Powerchip lasers provide short, reproducible ablation pulses at rates in excess of a kilohertz, but the pulse to pulse jitter resulting from their passive Q-switches has prevented previous intra-pulse gated measurements. We have performed time-resolved measurements of the analytical figures of merit for the powerchip LIBS determination of copper in aluminum under air, argon and helium atmospheres. Minimum limits of detection were measured 25–125 ns after ablation and were as much as 15× better than the ungated values. These optimal delays would not require an optical delay line in most cases as a result of the short insertion delay of modern ICCDs. Argon cover gas was found to improve LODs slightly. Using gated detection, 1064 nm laser ablation and argon cover gas, we calculate a LOD as low as 0.65 ppm of copper.

The Radon transform tomography is used for reconstruction of the emissivity distribution in single- (SP) and double-pulse (DP) laser induced plasmas in orthogonal geometry. The orthogonal DP plasma is intrinsically asymmetric and thus suitable for the Radon reconstruction. The DP plasma consists of two plasmas separated by a short time interval of ~1μs. The first plasma is created in air near the surface of a Si wafer and is followed (pre-ablation mode) or preceded (post-ablation mode) by the second plasma induced on this surface. A spectrometer moves in a semi-circular path around the plasma keeping the plasma in the rotation center. The optical detection is arranged so that a thin plasma layer parallel to the target surface is monitored. The axial symmetry of the SP plasma is investigated by comparing data from the Abel inversion taken at different angles and Radon reconstruction. The multi-angle measurements are used to estimate errors of the Abel reconstruction due to asymmetries of the plasma. Time-resolved Radon reconstruction in white light is performed for the DP plasma in both pre- and post-ablation modes. In the former case, the effect of ablated aerosol on the formation of the air plasma is monitored. In the latter case, a formation of an asymmetric compression shock created by the target plasma inside the air plasma is visualized. This observation is supported by computer simulations. An interaction of the two plasmas is studied by spectrally resolved Radon reconstruction revealing a complex distribution of target and ambient species inside the plasma at all studied delay times. Overall, it is demonstrated that Radon-based tomography is an informative tool to study transient asymmetric laser induced plasmas.

Multi-pulse laser-induced breakdown spectroscopy (LIBS) in the collinear pulse configuration with time-integrating detection was performed on metallic samples in ambient air in an effort to clarify the contributing processes responsible for the signal enhancement observed in comparison with single-pulse excitation. Complementary experiments were also carried out on another LIBS setup using detection by an imaging spectrograph with high time resolution. The effects of laser bursts consisting of up to seven ns-range pulses from Nd-doped solid-state lasers operating at their fundamental wavelength and separated by 8.5–50 μs time gaps was studied. The ablation and emission characteristics of the generated plasmas were investigated using light profilometry, microscopy, plasma imaging, emission distribution mapping, time-resolved line emission monitoring, and plasma temperature calculations. The experimental data suggest that the two contributing processes mainly responsible for the signal enhancement effect are the plume reheating caused by the sequential laser pulses and, more dominantly, the increased material ablation attributed to the lower breakdown threshold for the preheated (molten) sample surface and/or the reduced background gas pressure behind the shockwave of preceding pulses.

This paper describes the experimental realization and characterization of a versatile single particle detection apparatus. The system utilizes a novel particle beam inlet that can serve as either an on-line particle concentrator (i.e., all diameters confined in a narrow beam) or as a segregator (i.e., selected diameters confined in a narrow beam) and can be operated in a high-speed mode as well as in a low-speed mode, thus allowing different interaction times between the particles and the laser beam. An aerodynamic sizing technique has been incorporated into the system to provide rapid, real-time, and high-resolution sizing. Parameters such as transmission efficiency and size-segregation efficiency have been measured. The performance of the instrument has been demonstrated by on-line detection of spectrally resolved and time resolved fluorescence detection from airborne dye-doped particles and aerosolized endogenous fluorophores found in biological agents.

Materials analysis and characterization can provide important information as evidence in legal proceedings. The potential of laser induced breakdown spectroscopy (LIBS) for the discrimination of glass fragments for forensic applications is presented here. The proposed method is based on the fact that glass materials can be characterized by their unique spectral fingerprint. Taking advantage of the multielement detection capability and minimal to no sample preparation of LIBS, we compared glass spectra from car windows using linear and rank correlation methods. Linear correlation combined with the use of a spectral mask, which eliminates some high-intensity emission lines from the major elements present in glass, provides effective identification and discrimination at a 95% confidence level.

