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Kinetics of ion and neutral atom emission features were compared for nanosecond laser-produced plasmas generated from several metal targets (i.e., Al, Ti, Zr, Nb, Ta) and an alloy containing all of these as principal alloying elements. Plasmas were produced by focusing 6 ns, 1064 nm pulses from an Nd:YAG laser on the targets of interest in a vacuum. A Faraday cup was used for collecting ion temporal features while spatially and temporally resolved emission spectroscopy was used for measuring the optical time of flight of various neutral atomic transitions. Our results highlight that most probable ion and atom velocities decay with increasing atomic mass. Trends for ions from the alloy target represent a weighted average where all ions contribute. For both ions and atoms, velocities decrease with increasing heat of vaporization and melting temperature, consistent with the thermal mechanisms that contribute to nanosecond laser ablation. Kinetic energies for neutral atoms from pure metal targets have some variability with atomic mass, whereas kinetic energies for atoms from the alloy target are more similar. These more similar kinetic energies observed for neutral atoms in the multi-element plasma may be attributed to collisions between species from all elements in the Knudsen layer.

High-resolution tunable laser absorption spectroscopy is used to measure time-resolved absorption spectra for six neutral uranium transitions in a laser-produced plasma. Analysis of the spectra shows that kinetic temperatures are similar for all six transitions, but excitation temperatures are higher than kinetic temperatures from 10–100 μs, indicating departures from local thermodynamic equilibrium.

Broadband and high-resolution absorption spectra of molecular cerium oxide (CeO) are obtained in a laser-produced plasma using dual-comb spectroscopy. Simultaneous measurements of Ce and CeO are used to probe time-resolved dynamics of the system. A spectral resolution of 1.24 GHz (2.4 pm) over a bandwidth of 378.7–383.7 THz (781.1–791.5 nm) allows simultaneous detection of hundreds of closely spaced rotational transitions in complex CeO bands.

Ultrafast laser ablation coupled with optical emission spectroscopy is currently under development for standoff detection of elements and their isotopes for a wide range of application areas. In this work, we explore the role of ambient air pressure during the interaction of an ultrafast laser beam with two metallic targets (brass and uranium) with significantly different oxygen gas-phase reactivity. Plasma plumes were generated by focusing ∼800 nm, ∼35 fs pulses from an ultrafast laser system using a long focal length lens (f = 1 m). Two-dimensional spectral imaging was performed over the pressure range of 30 mTorr to 700 Torr air to evaluate emission dynamics, plasma chemistry, signal-to-background ratio, and characteristic parameters (i.e. excitation temperature, electron density). An increase in ambient air pressure during ultrafast laser ablation leads to plume confinement and subsequent changes in the emission dynamics due to plasma chemistry as well as changes in plasma generation conditions (i.e. focused fs laser ablation versus filament ablation). Ablation crater morphologies were also investigated via scanning electron microscopy. Results indicate atomic emission intensity and signal-to-background ratios peak at moderate pressure levels (∼50–100 Torr air) for both targets studied, although plasma chemistry influences uranium emission signatures. The emission features of the uranium plasma at pressures ≥10 Torr showed the presence of oxide molecules. We also find filament ablation leads to wider, more shallow craters compared to focused laser ablation. Our study provides unique insight into the interplay between plume dynamics, confinement, and plasma chemistry of fs laser-produced plasmas and how these phenomena evolve with changing ambient air pressure.

Tumor DNA sequencing data can be interpreted by computational methods that analyse genomic heterogeneity to infer evolutionary dynamics. A growing number of studies have used these approaches to link cancer evolution with clinical progression and response to therapy. Although the inference of tumor phylogenies is rapidly becoming standard practice in cancer genome analyses, standards for evaluating tumor phylogenies are lacking. To address this need, we systematically assess methods for reconstructing tumor sub-clonality. First, we elucidate the main algorithmic problems in subclonal reconstruction and develop quantitative metrics for evaluating them. Then we simulate realistic tumor genomes that harbor all known clonal and subclonal mutation types and processes. We benchmark 580 tumor reconstructions, varying tumor read-depth, tumour type, and somatic variant detection. Our analysis provides a baseline for the establishment of gold-standard methods to analyze tumor heterogeneity., Editors summary Methods for reconstructing tumor evolution are benchmarked in the DREAM Somatic Mutation Calling Tumour Heterogeneity Challenge using novel tools.

Rapid and standoff analysis of uranium is important to many applications. We present the recent advances in standoff detection of uranium using emission and laser-induced fluorescence spectroscopy in conjunction with ultrafast laser filament ablation-based solid sampling.

We demonstrate the use of dual-comb spectroscopy to perform time-resolved absorption measurements on cerium monoxide molecules formed in a laser-produced plasma.

Using the supercontinuum and second harmonic from a dual-comb spectrometer, simultaneous time-resolved absorption measurements of rubidium atomic transitions originating from ground and highly-excited states in the NIR (1529 nm) are obtained following a laser-produced plasma.

