an axisymmetric geometry, taking into account the gas streaming into a flowing ambient gas. A mixture of argon and helium is applied in the injector gas stream as it is often done in laser ablation ICP spectrometry. The results show a strong influence of the added helium on the center of the ICP, which is important for chemical analysis. The length of the central channel is significantly increased and the temperature inside is significantly higher than in the case of pure argon. This means that higher gas volume flow rates can be applied by addition of helium compared to the use of pure argon. This has the advantage that the gas velocity in the transport system towards the ICP can be increased, which allows shorter washout-times. Consequently, shorter measurement times can be achieved, e.g. for spatial mapping analyses in laser ablation ICP spectrometry. Furthermore, the higher temperature and the longer effective plasma length will increase the maximum size of droplets or particles injected into the ICP that are completely evaporated at the detection site. Thus, we expect an increase of the analytical performance of the ICP by helium addition to the injector gas.     

Double pulse laser ablation and laser induced breakdown spectroscopy: A modeling investigation

A numerical model, describing laser–solid interaction (i.e., metal target heating, melting and vaporization), vapor plume expansion, plasma formation and laser–plasma interaction, is applied to describe the effects of double pulse (DP) laser ablation and laser induced breakdown spectroscopy (LIBS). Because the model is limited to plume expansion times in the order of (a few) 100 ns in order to produce realistic results, the interpulse delay times are varied between 10 and 100 ns, and the results are compared to the behavior of a single pulse (SP) with the same total energy. It is found that the surface temperature at the maximum is a bit lower in the DP configuration, because of the lower irradiance of one laser pulse, but it remains high during a longer time, because it rises again upon the second laser pulse. Consequently, the target remains for a longer time in the molten state, which suggests that laser ablation in the DP configuration might be more efficient, through the mechanism of splashing of the molten target. The total laser absorption in the plasma is also calculated to be clearly lower in the DP configuration, so that more laser energy can reach the target and give rise to laser ablation. Finally, it is observed that the plume expansion dynamics is characterized by two separate waves, the first one originating from the first laser pulse, and the second (higher) one as a result of the second laser pulse. Initially, the plasma temperature and electron density are somewhat lower than in the SP case, due to the lower energy of one laser pulse. However, they rise again upon the second laser pulse, and after 200 ns, they are therefore somewhat higher than in the SP case. This is especially true for the longer interpulse delay times, and it is expected that these trends will be continued for longer delay times in the μs-range, which are most typically used in DP LIBS, resulting in more intense emission intensities.     

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.     

Comparison of Single Pulse and Double Simultaneous Pulse Laser Induced Breakdown Spectroscopy

A simple and cost-effective variant of laser induced breakdown spectroscopy is presented that involves a double simultaneous pulse configuration employing a single laser source. Its performance is compared with conventional single pulse configuration. Double simultaneous pulses were accomplished by splitting a Nd:YAG laser (1064 nm, 6 ns, 360 mJ) beam into two components that were focused on the sample surface to produce two concurrent breakdowns. Experiment was repeated for single pulse and double simultaneous pulses under different ambient pressures. The performance was evaluated on the basis of self-absorption, signal-to-noise ratio (SNR), and relative standard deviation (RSD) of the Mg II doublet (280.2704 nm, 279.553 nm). Optically thin emission lines of better profiles with higher signal-to-noise ratio resulted from double simultaneous pulses. The lowest relative standard deviations obtained by single pulse and double simultaneous pulse configurations were 18.89% and 12.01%, respectively. In fact, double simultaneous pulses have performed better than single pulse in all respects within the studied regime.     

Gas leak detection by diode laser absorption spectrometry

Diode laser atomic absorption measurements of argon traces in low-pressure discharges were carried out to detect and measure gas leaks in a test chamber. Helium flows as a carrier gas through the test chamber and the discharge. In the case of a leak, air and thus also its natural content of argon is mixed to the helium gas-flow through the chamber. The argon content of the mixed gas flow through the discharge is determined by wavelength modulation diode laser atomic absorption spectrometry. The resulting absorption signal is a measure for the existing leak-rate. For barometric pressure of ambient air lowest detectable leak rates are typically 10⁻⁶ mbar l s⁻¹. By application of pure Ar with pressures above 1 bar detectable leak rates can in practice be lower than 10⁻⁸ mbar l s⁻¹.     

Orthogonal Double-pulse versus Single-pulse laser ablation at different air pressures: A comparison of the mass removal mechanisms

The mass removal mechanisms occurring during the ablation of an aluminum target, induced by an Nd:YAG laser at λ = 1064 nm in air at different laser fluences, were investigated at different pressures and in the orthogonal double pulse configuration. Both the spectroscopic analysis of the plasma emission and the microscopic analysis of the craters, providing complementary information on the laser ablation process, were performed. The first technique allowed the calculation of the plasma thermodynamic parameters and an estimation of its atomized mass, while the latter led to the calculation of their volume, as well as a qualitative inspection of the craters profile and appearance. The results obtained at different fluences suggest a complex picture where the air pressure strongly drives the laser shielding effect, which in turn affects the relevance of melt displacement, melt expulsion and phase explosion mechanisms. The measurements performed in double pulse configuration suggest that in this case the ablation process is very similar to that induced at low air pressure. Phase explosion seems to occur in double pulse laser ablation while it seems inhibited in single pulse ablation at atmospheric pressure. Differently, melt splashing is much more efficient in single pulse ablation at atmospheric pressure than in double pulse ablation.     

