The absolute atomized mass of the neutral and first-ion reservoirs of a laser-induced yttrium plasma were measured with high resolution atomic-absorption spectroscopy mapping at various delays and under different noble and oxidizing cover gases. The maximum spatial extent of the plasma was inversely dependent on the molar mass of the cover gas. Time-resolved masses measured under oxidizing atmospheres showed continuous net loss from even the first measured delay (2 mu s). Though their total masses were stable at early delays, plasmas under helium and argon atmospheres also showed net loss of atomized yttrium from an unknown mechanism beyond 4 mu s. The measured total atomized masses during the early stable period under the noble atmospheres were approximately equal (similar to 30 ng) but higher than the masses measured under oxidizing atmospheres.

Atomic absorption spectroscopy was used to measure the absolute vanadium and titanium masses present as neutral atoms in the plume formed by laser ablation of a titanium alloy sample. At delays between two and twenty microseconds, the absolute masses and temperatures were measured with time-resolved Boltzmann plots under pure helium and an 80:20 He/O-2 mixture. The Boltzmann plots, which include the ground term (in contrast to emission-based measurements), produced high-quality estimates of the plasma composition. Under helium, the metal vapors persisted beyond the longest similar to 20 mu s delays measured. The V:Ti stoichiometry was approximately as expected from the nominal sample composition at all times under He. However, under He/O-2, the atomic vapor disappeared entirely in under 10 is and the stoichiometry became more enriched in vanadium at increasing delays. This enrichment is interpreted in terms of the difference in first oxide bond energies of the two metals. There was evidence of loss of metal even under helium within the investigated 20 mu s window.

Absorption spectroscopy hyphenations of the laser-induced plasma have been attempted by a handful of groups since the early 1990s. Despite the measurement's relatively long history, there are few reviews and the details of the experiment remain somewhat opaque. Though the early experiments were directed at understanding laser induced deposition physics, more recent measurements have begun to bend absorption spectroscopy to plasma diagnostics with an eye to analytical chemistry applications of laser ablation. This review discusses the benefits and difficulties of the different experimental approaches in addition to analytical and plasma characterization applications. Because the laser-induced plasma is so different from the usual atomic reservoirs used for atomic absorption spectroscopy, the assumptions of the Beer-Lambert Law are also discussed. Finally, with several groups currently working in the field, I provide an outlook for the future of the measurements.

A tunable ArF laser is used for deep ultraviolet LIF of arsenic in LIPs.

A new technique, pseudocontinuum source atomic absorption spectroscopy, is used to map neutral atomic populations in a titanium laser-ablation plasma under helium cover. Measurements show good reproducibility and spatial resolution adequate to measure inhomogeneity in atmospherically-confined LIBS plasmas. 160 +/- 20 ng of material is imaged across four terms (one ground and three metastable) of the neutral atom. This mass exceeds the crater volume, indicating substantial redeposition within the crater from pulse to pulse. At five microseconds comparison of the ground and the lowest metastable terms indicates departure from local thermodynamic equilibrium conditions. The technique shows promise for understanding the evolution of these inhomogeneous plasmas, particularly the fate of ablated material.

A new dual-beam atomic absorption technique is applied to laser-induced plasmas. The technique uses an optical parametric oscillator pseudocontinuum, producing emission that is both wider than the absorption line profile, but narrow enough to allow the use of an echelle spectrograph without order sorting. The dual-beam-in space implementation makes the technique immune to nonspecific attenuation of the probe beam and the structure of the pseudocontinuum. The potential for plasma diagnostics is demonstrated with spatially and temporally resolved measurements of magnesium metastable and lithium ground state optical depths in a laser-induced plasma under reduced pressure conditions. The lithium measurements further demonstrate the technique's potential for isotope ratio measurements.

A 193 nm ArF excimer pulse reexcites arsenic atoms in a cooled laser ablation plasma, lowering detection limits over LIBS.

Dual channel emission imaging of m-nitrobenzoic acid and benzoic acid was performed in order to visualize the morphology of the CN violet band emission of a TNT analogue. The CN channel was corrected for continuum emission using a simultaneously imaged background channel. Simultaneous dual channel imaging alleviated problems with shot to shot variation in the plasma morphology due to the friable substrates and showed differences between plasmas formed on the two targets.

