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.     

Plasma-cavity ringdown spectroscopy for analytical measurement: Progress and prospectives

Plasma-cavity ringdown spectroscopy is a powerful absorption technique for analytical measurement. It combines the inherent advantages of high sensitivity, absolute measurement, and relative insensitivity to light source intensity fluctuations of the cavity ringdown technique with use of plasma as an atomization/ionization source. In this review, we briefly describe the background and principles of plasma-cavity ringdown spectroscopy(CRDS) technology, the instrumental components, and various applications. The significant developments of the plasma sources, lasers, and cavity optics are illustrated. Analytical applications of plasma-CRDS for elemental detection and isotopic measurement in atomic spectrometry are outlined in this review. Plasma-CRDS is shown to have a promising future for various analytical applications, while some further efforts are still needed in fields such as cavity design, plasma source design, instrumental improvement and integration, as well as potential applications in radical and molecular measurements.     

Validation of the extended simultaneous kinetics and ringdown model by measurements of the reaction NH2 + NO

The determination of rate constants for fast chemical reactions from nonexponential cavity ringdown profiles requires a consideration of the interfering laser bandwidth effect that arises if the line width of the ringdown probe laser exceeds the absorption line width of the detected species. The deconvolution of the kinetics and the bandwidth effect can be accomplished with the extended simultaneous kinetics and ringdown (eSKaR) model presented by Guo et al. (Guo, et al. Phys. Chem. Chem. Phys. 2003, 5, 4622). We present a detailed validation of this eSKaR model by a corresponding investigation of the well-known rate constant for the reaction NH2 + NO. Line profiles of the pulsed ringdown probe laser and the NH2 absorption line were determined from forward modeling of experimental ringdown profiles and verified by narrow-bandwidth laser absorption measurements. In addition, the rate constant for the title reaction was evaluated using the eSKaR model and also by means of a conventional pump-probe approach with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses. The resulting room temperature rate constant for the NH2 + NO reaction, k1= (8.5 +/- 1.0) x 10(12) cm(3) mol(-1) s(-1), and the room temperature pressure broadening coefficient of NH2, = 2.27 GHz/bar, measured on the A2A1<-- X2B1 transition at wavelengths around lambda = 597 nm, were found to be in excellent agreement with the available literature data.     

Nonlinear effects in pulsed cavity ringdown spectroscopy of lithium vapour

We report cavity ringdown spectra of lithium vapour generated in the heat-pipe oven. Evaluation of ringdown decay curves in early and late time windows gives the change of absorption peak values as well as in the line shape profiles. Pronounced dips in the line center occur depending on molecular densities, injected laser pulse energies and chosen time window.     

利用激光腔内共振衰减技术研究CH2 CHO自由基的近紫外吸收光谱

Near-ultraviolet absorption spectrum of the B 2A" 0 ← X 2A" band of the vinoxy radical（CH2CHO）is recorded by cavity ringdown spectroscopy（CRDS）. The absorption spectrum shows a series of vibronic bands starting from 28786 cm - 1 and an increasing broad background towards higher photon energy. The CRDS absorption spectrum is similar to an early low-resolution absorption spectrum;and the vibronic peak positions match well with those in the laser-induced fluorescence and photofragment yield spectra.     

Radiative models of laser-induced plasma and pump-probe diagnostics relevant to laser-induced breakdown spectroscopy

The paper describes past and present efforts in modeling of laser-induced plasma and overviews plasma diagnostics carried out by pump-probe techniques. Besides general information on existing plasma models, the emphasis is given to models relevant to spectrochemical analysis, i.e. models of radiating plasma. Special attention is paid to collisional-radiative (CR) and collisional-dominated (CD) plasma models where radiative processes play an important role. Also, calibration-free (CF) models are considered which may endow with the possibility for standardless spectroscopic analysis. In the diagnostic part, only methods based on the use of additional diagnostic tools (auxiliary lasers, optics, and probes) are described omitting those based on plasma own radiation. A short review is provided on image-based diagnostics (shadowgraphy, schlieren, and interferometry), absorption and fluorescence, Langmuir probe, and less frequently used cavity ringdown and Thomson scattering methods.     

