Prasoon K. Diwakar

Experimental and computational study of complex shockwave dynamics in laser ablation plumes in argon atmosphere

We investigated spatio-temporal evolution of ns laser ablation plumes at atmospheric pressure, a favored condition for laser-induced breakdown spectroscopy and laser-ablation inductively coupled plasma mass-spectrometry. The 1064 nm, 6 ns pulses from a Nd:YAG laser were focused on to an Al target and the generated plasma was allowed to expand in 1 atm Ar. The hydrodynamic expansion features were studied using focused shadowgraphy and gated 2 ns self-emission visible imaging. Shadowgram images showed material ejection and generation of shock fronts. A secondary shock is observed behind the primary shock during the time window of 100-500 ns with instabilities near the laser cone angle. By comparing the self-emission images obtained using fast photography, it is concluded that the secondary shocks observed in the shadowgraphy were generated by fast moving target material. The plume front estimates using fast photography exhibited reasonable agreement with data obtained from shadowgraphy at early times ≤400 n...

Dynamics of femto- and nanosecond laser ablation plumes investigated using optical emission spectroscopy

We investigated the spatial and temporal evolution of temperature and electron density associated with femto- and nanosecond laser-produced plasmas (LPP) from brass under similar laser fluence conditions. For producing plasmas, brass targets were ablated in vacuum employing pulses either from a Ti:Sapphire ultrafast laser (40 fs, 800 nm) or from a Nd:YAG laser (6 ns, 1064 nm). Optical emission spectroscopy is used to infer the density and temperature of the plasmas. The electron density (ne) was estimated using Stark broadened profiles of isolated lines while the excitation temperature (Texc) was estimated using the Boltzmann plot method. At similar fluence levels, continuum and ion emission are dominant in ns LPP at early times (<50 ns) followed by atomic emission, while the fs LPP provided an atomic plume throughout its visible emission lifetime. Though both ns and fs laser-plasmas showed similar temperatures (∼1 eV), the fs LPP is found to be significantly denser at shorter distances from the target su...

The role of laser wavelength on plasma generation and expansion of ablation plumes in air

We investigated the role of excitation laser wavelength on plasma generation and the expansion and confinement of ablation plumes at early times (0–500 ns) in the presence of atmospheric pressure. Fundamental, second, and fourth harmonic radiation from Nd:YAG laser was focused on Al target to produce plasma. Shadowgraphy, fast photography, and optical emission spectroscopy were employed to analyze the plasma plumes, and white light interferometry was used to characterize the laser ablation craters. Our results indicated that excitation wavelength plays a crucial role in laser-target and laser-plasma coupling, which in turn affects plasma plume morphology and radiation emission. Fast photography and shadowgraphy images showed that plasmas generated by 1064 nm are more cylindrical compared to plasmas generated by shorter wavelengths, indicating the role of inverse bremsstrahlung absorption at longer laser wavelength excitation. Electron density estimates using Stark broadening showed higher densities for sh...

Assessment of soot particle vaporization effects during laser-induced incandescence with time-resolved light scattering

Although laser-induced incandescence (LII) has been successfully used for soot volume fraction and particle size measurements, uncertainties remain regarding issues of soot vaporization leading to mass loss and morphological changes occurring in soot due to intense heating. Prompt LII detection schemes are often based on the assumption that the associated time scale is shorter than the time scale of soot vaporization or sublimation. The validity of such assumptions is the focus of the current study. Time-resolved light-scattering measurements were made in combination with LII measurements to quantify soot particle vaporization effects resulting from the LII laser pulse. The light-scattering measurements revealed a sharp decrease in total soot particle mass during the time course of the 25 ns full-width LII laser pulse for fluences in the range of 0.5 J/cm2. Light-scattering theory was used to invert the scattering data, revealing approximately 80%-90% reductions in the soot particle volume for LII fluences of 0.47 and 0.61 J/cm2. In addition, the time-resolved scattering measurements show that the time scale of soot vaporization is completely confined to the LII laser pulse itself. Light scattering revealed no soot vaporization only for fluences of approximately 0.1 J/cm2, which is consistent with recent work on low-fluence LII. Possible mechanisms for soot vaporization are discussed, notably for near-threshold fluences.

100% Efficient Sub-Nanoliter Sample Introduction in Laser-Induced Breakdown Spectroscopy and Inductively Coupled Plasma Spectrometry: Implications for Ultralow Sample Volumes

Recently, nanoflow nebulizers with low-volume drain-free spray chambers became available for inductively coupled plasma-mass spectrometry application for analysis of very small sampling volumes. The present technical note reports on a different approach for 100% efficient subnanoliter sample introduction, the application of monodisperse piezoelectric microdroplet dispensers which generate 40-50 microm droplets with high reproducibility if nozzles of 30 microm diameter are applied. The droplets having volumes below 0.1 nL can be introduced loss-free and without plasma loading, with frequencies up to approximately 100 Hz into analytical plasmas. In this technical note, the analytical figures of merit of laser-induced breakdown spectroscopy and inductively coupled plasma-optical emission spectrometry with single droplet introduction are reported using Ca and Au standard solutions as examples. Future engineering is required to reduce the total sample volumes from the relatively large sample reservoir of the current study, thereby reducing potential issues of washout while enabling analysis of ultralow total sample volumes.

