Intensity enhancement in cross-beam pulsed laser ablation using two orthogonal targets

In this work we investigated a novel configuration of the orthogonal geometry for double pulse laser ablation. In this arrangement, a laser is focused onto a target generating a highly directed plume; after that, an additional laser produces a second plasma onto another perpendicular target. In this way, the second plume is expanded through the first plume region. Ablation of carbon was carried out in vacuum (10^− 4 Pa) by two delayed lasers. The first pulse corresponds to a Nd:yttrium–aluminum–garnet (YAG) (1064 nm) and the second one to an excimer (248 nm) laser. Results show that plasma interactions produce different species emission enhancement depending on the delay between lasers, laser fluences and the spatial overlapping between plumes. Approximately an 100-fold increase in emission signal was measured as the observation distance grows.     

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 oxygen on laser-induced elemental fractionation in LA-ICP-MS analysis

Elemental fractionation poses serious difficulties in obtaining accurate concentration and isotope ratio data when using laser ablation sampling. One of the factors that control the extent of laser-induced elemental fractionation is the composition of sample carrier gas in the sample cell. This study demonstrates that the presence of small amounts of oxygen in the He carrier gas has a significant effect on elemental fractionation during the ablation of silicate (NIST 612 glass and zircon 91500) and sulphide (NiS fire assay) samples. The extent of elemental fractionation for a given amount of ablated material and concentration of oxygen in the He carrier gas was related to the volume of the plasma plume that forms above the sample surface. This indicates that an oxidation reaction takes place in the plasma plume. It has been reported that oxidation can affect the particle size distribution during laser sampling and hence change the extent of elemental fractionation. The purity of the carrier gas used in laser ablation-ICP-MS, as well as the amount of oxygen released from silicate and oxide samples during the ablation in "oxygen-free" ambient gas, is shown to contribute significantly to elemental fractionation.     

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.     

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.     

Emission spectroscopic monitoring of particle composition,size and transport in laser ablation inductively coupled plasma spectrometry

The spectroscopic line emissions of copper and zinc from atomized particles generated by orthogonal pre-pulse laser ablation of brass and transported from the ablation cell through tubes into an ICP have been simultaneously measured end-on with fast photomultipliers. It was shown that simultaneous line monitoring of major elements provides not only information on the aerosol transport in laser ablation ICP-spectrometry, but also on the ratios of small to single larger particles and their respective elemental compositions and, therefore, on possible elemental fractionation problems. Furthermore, the spectroscopic information can be easily exploited for proper adjustment of the laser fluence in order to minimize the production of large particles, to improve the transport efficiency and to reduce the noise of analytical signals in laser ablation ICP-spectrometry. The present particular experiment on orthogonal pre-pulse laser ablation of brass confirms the recent finding that such kind of double-pulse arrangement produces predominantly ultra-fine particles. Individual brass particles with diameters ≳ 250 nm could be analyzed. They showed large Zn depletions as expected. Finally, strong accumulations of aerosol particles were found in the ablation cell used even at low laser pulse frequencies.     

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.     

Infrared laser post-ionization of large biomolecules from an IR-MALD(I) plume

A two-infrared laser desorption/ionization method is described. A first laser, which was either an Er:YAG laser or an optical parametric oscillator (OPO), served for ablation/vaporization of small volumes of analyte/matrix sample at fluences below the ion detection threshold for direct matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). A second IR-laser, whose beam intersected the expanding ablation plume at a variable distance and time delay, was used to generate biomolecular ions out of the matrix-assisted laser desorption (MALD) plume. Either one of the two above lasers or an Er:YSGG laser was used for post-ionization. Glycerol was used as IR-MALDI matrix, and mass spectra of peptides, proteins, as well as nucleic acids, some of which in excess of 10(5) u in molecular weight, were recorded with a time-of-flight mass spectrometer. A mass spectrum of cytochrome c from a water ice matrix is also presented. The MALD plume expansion was investigated by varying the position of the post-ionization laser beam above the glycerol sample surface and its delay time relative to the desorption laser. Comparison between the OPO (pulse duration, tau(L) = 6 ns) and the Er:YAG laser (tau(L) approximately 120 ns) as primary excitation laser demonstrates a significant effect of the laser pulse duration on the MALD process.     

UV-laser ablation of ionic liquid matrices

Ionic liquid matrices are a new class of matrices used in MALDI mass spectrometry. The ablation process of several ionic liquid matrices was studied by determining the velocity distribution of ablated neutral matrix molecules. This was done by a postionization approach, where the neutrals were ionized in the ablation plume by a second laser pulse. It was found that a second, time-delayed ablation event occurs consisting completely of neutral molecules. To explain this, the reflected-shockwave model is used, which assumes that the shockwave emerging from the laser ablation is reflected at the sample holder surface. When the shockwave arrives at the sample surface it causes a second ablation.     