The hollow cathode plume: A plasma emission source for solids

A hollow cathode discharge serves to form an energetic plasma plume which exhibits intense emission characteristics. The sample cathode is sputtered in the discharge to produce an atomic population which then flows into the plume through an exit orifice for subsequent excitation. Atomic absorption and emission profiles in the plume are shown. Emission spectra of major and minor elements show strong atomic and ionic lines of sputtered species, particularly from the inner core of the plume. A sample in the form of a small disc can serve as the base of the hollow cathode for more convenient sample preparation. The source is believed to have potential also in elemental analysis by mass spectrometry.     

Effect of laser parameters on laser ablation and laser-induced plasma formation: A numerical modeling investigation

A comprehensive numerical model has recently been developed for nanosecond (ns) laser ablation of metallic targets, describing the processes of target heating, melting and vaporization, the resulting plume expansion in 1 atm helium gas, as well as plasma formation in the plume. In the present paper, we investigate the influence of laser parameters, i.e., laser irradiance, pulse duration and wavelength, on typical calculation results, such as the target temperature, melt and evaporation characteristics, the plume expansion velocity, plume (plasma) temperature and ionization degree, densities of neutrals, ions and electrons in the plume, as well as the laser absorption characteristics in the plume (plasma shielding). Comparison is made with experimental data from literature, whenever available, and in general, good agreement is reached between our model predictions and experimental results. Therefore, the model can be useful to predict trends in target and plume (plasma) characteristics, which are difficult to obtain experimentally.     

Absorption and scattering characterization of airborne microparticulates by a cavity ringdown technique

Nonresonant cavity ringdown laser absorption spectroscopy (CRLAS) was applied for detection and characterization of airborne particulates. Sensitive detection of a variety of aerosols under ambient conditions was achieved. The method provides, for the first time, time-resolved absolute aerosol concentration, with spatial resolution (along a line). The first report on absorption spectroscopy of monodispersed aerosols (in the size range 100–200 nm) is provided, and comparisons are made with the bulk data. The results indicate the possibility of applying CRLAS for selective analysis of aerosols. A new method for estimating the aerosol refraction index is also obtained from the ringdown data.     

Laser induced breakdown spectroscopy on metallic alloys: Solving inhomogeneous optically thick plasmas

In this work, laser-induced breakdown spectroscopy (LIBS) has been applied to the characterization of a plasma generated on a ternary Co–Cr–Mo alloy commonly used on hip prosthesis in air at atmospheric pressure. A method to achieve analytical results without employing any reference sample was implemented within a two-region plasma picture of a hot dense core surrounded by a colder periphery, where both self-absorption and inhomogeneity effects were taken into account. High resolution spectra of three strong Co I–II lines from different regions of the plasma plume were recorded and the analysis was carried out by means of a least-squares calibration-free algorithm. In this approach, theoretical spectra were matched to the experimental line profiles. Thus, the plasma parameters (temperature, atom, ion and electron densities) and the line widths were obtained, demonstrating the feasibility of the method to characterize the physical state of a laser-induced plasma.     

Two-dimensional observation of emission spectra excited from laser induced plasmas and the application to emission spectrometric analysis.

The spatial distribution analysis of emission signals from a laser-induced plasma can provide information on the excitation mechanism as well as on the optimization of the analytical conditions when it is employed as a sampling and excitation source in optical emission spectrometry. A two-dimensionally imaging spectrometer system was employed to measure spatial variations in the emission intensities of a copper sample and plasma gases when krypton, argon, or helium was employed under various pressure conditions. The emission image of the Cu I 324.75-nm line consists of a breakdown spot and a plasma plume, where the breakdown zone expands toward the surrounding gas. The shape and the intensities of the plasma plume are strongly dependent on the kind and pressure of the plasma gas, while those of the breakdown zone are less influenced by these experimental parameters. This effect can be explained by the difference in the cross-section of collisions between krypton, argon, and helium. The signal-to-background ratio of the Cu I 324.75-nm line was estimated over two-dimensional images to determine the optimum position for analytical applications.     