Laser-induced breakdown spectroscopy for analysis of micro and nanoparticles

The use of laser-induced breakdown spectroscopy (LIBS) for analysis of micro- and nanoparticles is explored, including a brief review of the recent research, both fundamentals and applications, along with new experimental work regarding aerosol particle sampling statistics, analysis of laser ablation particles via aerosol LIBS for matrix effect minimization for bulk solids analysis, and a novel aerosol particle concentration scheme that is suited for near real-time analysis of aerosol nanoparticles. The statistical analysis reveals that the LIBS particle sampling physics are well modeled using Poisson sampling statistics, as based on analysis of calcium-rich ambient air particles. The laser-ablation LIBS (LA-LIBS) methodology was explored for a range of disparate metallic and non-metallic bulk samples, revealing a linear calibration curve for all six samples over the range of relative Mn/Fe mass concentrations. Finally, the microneedle concentration technique for aerosol nanoparticle analysis was successfully demonstrated with linear mass calibration curves for copper-rich nanoparticles. Overall, a fundamental understanding of the plasma–particle physics has enabled the formulation of robust LIBS-based nanoparticle schemes.

Ultrafast laser ablation ICP-MS: role of spot size, laser fluence, and repetition rate in signal intensity and elemental fractionation

Ultrafast laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has gained prominence in recent times owing to its superior capabilities in eliminating fractionation effects providing the possibility of developing a non-matrix matched calibration system. Though ultrafast laser ablation sample introduction provides better accuracy and precision compared to long-pulse nanosecond lasers, the fundamental ablation mechanisms using femtosecond lasers are still not clearly understood. In this study, the role of spot size, fluence and repetition rate has been studied using brass and NIST 612 samples to understand their signal intensity and fractionation effects. A white light interferometric microscope has been used for crater analysis and the results are correlated with the LA-ICP-MS signal intensity and measured elemental ratios. The results show that the spot size, laser fluence and repetition rate can play an important role in overall performance of the LA-ICP-MS system as well as in elemental fractionation. Crater measurements also show that the ablation mechanisms are different for the NIST 612 sample and brass and the observed differences are discussed using ultrafast ablation theories.

New Approach for Near-Real-Time Measurement of Elemental Composition of Aerosol Using Laser-Induced Breakdown Spectroscopy

A new approach has been developed for making near-real-time measurement of elemental composition of aerosols using plasma spectroscopy. The method allows preconcentration of miniscule particle mass (pg to ng) directly from the sampled aerosol stream through electrostatic deposition of charged particles (30–900 nm) onto a flat-tip microneedle electrode. The collected material is subsequently ablated from the electrode and monitored by laser-induced breakdown spectroscopy. Atomic emission spectra were collected using a broadband spectrometer with a wavelength range of 200–980 nm. A single-sensor delay time of 1.3 μs was used in the spectrometer for all elements to allow simultaneous measurement of multiple elements. The system was calibrated for various elements including Cd, Cr, Cu, Mn, Na, and Ti. The absolute mass detection limits for these elements were experimentally determined and found to be in the range of 0.018–5 ng. The electrostatic collection technique has many advantages over other substrate-based methods involving aerosol collection on a filter or its focused deposition using an aerodynamic lens. Because the particle mass is collected over a very small area that is smaller than the spatial extent of the laser-induced plasma, the entire mass is available for analysis. This considerably improves reliability of the calibration and enhances measurement accuracy and precision. Further, the aerosol collection technique involves very low pressure drop, thereby allowing higher sample flow rates with much smaller pumps—a desirable feature for portable instrumentation. Higher flow rates also make it feasible to measure trace element concentrations at part per trillion levels. Detection limits in the range of 18–670 ng m−3 can be achieved for most of the elements studied at a flow rate of 1.5 L min−1 with sampling times of 5 min. Copyright 2012 American Association for Aerosol Research

The effect of ultrafast laser wavelength on ablation properties and implications on sample introduction in inductively coupled plasma mass spectrometry.

We investigated the role of femtosecond (fs) laser wavelength on laser ablation (LA) and its relation to laser generated aerosol counts and particle distribution, inductively coupled plasma-mass spectrometry (ICP-MS) signal intensity, detection limits, and elemental fractionation. Four different NIST standard reference materials (610, 613, 615, and 616) were ablated using 400 nm and 800 nm fs laser pulses to study the effect of wavelength on laser ablation rate, accuracy, precision, and fractionation. Our results show that the detection limits are lower for 400 nm laser excitation than 800 nm laser excitation at lower laser energies but approximately equal at higher energies. Ablation threshold was also found to be lower for 400 nm than 800 nm laser excitation. Particle size distributions are very similar for 400 nm and 800 nm wavelengths; however, they differ significantly in counts at similar laser fluence levels. This study concludes that 400 nm LA is more beneficial for sample introduction in ICP-MS, particularly when lower laser energies are to be used for ablation.

Electron-ion relaxation time dependent signal enhancement in ultrafast double-pulse laser-induced breakdown spectroscopy

We investigated the emission properties of collinear double-pulse compared to single-pulse ultrafast laser induced breakdown spectroscopy. Our results showed that the significant signal enhancement noticed in the double pulse scheme is strongly correlated to the characteristic electron-ion relaxation time and hence to the inter-pulse delays. Spectroscopic excitation temperature analysis showed that the improvement in signal enhancement is caused by the delayed pulse efficient reheating of the pre-plume. The signal enhancement is also found to be related to the upper excitation energy of the selected lines, i.e., more enhancement noticed for lines originating from higher excitation energy levels, indicating reheating is the major mechanism behind the signal improvement.