Analysis of aluminum alloys by resonance-enhanced laser-induced breakdown spectroscopy: How the beam profile of the ablation laser and the energy of the dye laser affect analytical performance

In resonance-enhanced laser-induced breakdown spectroscopy, the sample was ablated by a laser pulse and the expanding plume was photoresonantly rekindled by a dye laser pulse. By sampling aluminum alloys for Mg, Pb, Si, and Cu, we showed that for the ablation step, Gaussian beams gave 2 to 3× better signal-to-noise ratio (SNR) than non-uniform beams. For the rekindling step, if no further sample destruction was allowed, dye laser pulses that intercepted the plume transversely gave 6 to 12× higher SNR than the longitudinal case. By combining Gaussian beams and transverse rekindling, the mass limit-of-detection for Mg was about 100 amol while non-resonant analysis was 10× more destructive. Sub-monolayer of oxides grown on laser-cleaned aluminum surfaces was detected by monitoring the AlO emissions of rekindled plumes; without resonant enhancements, they were not detectable no matter how destructive was the analysis. Time resolved studies showed that the Gaussian beam produced less dispersed plumes and that a stronger dye laser beam directed transversely heated up a bigger plume mass without over-heating the plume core. The analyte emissions were sustained while the continuum background remained low.     

A comparative study of the enhancement of molecular emission in a spatially confined plume through optical emission spectroscopy and probe beam deflection measurements

The spatial confinement effects of shock wave on the expansion of a carbon plume induced by pulsed laser ablation of graphite in air and the enhancement of the plume emission were studied by optical emission spectroscopy and probe beam deflection measurements. A metal disk was set in the way of the ablation-generated shock wave to block and reflect the supersonically propagating shock wave. The reflected shock wave propagated backwards and confined the expanding plume. The optical emission of CN molecules was enhanced in contrast to the case without the block disk and the emission enhancement was dependent on the position of the disk. Based on the results of time-integrated and -resolved optical emission spectroscopy, and the time- and space-resolved probe beam deflection measurements, the processes occurring in the plume were discussed and the mechanisms responsible for the enhancement of molecular emission in the spatially confined plume were investigated.     

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.     

Comparison of two laser-induced breakdown spectroscopy techniques for total carbon measurement in soils

The potentials of two advanced laser-induced breakdown spectroscopy (LIBS) techniques which are used to determine the total carbon content in soils have been examined. The first one is the combination of a single-pulse laser ablation with spark excitation of plasma plume triggering the gap between electrodes close to the target surface. The second one is a more conventional double-pulse LIBS. In both modes the calibration graphs have a nonlinear trend in the actual range of carbon contents and present a good R² value (0.97). In the combined laser-spark approach, using low-cost and portable laser instrumentation is possible, as well as inducing a micro-damage on the target surface. Certain regularities in the spectral line intensities of soil nutritious elements have been detected and appear to be connected to the total carbon content and to the soil origin.     

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.     

Comment on “three-dimensional analysis of laser induced plasmas in single and double pulse configuration”

Laser induced breakdown spectroscopy (LIBS) is an effective technique for real-time chemical analysis of samples in the laboratory and in the field. The performance of LIBS can be significantly improved by replacing the conventional LIBS configuration from single pulse laser to double pulse laser ablation. Corsi et al. showed that by firing two lasers with microsecond order delay can increase LIBS sensitivity [M. Corsi, G. Cristoforetti, M. Giuffrida, M. Hidalgo, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni, C. Vallebona, Three-dimensional analysis of laser induced plasmas in single and double pulse configuration, Spectrochimica Acta, Part B 59 (2004) 723–735] [1]. By studying plume evolution, they attribute this enhancement to the faster plume expansion in double pulse laser ablation. Blast wave theory was used in Corsi's paper to explain the higher expansion speed observed in double pulse laser ablation. However, it is questionable whether the blast wave theorem applies in laser ablation where the shockwave is driven by a vapor plume of mass. We introduce an alternative way to explain the faster plume expansion during double pulse laser through a more general thermodynamic relation.     

Cavity ringdown spectroscopy of collinear dual-pulse laser plasmas in vacuum

The plasma plume induced by dual-pulse laser ablation of a titanium target in vacuum was analyzed by the technique of cavity ringdown spectroscopy (CRDS). Large Doppler-splitting of the absorption spectral lines was observed which is due to increase of the velocity components parallel to the optical axis and specific features of the CRDS measurements. Vertical velocity component, the particle number density and plasma volume also show increase compared to the single-pulse laser ablation. The forward convolution best fit of absorption lineshapes was used to extract parameters describing dual-pulse laser ablation plasma plume.     