Detection of explosives, explosive precursors, or other threat agents presents a number of technological challenges for optical sensing methods. Certainly detecting trace levels of threat agents against a complex background is chief among these challenges; however, the related issues of multiple target distances (from standoff to proximity) and sampling time scales (from passive mines to rapid rate of march convoy protection) for different applications make it unlikely that a single technique will be ideal for all sensing situations. A number of methods for spanning the range of optical sensor technologies exist which, when integrated, could produce a fused sensor system possessing a high level of sensitivity to threat agents and a moderate standoff real-time capability appropriate for portal screening of personnel or vehicles. In this work, we focus on several promising, and potentially synergistic, laser-based methods for sensing threat agents. For each method, we have briefly outlined the technique and report on the current level of capability.

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.

Our measurements of micro-plasma following laser-induced optical breakdown of nitro compound explosive simulants, here 3-nitrobenzoic acid, show well-developed molecular spectra during the first several hundreds of nanoseconds. Analysis of recorded carbon spectra is accomplished using accurate line strengths for the diatomic molecular Swan system. Presence of hydrogen-beta allows us to infer electron density in the plasma evolution. Computational challenges include accounting for background variation and appropriate modeling of hydrogen embedded in molecular spectra. Recorded and computed spectra agree nicely for time delays on the order of 1.6 μs from optical breakdown when using a single temperature for local thermodynamic equilibrium plasma.

Detection of SWCNTs in complex matrices presents a unique challenge as common techniques lack spatial resolution and specificity. Near infrared fluorescence (NIRF) has emerged as a valuable tool for detecting and quantifying SWCNTs in environmental samples by exploiting their innate fluorescent properties. The objective of this study was to optimize NIRF-based imaging and quantitation methods for tracking and quantifying SWCNTs in an aquatic vertebrate model in conjunction with assessing toxicological end points. Fathead minnows (Pimephales promelas) were exposed by single gavage to SWCNTs and their distribution was tracked using a custom NIRF imaging system for 7 days. No overt toxicity was observed in any of the SWCNT treated fish; however, histopathology observations from gastrointestinal (GI) tissue revealed edema within the submucosa and altered mucous cell morphology. NIRF images showed strong SWCNT-derived fluorescence signals in whole fish and excised intestinal tissues. Fluorescence was not detected in other tissues examined, indicating that no appreciable intestinal absorption occurred. SWCNTs were quantified in intestinal tissues using a NIRF spectroscopic method revealing values that were consistent with the pattern of fluorescence observed with NIRF imaging. Results of this work demonstrate the utility of NIRF imaging as a valuable tool for examining uptake and distribution of SWCNTs in aquatic vertebrates.

Time-resolved emission experiments are reported in the fast-decaying transient plasma induced by a microchip laser on an aluminum target in three different cover gases, i.e., air, argon and helium. The laser operates at 532 nm, with a repetition frequency of 1 kHz and a pulse width of less than 0.5 ns. The overall persistence of plasma emission is of the order of 100 ns.We examine the existence of local thermodynamic equilibrium (LTE) by evaluating the temporal criteria required (in addition to the McWhirter criterion), as recommended by Cristoforetti et al. (Spectrochim. Acta Part B 65, 2010, 86-95). The temporal criteria examine the evolution of temperature and electron number density and compare their rate of change to the rate at which electron collisions can thermalize the change. These considerations are used to determine time windows in which LTE may be present. Our results suggest that calibration-free LIBS measurements with these lasers may be possible for some elements at early times, especially under argon. (c) 2013 Elsevier B.V. All rights reserved.

The spatial and temporal evolution of the CN molecular emission following laser ablation of a TNT analog (3- nitrobenzoic acid) has been studied along with ablation of targets that contain neither nitro groups nor C-N bonds. At a fluence of ~104 J/cm2, behavior indicative of the ablation of native CN bonds has been observed in samples containing no native CN bonds. The recorded data show significant plasma background emissions that pose difficulties for direct spectral imaging. Spatially resolved images suggest that some of the observed phenomena are simply the result of the interaction of the plasma and the observation volume of the collection optics.

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.