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.     

Time-resolved spectroscopy and imaging diagnostics of single pulse and collinear double pulse laser induced plasma from a glass sample

The characterization of laser-induced plasma from a glass sample was performed in the single- and double-pulse excitation regimes. The detailed information about density distributions of excited atoms and ions in the expanding plasma was obtained by using the imaging detection system providing measurements of the spatial, temporal, and spectral plasma emission characteristics. The expansion dynamics was shown to differ strongly between two excitation regimes. The enhancement factors of the line emissions in the double-pulse mode were found to be spatial dependent and to differ for the different elements in the plasma plume. The obtained results are useful for a better understanding of the main physical processes leading to the analytical improvement achieved by the use of double-pulse laser-induced breakdown spectroscopy (LIBS).     

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.     

Material removal and chemical and structural changes induced by irradiation of polymer surfaces with KrF-excimer laser radiation

Holes in polypropylene (PP) and polymethylmethacrylate (PMMA) plates, 0.5 mm in thickness were drilled by irradiation with up to 3000 pulses of KrF-excimer laser radiation (λ = 248 nm) at fluences per pulse ? in the range 0.1–10 J/cm², conditions which yield a laser-induced plasma/vapor plume. The process was analyzed experimentally in terms of material removal rate, optical emission of the laser-induced plasma, hole geometry, debris production at the hole edge, and chemical changes in the polymer induced by the laser irradiation. Additionally, the process was simulated using a model based on degradation of the polymer induced by optical absorption and heating, leading to gas-phase products. Such characteristics as the material removal rate as a function of fluence, the nature of the gas phase products and the deposition of debris were calculated.     

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

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

Rubidium isotope measurements in solid samples by laser ablation-laser atomic absorption spectroscopy

Laser atomic absorption was used to measure the rubidium isotopes in a laser-induced plasma. An ⁸⁵Rb/⁸⁷Rb isotope ratio of 2.7±0.2 was determined in solid calcium carbonate samples. A Nd:YAG laser was used to produce the plasma on the surface of solid samples placed inside a low pressure chamber with a controlled atmosphere of 150 mtorr to 10 torr. The plasma conditions were optimized in order to provide the best sensitivity and resolution. A narrowband Ti:Sapphire laser was scanned across the 780.02-nm transition of the rubidium isotopes. The resolved isotope spectrum is shown, as well as the isotope selective calibration plots. A detection limit of 25 ppm for the individual isotopes was obtained. The optimization studies and the likely mechanisms of line broadening are discussed.     

Sampling statistics and considerations for single-shot analysis using laser-induced breakdown spectroscopy

A statistical analysis of single-shot spectral data is reported for laser-induced breakdown spectroscopy (LIBS). Fluctuations in both atomic emission and plasma continuum emission are investigated in concert for a homogenous gaseous flow, and fluctuations in plasma temperature are reported based on iron atomic emission in an aerosol-seeded flow. Threshold irradiance for plasma initiation and plasma absorption were investigated for pure gaseous and aerosol streams, with detailed statistical measurements performed as a function of pulse energy in the breakdown regime. The ratio of the analyte atomic emission intensity to the continuum emission intensity (peak/base) provided a robust signal for single-shot LIBS analysis. Moreover, at optimal temporal delay, the precision of the LIBS signal was maximized for pulse energies within the saturation regime with respect to plasma absorption of incident energy. Finally, single-shot temperature measurements were analyzed, leading to the conclusion that spatial variations in the plasma volume formation and subsequent plasma emission collection, play important roles in the overall shot-to-shot precision of the LIBS technique for gaseous and aerosol analysis.     

Some causes of bending of analytical curves in atomic absorption spectroscopy

Some factors affecting the shape of analytical curves in atomic absorption spectroscopy are considered and the influence of the emission and absorption line profiles is discussed in detail. An empirical equation expressing the analytical curves for different ratios of emission line width to absorption line width is given. The possible influence of resonance line broadening and resonance line shift in atomic absorption flame photometry is also discussed.