Phenomenological studies on structure and elemental composition of nanosecond and femtosecond laser-generated aerosols with implications on laser ablation inductively coupled plasma mass spectrometry

In laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), the properties of laser-generated aerosols, such as size and composition, are crucial for matrix-independent quantification. In this study, the aerosol particle morphology and elemental composition generated by two state-of-the-art laser systems (ArF excimer nanosecond-UV laser and Ti:sapphire femtosecond-IR laser) were investigated by electron microscopic techniques. Electrostatic sampling of the aerosols directly onto transmission electron microscopy (TEM) grids allowed us to study the morphology and elemental composition of the aerosols using TEM and TEM–EDX (energy dispersive X-ray spectroscopy) analyses, respectively. The results of the electron microscopic studies were finally compared to the LA-ICPMS signals of the main matrix components. The investigations were carried out for non-conducting materials (glass and zircon), metallic samples (steel and brass) and semiconductors (sulfides). The studies confirm that ns-LA-generated aerosols dominantly consist of nanoparticle agglomerates while conducting samples additionally contain larger spherical particles (diameter typically 50 to 500 nm). In contrast to ns-laser ablation, fs-LA-generated aerosols consist of a mixture of spherical particles and nanoparticle agglomerates for all investigated samples. Surprisingly, the differences in elemental composition between nanoparticle agglomerates and spherical particles produced with fs-LA were much more pronounced than in the case of ns-LA, especially for zircon (Si/Zr fractionation) and brass (Cu/Zn fractionation). These observations indicate different ablation and particle formation mechanisms for ns- and fs-LA. The particle growth mechanism for ns-LA is most likely a gas-to-particle conversion followed by agglomeration and additional hydrodynamic sputtering for conducting samples. On the other hand, phase explosion is assumed to be responsible for the mixture of large spherical particles and nanoparticle agglomerates as found for fs-LA-generated aerosols. Based on these mechanisms, the overall temporal elemental fractionation effects in ns-LA-ICPMS seem to occur mainly during the ablation. This effect was not observed for fs-LA-ICPMS despite the element separation into different particle fractions, which, on the other hand, could induce severe ICP-induced fractionation.     

Analysis of Soil by Magnetic Field Assisted Calibration-Free Laser Induced Breakdown Spectroscopy (CF-LIBS) and Laser Ablation – Time-of-Flight Mass Spectrometry (LA-TOF-MS)

The qualitative and quantitative analysis of soil samples collected from Sialkot, Pakistan (which contains leather industrial plants), has been performed using laser-induced breakdown spectroscopy (LIBS) and laser ablation time of flight mass spectrometry (LA-TOF-MS). The focused beam of a Q-switched Nd: YAG laser (532?nm) was used to ablate the soil samples in air at atmospheric pressure. The optical emission spectra demonstrate the presence of the spectral lines of Si, Fe, Al, Ca, Ti, K, Cr, Mg, Na, Ba, and Li in all of the samples. The emission lines intensities, electron number densities, and excitation temperatures were significantly enhanced in the presence of an external 0.3 T magnetic field applied perpendicular to the plasma plume. A maximum enhancement factor of approximately 8 was observed in the emission intensity. The emergence of several additional lines has also been detected using the magnetic field-assisted LIBS approach. The elemental composition determined using calibration-free laser-induced breakdown spectroscopy (CF-LIBS), with and without magnetic field, reveals that the external magnetic field only adjusts the laser-generated plasma dynamics without affecting the quantitative analysis of the samples. Importantly, the toxic and heavy elements such as chromium and barium were detected and quantified in all of the soil samples by both of these techniques. The variations in the compositional analysis using CF-LIBS with and without the applied magnetic field and LA-TOF-MS were less than 10%.     

Diagnostic and analytical study of post ablation ionization of neutral atoms of major and minor constituents from a low alloy steel

Post ablation ionization (PAI) of neutral atoms from a low alloy steel has been investigated using non-resonant laser ionization in a time-of-flight mass spectrometer. By varying the delay between the ablation and ionization lasers, the velocity distributions of the Ti, V, Cr, Mn and Fe atoms have been determined simultaneously. These distributions have been recorded as a function of ablation laser fluence. The half-range Maxwell-Boltzmann velocity distribution has been used to fit the data and different characteristic temperatures have been determined for the various elements in the sample. The quantitative capability of this method for bulk and surface analysis has been evaluated by calculating the relative sensitivity factors (RSFs) for the various constituent elements. The RSFs for all of the elements are seen to be highly dependent on the delay between the ablating and ionizing lasers. This dependence was reduced by integrating the temporal dependent ion yield, leading to a significant improvement in the calculated RSF values. It was also found that the RSFs were not highly dependent on the power density of the ablation laser beam.     

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. Received: 25 January 1999 / Revised: 7 April 1999 / Accepted: 30 April 1999