Influence of ambient gas pressure on laser-induced breakdown spectroscopy technique in the parallel double-pulse configuration

Single-pulse and double-pulse laser-induced breakdown spectroscopy experiments have been performed using two Nd:YAG lasers in the fundamental mode on a brass sample at different air pressures, ranging from 0.1 Torr to atmospheric conditions, in order to obtain information about the different ablation and plasma evolution processes in the different configurations. Neutral and ionized lines originated both by species deriving from the target and from the air environment were analysed. The temperature and electron density values were estimated in all the experimental conditions. A different behavior of the plasma emission versus the air pressure, in the case of lines deriving from the target, was observed in the single-pulse and double-pulse configurations, suggesting that the different environmental conditions in the first and the second laser ablation may be responsible in determining the plasma emission in the two cases. An interpretative model based on the cavity produced in air by the laser-induced shock wave, according to the Sedov theory of the blast wave expansion, was able to qualitatively describe the effects observed in single-pulse and double-pulse experiments.

Besides, the influence of the interpulse delay time between the two laser pulses was explored in the range between 0 and 20 μs. The results, according to the model proposed, provide information on the plume evolution in the single-pulse and double-pulse configurations at different air pressures. In particular, different optimum interpulse delays were found for the observation of neutral lines and ionic lines.     

Effects of atmosphere on laser vaporization and excitation processes of solid samples

Effects of atmosphere on the laser vaporization and excitation processes were investigated with spectral measurements and with the direct measurement of vaporized weight of samples. The samples, metals and ceramics, were positioned in three different atmospheres, i.e. air, argon and helium, from atmospheric pressure to a pressure reduced to a few torr. The time-resolved emission intensities of the Fe I lines and the Al I lines of the Al alloy and Al metal samples were measured in two time windows, i.e. 0–1 μs and 1 μs-1 ms. The excitation temperatures and the electron number densities of the plasmas were also estimated. The emission spectra, the excitation temperatures, and the electron densities were shown to be appreciably influenced by the ambient atmosphere. The amount of sample vaporized which was measured directly with an electric micro-balance after irradiation by 500 laser shots changed considerably with the atmosphere, e.g. from 12 ng/pulse at atmospheric pressure to 330 ng/pulse at 10 torr in argon.

The results are discussed in the scope of the possibility of ambient gas breakdown before sample vaporization and a change in the laser radiation coupling to the solid surface. It is revealed that the control of the interaction between laser radiation and plasmas and the prevention of the preceding gas breakdown are important for effective laser vaporization and for the emission measurement of the plasmas. The sample characteristics also influence the initiation stage of the plasmas and the effects from the atmosphere.     

The effect of sequential dual-gas testing on laser-induced breakdown spectroscopy-based discrimination: Application to brass samples and bacterial strains

Four Cu–Zn brass alloys with different stoichiometries and compositions have been analyzed by laser-induced breakdown spectroscopy (LIBS) using nanosecond laser pulses. The intensities of 15 emission lines of copper, zinc, lead, carbon, and aluminum (as well as the environmental contaminants sodium and calcium) were normalized and analyzed with a discriminant function analysis (DFA) to rapidly categorize the samples by alloy. The alloys were tested sequentially in two different noble gases (argon and helium) to enhance discrimination between them. When emission intensities from samples tested sequentially in both gases were combined to form a single 30-spectral line “fingerprint” of the alloy, an overall 100% correct identification was achieved. This was a modest improvement over using emission intensities acquired in argon gas alone. A similar study was performed to demonstrate an enhanced discrimination between two strains of Escherichia coli (a Gram-negative bacterium) and a Gram-positive bacterium. When emission intensities from bacteria sequentially ablated in two different gas environments were combined, the DFA achieved a 100% categorization accuracy. This result showed the benefit of sequentially testing highly similar samples in two different ambient gases to enhance discrimination between the samples.     

Laser-induced breakdown spectroscopy—From research to industry,new frontiers for process control

This paper presents R&D activities to explore new laser parameter ranges in pulse energy, time and space for laser-induced breakdown spectroscopy. The collinear double pulse effect, which is well studied for pulses of typically several 100 mJ energy can also be observed for laser pulses having a pulse energy two orders of magnitude lower. In this case, maximum line emission intensity occurs at interpulse separations of a few 100 ns. Temporal pulse tailoring to improve the performance of LIBS is only a first step. A comprehensive approach includes spatial pulse shaping to generate craters with predefined shape or to improve spatial averaging for the analysis of inhomogeneous samples. High performance components for LIBS systems such as spectrometers, electronics and sample stands are required to enable industrial applications. Latest developments offer wide-band single spectra acquisition with a high spectral resolution at a measuring frequency of up to 500 Hz. The next generation of multi-channel integrator electronics for Paschen–Runge spectrometers equipped with PMT detectors will further push the measuring speed to up to 5 kHz, thus opening a new area of high-speed LIBS microanalysis. Novel LIBS devices for various industrial applications presented include analysis of metallic process control samples with scale layers, on-site analysis of slag samples in secondary metallurgy, high-speed identification of Al scrap, mix-up detection of pipe fittings as well as recent work towards in-process identification of hot coils in a rolling mill.