A thallium see-through hollow cathode lamp, or galvatron, was studied to evaluate its potential as a narrow band atomic line filter. Time-resolved laser-induced saturated fluorescence was used to evaluate the ground state number density of this glow discharge as a function of current. It was found to produce a sufficient number density at 16.0mA to absorb 99.9% of incident light from a line source based on an absorption curve-of-growth calculation. A saturation curve was experimentally obtained and modeled with a time-dependent two-level model, as well as a time-dependent three-level model in the presence of a trap. The three-level model showed excellent agreement with the experimental data when a 10ns pulse duration was used and when collisional rate constants were set to zero. The quantum efficiency of the system was found to be limited only by the spontaneous transition probabilities. Evaluation of these two parameters has shown that a thallium galvatron is an attractive atom reservoir for the applications as a narrow band atomic line filter.

The lifetimes of several states in a thallium see-through hollow cathode discharge, or galvatron, are obtained to characterize its potential as an atomic line filter. The lifetimes of the thallium 6(2)D(3/2), 6(2)D(5/2), and 7(2)S(1/2) states are measured by time-resolved single-step laser-excited fluorescence by use of a 276.787 nm laser pulse or a 535.046 nm laser pulse and measuring the resulting fluorescence waveform at the appropriate wavelength. Values of 6.4 +/- 0.1, 7.5 +/- 1.1, and 7.7 +/- 0.2 ns were obtained for the 6(2)D(3/2), 6(2)D(5/2), and 7(2)S(1/2) states, respectively, which agree with values obtained by previous authors, as well as calculated values. No current dependence was observed for each of these states. The lifetime of the long-lived thallium 6(2)P(3/2) degrees metastable state was measured by two-step laser-excited fluorescence at various applied currents. The metastable level was pumped by a 276.787 nm laser pulse, and a temporally delayed 535.046 nm laser pulse interrogated the population of the metastable state. Relating the fluorescence intensity to the population of the metastable state as a function of delay time yielded a decay curve for the 6(2)P(3/2) degrees metastable state. Values of 2.1 +/- 0.2, 2.8 +/- 0.1, 3.1 +/- 0.3, 3.8 +/- 0.4, and 4.8 +/- 0.6 micros were found for applied currents of 14.0, 12.0, 10.0, 8.0, and 6.0 mA, respectively. The resulting lifetimes for the 6(2)P(3/2) degrees metastable state clearly show a dependence on the applied current and are expected to be due to collisions with the wall of the cathode, as well as a contribution due to collisions with electrons.

A thallium see-through hollow cathode lamp, or galvatron, is investigated for applications as a narrow band atomic line filter. In order to determine the resolving power of this atom reservoir, high resolution emission profile measurements of the 535.046 nm line were obtained by a scanning Fabry–Pérot interferometer coupled to a spectrometer. The resulting spectra are modeled by a two-layer model which takes into account line broadening due to self-absorption and allows the calculation of the Doppler temperature and optical depths of the two layers. The results obtained for the galvatron are compared to a traditional thallium hollow cathode lamp and a thallium electrodeless discharge lamp. The galvatron was found to produce Doppler temperatures from 495 K to 630 K from currents of 10.0 mA to 30.0 mA, respectively, which were lower than the values found from the traditional hollow cathode lamp at similar currents. These results make this atomic reservoir attractive for applications as an atomic line filter.

Silver colloids have been commonly used as substrates for surface enhanced Raman spectroscopy (SERS). It has been shown that SERS requires partial aggregation of the silver colloids. This study evaluates factors affecting the aggregative state of the silver colloids such as the age of the silver colloids and the aggregation as a result of addition of the analyte. The silver colloids are obtained from the chemical reduction of silver nitrate by sodium borohydride. Further oxidation of the sodium borohydride solution at room temperature results in concentration changes of the resulting silver colloids. Methods of controlling the sodium borohydride depletion are presented in this paper. The analyte used is dipicolinic acid, a molecular signature of Bacillus spores.

A see-through hollow cathode lamp, or galvatron, is investigated. A novel method is presented for the measurement of an atomic absorption profile using a quasi-continuum source created by the combination of two line sources and a high-resolution Fabry–Pérot interferometer coupled to a spectrometer. Number densities are calculated from the resulting absorption profiles by the peak absorption coefficient relationship and compare well with results obtained from high-resolution emission measurements. Number densities are also determined for the lead 3 P 1 metastable state and thallium 2 P 1/2 o ground state by conventional atomic absorption. A hollow cathode lamp is used as an emission source and is set at a relatively low current to approximate as a line source relative to the galvatron. Due to the relative line widths of the source and absorber, only the lead metastable state results compare to results obtained by saturated fluorescence.

