Diagnostics of silicon plasmas produced by visible nanosecond laser ablation

The second harmonic of a pulsed Nd:YAG laser (532 nm) has been used for the ablation of silicon samples in air at atmospheric pressure. In order to study the interaction for silicon targets, the laser-induced plasma characteristics were examined in detail with the use of a space- and time-resolved technique. Electron temperatures, ionic temperatures and electron number densities were determined. A discussion of thermodynamic equilibrium status of the silicon-microplasma is presented. Electron number densities are deduced from the Stark broadening of the line profiles of atomic silicon. Plasma ionization and excitation temperatures were determined from the Boltzmann plot and the Saha–Boltzmann equation, respectively. A limited number of suitable silicon lines for the studies of temperatures were found and the effect of these lines on the temperature measurements is discussed. Electron temperatures in the range of 6000–9000 K and ionic temperatures of 12 000–17 000 K with electron number densities of the order of 10¹⁸ cm⁻³ were observed. The breakdown threshold fluence has been also measured. Silicon plasmas were also characterized in terms of their morphology (shape and size) as a function of laser energy and delay time.     

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.     

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.     

Metal particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266 nm) was used to generate metal particles of Zn and Al alloys using femtosecond (150 fs) and nanosecond (4 ns) laser pulses with identical fluences of 50 J cm⁻². Characterization of particles and correlation with inductively coupled plasma mass spectrometer (ICP-MS) performance was investigated. Particles produced by nanosecond laser ablation were mainly primary particles with irregular shape and hard agglomerates (without internal voids). Particles produced by femtosecond laser ablation consisted of spherical primary particles and soft agglomerates formed from numerous small particles. Examination of the craters by white light interferometric microscopy showed that there is a rim of material surrounding the craters formed after nanosecond laser ablation. The determination of the crater volume by white light interferometric microscopy, considering the rim of material surrounding ablation craters, revealed that the volume ratio (fs/ns) of the craters on the selected samples was approximately 9 (Zn), 7 (NIST627 alloy) and 5 (NIST1711 alloy) times more ablated mass with femtosecond pulsed ablation compared to nanosecond pulsed ablation. In addition, an increase of Al concentration from 0 to 5% in Zn base alloys caused a large increase in the diameter of the particles, up to 65% while using nanosecond laser pulses. When the ablated particles were carried in argon into an ICP-MS, the Zn and Al signals intensities were greater by factors of ∼50 and ∼12 for fs versus ns ablation. Femtosecond pulsed ablation also reduced temporal fluctuations in the ⁶⁶Zn transient signal by a factor of 10 compared to nanosecond laser pulses.     

Glass particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266 nm) was used to generate glass particles from two sets of standard reference materials using femtosecond (150 fs) and nanosecond (4 ns) laser pulses with identical fluences of 50 J cm⁻². Scanning electron microscopy (SEM) images of the collected particles revealed that there are more and larger agglomerations of particles produced by nanosecond laser ablation.In contrast to the earlier findings for metal alloy samples, no correlation between the concentration of major elements and the median particle size was found. When the current data on glass were compared with the metal alloy data, there were clear differences in terms of particle size, crater depth, heat affected zone, and ICP-MS response. For example, glass particles were larger than metal alloy particles, the craters in glass were less deep than craters in metal alloys, and damage to the sample was less pronounced in glass compared to metal alloy samples. The femtosecond laser generated more intense ICP-MS signals compared to nanosecond laser ablation for both types of samples, although glass sample behavior was more similar between ns- and fs-laser ablation than for metal alloys.     

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.     

Laser ablation of polished and nanostructured titanium surfaces by nanosecond laser pulses

A comparison of the IR nanosecond laser ablation parameters for polished and nanostructured titanium samples has been performed. The titanium foil was mechanically polished and pres-structured by multiple 744-nm femtosecond laser pulses producing large surface spots covered by ripples with periods in range of 400–500 nm. In order to evaluate the influence of such nanoripples, the nanosecond laser ablation and laser plasma properties were compared for polished surface, surface with nanoripples parallel and orthogonal to the laser beam polarization. A substantial decrease of the nanosecond ablation threshold was observed for the nanostructured in contrast to polished surface was detected while no influence of the ripple orientation vs. beam polarization was revealed. The comparison of plasma spectra for the ablation cases demonstrated that intensity of basic atomic lines and plasma emission duration were 2–5 times larger for the polished sample while spectra evolution was faster for the nanostructured sample. Plasma temperature and electron density were slightly lower for nanostructured sample while laser beam polarization has no effect on plasma properties.     

Surface analysis using ultrashort laser pulses and time-of-flight mass spectrometry

An all-optical mass spectrometric system is presented which combines the advantages of picosecond-laser desorption and post-ionization with the high performance of a time-of-flight mass-spectrometer. System studies show directly that the ionization process is saturated at focal power densities of about 10¹²W/cm². The saturated ionization is used to quantificate the amount of desorbed particles. Typical desorption rates far below one single monolayer per laser pulse could been achieved.     