Laser induced breakdown spectroscopy is a promising, rapid analysis method for the detection and quantification of Li and its isotopes needed in geochemical, nuclear, and energy storage applications. However, spectral broadening in laser produced plasmas, presence of fine and hyperfine structures, and self-reversal effects make Li isotopic analysis via laser induced breakdown spectroscopy challenging. The present study explores the influence of Ar, N2, and He ambient gases over the pressure range of 0.05 - 100 Torr on line broadening and self-reversal of the Li I transition with the greatest isotopic shift in the VIS spectral region (i.e., ≈670.8 nm, ≈15.8 pm isotopic shift). We perform spatially and temporally resolved optical emission spectroscopy of plasmas produced via laser ablation of LiAlO2 substrates. Our results show that the self-reversal and linewidth is reduced at lower pressures for all gases, and using optimized plasma conditions with chemometric methods, the 6Li/7Li isotopic ratios can be predicted.

Chemistry evolution in transient plasma or fireball systems in air comprised of multiple elements is extremely complex, owing to numerous ionic, atomic, and molecular species present, overlapping spectral lines, and the gas-phase oxidation. Although plasma chemistry is widely studied for analytical and material detection applications, tracking the chemical evolution from atoms and ions to diatoms, polyatoms, and larger molecules, nanoparticles, and agglomerates in a multi-component system remains largely unexplored.

Gas-phase oxidation of uranium (U), plume chemistry, and the corresponding impact on optical emission features of the U plasma are investigated. Plasmas were produced via nanosecond laser ablation of a natural U target in a chamber where U oxidation was controlled by varying the oxygen partial pressure in an argon cover gas. Monochromatic imaging of U atoms and monoxide (UO) molecules was performed using narrow bandpass optical filters. Results reveal the spatio-temporal evolution of atomic and molecular species in the plasma. U oxides are found to be formed further from the target (in comparison to U atoms), where lower temperatures favor molecular recombination. Segregation between the distribution of U atoms and UO species is observed at later times of plasma evolution, and is more apparent at lower oxygen partial pressures. At higher oxygen partial pressures, significant variation in plume morphology is noticed for UO species, which can be attributed to the higher oxide (UxOy) formation further from the target. The monochromatic images of U atoms and UO molecules and corresponding spectral features at various oxygen partial pressures presented here provide unique insight into gas-phase, high-temperature U oxidation and chemistry, with implications for a wide range of nuclear applications, from stand-off detection of radioisotopes to forensics and safeguards.

Dual-comb spectroscopy (DCS) represents a novel method of using absorption spectroscopy as a diagnostic tool for multispecies analysis of excitation temperatures and column densities in laser-produced plasmas (LPPs). DCS was performed on a LPP generated by ablating a multielement alloy containing Nd, Gd, and Fe. Transitions from all three elements were observed in absorption spectra measured from 530.08 to 535.19 nm at seven time-delays from 31 to 250 μs after ablation. The spectra were fit using a nonlinear regression algorithm to determine peak areas, and excitation temperatures and column densities were determined for the three atomic species separately using Boltzmann plots. The measured excitation temperatures of Nd I and Gd I showed good agreement at all time-delays, whereas the Fe I temperature was found to be higher, and the ratios between the column densities varied with delay. The observations are understood via effects of LPP spatial averaging, elemental fractionation, and molecular formation and are compared and contextualized with previous work studying LPPs using other spectroscopic techniques. A brief discussion of the precision and accuracy of the determined excitation temperatures and column densities is also presented.

Laser-induced breakdown spectroscopy is a promising method for rapidly measuring hydrogen and its isotopes, critical to a wide range of disciplines (e.g. nuclear energy, hydrogen storage). However, line broadening can hinder the ability to detect finely spaced isotopic shifts. Here, the effects of varying plasma generation conditions (nanosecond versus femtosecond laser ablation) and ambient environments (argon versus helium gas) on spectral features generated from Zircaloy-4 targets with varying hydrogen isotopic compositions were studied. Time-resolved 2D spectral imaging was employed to detail the spatial distribution of species throughout plasma evolution. Results highlight that hydrogen and deuterium isotopic shifts can be measured with minimal spectral broadening in a ∼ 10 Torr helium gas environment using ultrafast laser-produced plasmas.

We introduce a new, to the best of our knowledge, modality of dual-comb spectroscopy (DCS) that enables a simplified and powerful new approach for time-resolved measurements with increased acquisition rates. This “burst mode” form of DCS relies on the multiplexing of each probe pulse into a short train of pulses. With this approach we demonstrate a time-resolved series of absorption-based spectroscopic measurements of a laser-induced plasma using only a single laser ablation shot and identify 22 Nd lines not previously reported in the literature. The transmission spectra spanned 3.1 THz and were acquired at an effective acquisition rate of 25 kHz with 40 µs time resolution. This simple modification to ∼ 100 M H z level dual-comb systems provides a flexible approach for studying transient and low-duty-cycle events such as laser-induced plasmas, combustion, and explosive reactions.

We performed simultaneous measurement of absorption, emission, and laser-induced fluorescence spectroscopic signatures for determining femtosecond laser-produced plasma’s physical properties throughout its lifecycle.