The dynamics of the radiative plasma expansion into an ambient gas is considered. The model describes the evolution of the plasma emission spectrum and the dynamics of the resulting shock wave. The time frame for the applicability of the model is in the tens of nanoseconds after the laser pulse is terminated, until a few microseconds later when the plasma ceases to emit. It is assumed that local thermodynamic equilibrium is established and that the plume expands with spherical symmetry. The model outputs are spatial and temporal distributions of atoms, ions, and electron number densities, evolution of atom and ion line profiles, and the shock wave. The model should be applicable to spectroscopic analysis of the initial plasma state and plasma dynamics.

Metal ablation with a short pulse, low energy microchip laser was investigated with respect to its application to laser induced breakdown spectroscopy (LIBS). Target surface modi. cations and crater parameters as a function of laser pulse properties were studied. The effect of the laser pulse is limited to the focal spot, but surface modi. cation by the laser-induced plasma can extend several micrometers beyond the focal spot depending on the target's thermal properties. Mass removal per shot was found to depend upon the heat of fusion of the target, while appreciable plasma emission was observed only at high pulse energies. Plasma composition and emission intensity can change significantly with the surface properties, requiring a fresh,. at surface to be exposed to each laser pulse. Increasing the temperature of the target resulted in a corresponding increase in plasma emission due to an increased mass removal per laser shot: however, selective ablation was not observed at temperatures up to 550 degrees C. Fractionation was observed at low laser irradiances and inside deep craters, but it was minimal compared with the results reported for other laser ablation systems. Characteristics such as precision in the mass removal process, well-defined crater parameters, and good spatial resolution make the Powerchip laser an attractive laser sampling tool.

A radiation dynamic model of the postbreakdown stage of laser-induced plasma solves a twofold task: first, the direct problem, it yields an analytical expression for the plasma radiation dynamics under arbitrarily chosen initial conditions allowing the computation of synthetic spectra; second, the inverse problem, it allows finding of the initial conditions by a direct comparison of calculated synthetic spectra with experimentally measured ones. In this work, we carry out experimental verification of the model, thus dealing with the inverse problem. We vary the initial parameters of the model (plasma initial temperature and the initial concentrations of species) until a close fit between the synthetic and the experimental spectrum is obtained. Some of the model inputs (e.g., the initial radius of the plasma) are measured and introduced into the model as fixed constants. Calculations and measurements are performed on a binary SiC system; on a series of multicomponent aluminum samples doped with Si, Mg, Cu, Zn, and Fe; and on pure iron, silicon, and carbon. From two to six elements and up to 500 spectral lines were involved in the calculations. The Monte Carlo optimization (the simulated annealing method) is used for finding initial plasma temperature and number densities. A reasonably good agreement is obtained between the computed and the experimental spectra. This approach can be considered as a valuable step towards the achievement of absolute analysis.

The development of a compact laser induced breakdown spectroscopy (LIBS) system increases the possibilities of applying the technique in industrial arenas, field applications and process monitoring. Significant progress has been achieved in miniaturization of optical detectors and lasers, allowing portable, low-cost LIBS equipment to be devised. Conventional lasers for LIBS, like actively Q-switched Nd:YAG lasers are limited by their bulkiness, the need for a cooling system and high power consumption. The use of a miniature solid state microchip laser overcomes these drawbacks and offers further advantages of good beam quality, high pulse repetition frequency and less damage to target. In this work we studied the quantification of elemental composition of low alloy steel samples using a higher power microchip (“powerchip”) laser. The possibility of real time, in situ quantification of such materials by powerchip LIBS enhances the applicability of the technique to process monitoring in the steelmaking industry. The performance of the LIBS technique based on a powerchip laser and a portable non-intensified, non-gated detector for elemental quantification is evaluated and compared to that obtained using an intensified detector. Calibrations were achieved for Cr, Mo, Ni, Mn and Si with linear regression coefficients between 0.98–0.99 and limits of detection below 100 ppm in most cases.

A commercial, 7 μJ/pulse, 550 ps microchip laser is used to induce plasma on Pb, Si, Cu, Fe, Ni, Ti, Zn, Ta, and Mo foils and a Si wafer. The measured plasma lifetime is comparable with the duration of the laser pulse (a few ns). The plasma continuum radiation is low, while some of the strong resonance lines (e.g., Zn 213.86 nm) show self-reversal. Quantitative analysis is possible using non-gated detectors but analytical lines should be chosen with care to avoid reduction in the linear dynamic range. The mass removed (0.5–20 ng/pulse) is sufficient to yield spectra that are detectable with portable grating spectrometers equipped with non-gated, non-intensified detector arrays. The spectrum of Cd is detected with a broadband portable spectrometer (200–950 nm). The combination of the broadband spectrometer and the microchip laser is very promising for material identification, especially in field applications.