Spectroscopic and shadowgraphic analysis of laser induced plasmas in the orthogonal double pulse pre-ablation configuration

This work focuses on the study of the plumes obtained in the double pulse orthogonal Laser Induced Breakdown Spectroscopy (LIBS) in the pre-ablation configuration using both spectroscopic and shadowgraphic approaches. Single and double pulse LIBS experiments were carried out on a brass sample in air. Both the distance of the air plasma from the target surface and the interpulse delay were varied (respectively in the range 0.1–4.2 mm and up to 50 μs) revealing a significant variation of the plasma emission and of the plume-shock wave dynamical expansion in different cases. The intensity of both atomic and ionized zinc lines was measured in all the cases, allowing the calculation of the spatially averaged temperature and electron density and an estimation of the ablated mass. The line intensities and the thermodynamic parameters obtained by the spectroscopic measurements were discussed bearing in mind the dynamical expansion characteristics obtained from the shadowgraphic approach. All the data seem to be consistent with the model previously proposed for the double pulse collinear configuration where the line enhancement is mainly attributed to the ambient gas rarefaction produced by the first laser pulse, which causes a less effective shielding of the second laser pulse.     

Investigation of multi-layered silicate ceramics using laser ablation optical emission spectrometry,laser ablation inductively coupled plasma mass spectrometry,and electron microprobe analysis

The applicability of laser ablation (LA) inductively coupled plasma (ICP) spectrometry for assessing elemental distributions in layered ceramics was investigated and compared with electron probe microanalysis (EPMA). Ordinary glazed wall tiles were employed as model specimens due to their defined structure and composition. They were used for calibration in the analysis of ancient pottery. A qualitative depth profile was acquired by single-spot laser drilling perpendicular to coatings with a Nd:YAG (1064 nm) laser coupled with an ICP optical emission spectrometer (OES). The lower lateral resolution associated with the laser spot diameter of 1.0 mm led to smoothing of the depth profile due to the averaging of local irregularities. In addition, transverse line scans by ablation across the tile section using an ArF* (193 nm) laser coupled with an ICP mass spectrometer (MS) were performed. LA-ICP-OES depth profiles and LA-ICP-MS transverse scans were validated by EPMA section scans and 2D back-scattered electrons images. The LA-ICP-OES acquisition was less dependent on sample surface and layer irregularities, whereas the transverse line scan over the tile section with the small-spot beam offered insight into the micromorphology of the individual layer. The combined approach revealed the occurrence of individual mineral grains, micro-heterogeneities and the character of interfaces between layers.     

Multielement analysis of coal by ICP techniques using solution nebulization and laser ablation

The combination of inductively coupled plasma atomic emission spectrometry and high resolution inductively coupled plasma mass spectrometry for the determination of 70 elements in coal were studied. Four microwave-assisted digestion procedures with different dissolution mixtures (nitric and hydrofluoric acids, aqua regia and hydrogen peroxide), lithium metaborate fusion with and without previous sample ashing as well as direct sampling by laser ablation (LA) have been tested. Examples of spectral interferences are given and different correction procedures are discussed. Detection limits in the low ng g(-1) range were obtained for most of the elements investigated by using high-purity reagents and by taking special care to prevent sample contamination during preparation. The precision was assessed from replicate analysis (including sample preparation) of coal samples and was found to be, as average values far all elements, 4-5% RSD and 10-15% RSD for procedures including sample digestion and LA sampling, respectively. The accuracy of the overall analytical procedures was estimated by analysis of certified reference materials and of a coal sample obtained from the Interlab Trace round robin test. Among the dissolution mixtures tested, the combination of nitric and hydrofluoric acids with hydrogen peroxide provide the best agreement with certified, recommended, literature-compiled or consensus values, though fusion is necessary to obtain quantitative recoveries for Si, Cr, Hf, W, Zr, Y. In general, results obtained by LA fall within +/-20% of those obtained after digestion.     

Investigation of laser ablated metal and polymer plasmas in ambient gas using fast photography

We report the studies on interaction of laser ablated Teflon and aluminum plasmas in various ambient atmospheres. Intensified Charge Couple Device (ICCD) was used to image the laser ablated plume. The stratification of plasma at moderate laser intensities in the case of metal as well as polymer was observed. The possible cause of stratification is discussed. The reason for expansion and diffusion within the plasma and shock formation in the presence of ambient gas at different pressures is discussed based on hydrodynamic model.     

Measurements of sub-nanosecond lifetimes from biphotonic fluorescence produced by nanosecond laser pulses

When the absorption of light from nanosecond laser pulses gives rise to a short-lived biphotonic fluorescence the lifetime of the excited state can be derived from the dependence of the intensity of the fluorescence on the energy of the laser pulse. The results of investigations by Müller and Pflüger on cryptocyanine in methanol and glycerol are used to illustrate the principle of the method. An analysis of their data leads to a lifetime for the lowest singlet excited state of about 15 psec and 200 psec for methanol and glycerol, respectively.     