A two-grating high-resolution spectrometer for dual wavelength imaging is demonstrated based on the standard Czerny-Turner mounting with an auxiliary grating and a mirror. A two-dimensional charge-coupled device (CCD) detector in the spectrometer focal plane allows simultaneous detection of two spectral intervals. Each spectrometer grating is driven by a high-precision stepper motor interfaced to a computer via home-made software. The software allows fast tuning of the gratings to a desirable spectral interval anywhere between 200 nm and 800 nm. The spectral interval widths are 2-3 nm for a ''high-resolution'' (2400 grooves/mm) grating and 4-5 nm for a ''low-resolution'' (1200 grooves/mm) grating. The resolution varies between 0.01 nm and 0.02 nm depending on the grating used. The performance of the spectrometer is demonstrated by detecting spectrally resolved images from a back-illuminated template and from a laser-induced plasma. The spectrometer can be useful for two-line spectroscopic diagnostics or can be expanded for multi-element spectral analysis.

Resonance ionization imaging detectors (RIIDs) are photon detection devices based on narrow band resonance absorption of laser radiation (scattered by the object investigated) by an atomic vapor followed by ionization and measurement of the resulting ion or electron signal. The figure of merit of these detectors as well as their potential applications have been extensively discussed.1 An RIID based on Hg vapor was first described by Matveev et al.2 and subsequently optimized by Pappas et al.3 Recently, another RIID using Cs as an active element was successfully demonstrated and its operational characteristics were evaluated.4 The present note, as a continuation of the RIID development, describes a detector with two active elements (Cs and Hg), which exhibits dual-wavelength detection capability. A unique excitation/ionization scheme, based on ultraviolet (254 nm) and near-infrared (852 nm) resonance excitation, was devised and is discussed here. Simultaneous imaging at the two corresponding resonance wavelengths, coupled with time-resolved ionization measurements, is shown to be possible with this scheme.

The recently developed mercury resonance ionization imaging detector (RIID) has many potential applications in the field of imaging science. We have demonstrated that useful information can be obtained from the time-resolved ionization signal detected along with the image of the object. Clearly distinguishable time-resolved signals from resonance ionization of mercury atoms and photoelectrons created within the channels of a microchannel plate by a UV signal transition of Hg at 253.7 nm were observed. Also, a new source of noise has been identified as low-mass ion desorption by 253.7 nm radiation from the inner parts of the Hg RIID. The time-resolved signal detection allowed temporal correction for the additional noises caused by nonresonant ionization processes inside the RIID, such as photoelectric effect and low-mass ion desorption. The temporal resolution of the RIID could be used for frequency shifted radiation detection and imaging.

A radiation dynamic model is developed for a post-breakdown stage of a laser induced plasma expanding into vacuum. The model describes the plasma formed on a small solid particle, which is completely vaporized by a laser. The symmetry of the expanding plasma is spherical. The time frame for the applicability of the model is somewhat between a hundreds of nanoseconds, after the laser pulse is terminated, and a few microseconds, when the plasma ceases to emit. The model is based on a system of gas dynamic equations coupled with the equation of radiative transfer. Local thermodynamic equilibrium is assumed, allowing the application of the collision-dominated plasma model as well as standard statistical distributions. Calculations are performed for a dual SiC system, although calculations for any arbitrary number of system's components are permitted. The model has two implications. First, an analytical expression for the plasma radiation dynamics is obtained by artificially setting the initial conditions. Second, from experimentally measured plasma parameters, information is deduced about the initial state of the plasma. The main model input parameters are the total number and distribution of plasma species and the initial distribution of temperature. Some of the other model inputs, such as the speed of the plasma front and the temperature profile across the plasma can be directly measured, thus providing valuable experimental feedback to the model. The model outputs are the evolution of plasma temperature, the spatial and temporal distributions of atoms, ions and electron number densities and the evolution of the plasma spectrum in a desirable spectral window (e.g. 280–290 nm for the chosen in this work SiC system).

A method of temperature measurement based on the model developed by Bartels of an optically thick inhomogeneous plasma was applied to a laser plasma induced on a target containing barium. The method involves the intensity ratio measurement of two self-reversed Ba(II) lines. The temperature thus determined corresponds to the maximum temperature in the plasma center. The plasma temperature was measured for delay times between 0.5 micros and 10 micros in two spectrometer operating modes: the scanning mode and the dual-wavelength mode, the latter resulting in better precision. A detailed analysis of experimental errors was performed. The error strongly depended on the wavelength separation of the lines used. The most accurate results were obtained for the largest line separation. Using one line in the UV and the other in the visible region, the relative error was 2-6% for temperatures between 8000 K and 20 000 K. The distribution of the plasma temperature along the plasma height was measured in the same delay time range. The temperature was found to be uniform along the plasma vertical axis, thus confirming the plasma cylindrical symmetry.