Characterization and surface-enhanced Raman spectral probing of silver hydrosols prepared by two-wavelength laser ablation and fragmentation

A four step Ag foil laser ablation-Ag nanoparticle fragmentation procedure in ultrapure water was carried out both under argon and in air. Pulses of a high power Nd/YAG laser were used for laser ablation (1064 nm) and for the three step Ag hydrosol treatment in the absence of Ag foil in the sequence 1064-532-1064 nm. Transmission electron microscopy (TEM) and surface plasmon (SP) extinction spectra provide evidence of Ag nanoparticle fragmentation in the second and third step of the procedure carried out under argon. While polydispersity of Ag hydrosol increases in the second step, both the polydispersity and the mean size of the nanoparticles are reduced in the third step. Qualitative and quantitative surface-enhanced Raman scattering (SERS)/surface-enhanced resonance Raman scattering (SERRS) spectral probing of systems with Ag hydrosols and the selected adsorbates at 514.5 nm excitation shows that Ag hydrosols obtained in the second step of the preparation procedure carried out in air are the most suitable substrates for SERS/SERRS experiments performed at this excitation wavelength.     

Hydrogenation of fragment cations produced by femtosecond laser ablation of boron nitride

Cations (positive ions) produced by laser ablation of boron nitride (BN) have been mass analyzed and the size-dependent hydrogenation reactivity is revealed for the first time. The main product cations determined by femtosecond laser ablation (fsLA) were a series of B(BN)(n)(+), with much lesser production of B(2)(BN)(k)(+) and N(BN)(m)O(+) series cations. Least-squares fitting of the relative yields of hydrogenated cations indicates that the yield of B(BN)(n)H(+) almost diminishes for n ≥ 5 and that of B(BN)(n)H(2)(+) increases as n increases. Based on the different n-dependence and electronic structures of B(BN)(n) and B(BN)(n)(+), B(BN)(n) is likely to be the precursor of B(BN)(n)H(+), and B(BN)(n)(+) that of B(BN)(n)H(2)(+). In contrast to fsLA, the production of H(+) by nanosecond laser ablation is not observed and the production of various cationic species makes it difficult to identify either the fragment species or their hydrogenated products. This observation highlights the significant efficiency of fsLA in producing H(+) (and presumably H) from the surface adsorbates.     

苯甲酸钠团簇离子的形成和结构

用串级飞行时间质谱(Tandem TOF)仪研究了激光蒸发方法产生的苯甲酸钠及裂解产物团簇离子的形成和可能的构型。通过对苯甲酸钠团簇离子的紫外光解的研究, 决定了反应通道及分支比, 并定性地讨论了其结构特征。     

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.     

UV laser induced proton-transfer of protein molecule in the gas phase produced by droplet-beam laser ablation

Droplet-beam laser-ablation mass-spectrometry was applied for a study of the UV-laser induced proton-transfer reaction of protonated lysozyme hydrated clusters in the gas phase. Protonated lysozyme hydrated clusters were produced by irradiation of an IR laser onto a droplet-beam of an aqueous solution of lysozyme and were subsequently irradiated by a UV laser. It is found that H⁺ and H₃O⁺ are produced through photodissociation of protonated lysozyme hydrated clusters. The mechanism of the proton-transfer reaction is discussed.     

Performance of laser ablation: quadrupole-based ICP-MS coupling for the analysis of single micrometric uranium particles

In this paper we describe the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) coupling to particle analysis, i.e., the determination of the isotopic composition of micrometric uranium particles. The performances of this analysis technique are compared with those of the two reference particle analysis techniques: secondary ion mass spectrometry (SIMS) and fission track-thermo-ionization mass spectrometry (FT-TIMS), based on the measurement of the isotopic ratios of ²³⁵U/²³⁸U in particles present in an inter-comparison particulate sample. The agreement of the results obtained using LA-ICP-MS with target values and with the results obtained using FT-TIMS and SIMS was good. Accuracy was equivalent to that of the other two techniques (±3 % deviation). However, relative experimental uncertainties present with LA-ICP-MS (7 %) were higher than those present with FT-TIMS (4.5 %) and SIMS (3 %). Furthermore, measurement yield of LA-ICP-MS coupling was close to that obtained with the same quadrupole ICP-MS for the measurement of a liquid sample (~10^?4), but lower than that obtained with FT-TIMS and SIMS, respectively, by a factor of 10 and 20, although the particles analyzed using LA-ICP-MS were most likely smaller (diameter ~0.6 μm, containing 4–7 fg of ²³⁵U). Nevertheless, thanks to the brevity of the signals obtained, the detection capacity for low isotopic concentrations by LA-ICP-MS coupling is equivalent to that of FT-TIMS, although it remains well below that of SIMS (×15). However, with more sensitive double focusing ICP-MS, performances equivalent to those achieved using SIMS could be obtained.