This Letter reports dual-frequency-comb spectroscopy in the deep ultraviolet used to characterize transient laser-produced plasmas. Dual-comb spectroscopy at these wavelengths enables access to a large number of strong electronic transitions in neutral and ionized atoms and molecules. This broadband dual-frequency-comb system at 265 nm measures multiple transitions in neutral and singly ionized iron atoms in a laser-produced plasma. Analysis of absorption spectra yields time-resolved ion and neutral column densities, excitation temperatures, and electron densities, which are measured down to 10¹⁴cm⁻³levels. The results provide insights into low-temperature plasma properties.

Time-resolved tunable laser absorption spectroscopy is used to characterize the physical properties of ultrafast laser-produced plasmas. The plasmas were produced from an Inconel target, with $\le 0.4\mathrm{wt}\%$ Al, using $\sim 35\mathrm{fs}$, $\sim 800\mathrm{nm}$, $\sim 5\mathrm{mJ}$ laser pulses at varying Ar background pressures from 1 to 100 Torr. The absorption spectrum of atomic Al is measured with high spectral and temporal resolution when the probe laser is stepped across the selected Al transition at 394.4 nm. Spectral fitting is used to infer linewidths, kinetic temperature, Al column density, and pressure broadening coefficient. The late time physical properties of plasmas are compared for various pressure levels. Our studies highlight that a significant lower state population exists even at early times of ultrafast laser-produced plasma evolution, and lower state population persistence decreases with increasing ambient pressure. We also show that the fundamental optical properties, such as pressure broadening, can be measured using ultrafast laser-produced plasmas combined with laser absorption spectroscopy.

We report spatiotemporal evolution of emission and absorption signatures of Al species in a nanosecond (ns) laser-produced plasma (LPP). The plasmas were generated from an Inconel target, which contained $\sim 0.4$ wt. % Al, using 1064 nm, $\approx 6$ ns full width half maximum pulses from an Nd:YAG laser at an Ar cover gas pressure of $\approx 34$ Torr. The temporal distributions of the Al I (394.4 nm) transition were collected from various spatial points within the plasma employing time-of-flight (TOF) emission and laser absorption spectroscopy, and they provide kinetics of the excited state and ground state population of the selected transition. The emission and absorption signatures showed multiple peaks in their temporal profiles, although they appeared at different spatial locations and times after the plasma onset. The absorption temporal profiles showed an early time signature representing shock wave propagation into the ambient gas. We also used emission and absorption spectral features for measuring various physical properties of the plasma. The absorption spectral profiles are utilized for measuring linewidths, column density, and kinetic temperature, while emission spectra were used to measure excitation temperature. A comparison between excitation and kinetic temperature was made at various spatial points in the plasma. Our results highlight that the TOF measurements provide a resourceful tool for showing the spatiotemporal LPP dynamics with higher spatial and temporal resolution than is possible with spectral measurements but are difficult to interpret without additional information on excitation temperatures and linewidths. The combination of absorption and emission TOF and spectral measurements thus provides a more complete picture of LPP spatiotemporal dynamics than is possible using any one technique alone.

Rapid and sensitive standoff measurement techniques are needed for detection of trace chemicals in outdoor plume releases, for example from industrial emissions, unintended chemical leaks or spills, burning of biomass materials, or chemical warfare attacks. Here, we present results from 235 m standoff detection of transient plumes for 5 gas-phase chemicals: Freon 152a (1,1-difluoroethane), Freon 134a (1,1,1,2-tetrafluoroethane), methanol (CH₃OH), nitrous oxide (N₂O), and ammonia (NH₃). A swept-wavelength external cavity quantum cascade laser (ECQCL) measures infrared absorption spectra over the range 955-1195 cm⁻¹ (8.37- 10.47 µm), from which chemical concentrations are determined via spectral fits. The fast 400 Hz scan rate of the swept-ECQCL enables measurement above the turbulence time-scales, reducing noise and allowing plume fluctuations to be measured. For high-speed plume detection, noise-equivalent column densities of 1-2 ppm*m are demonstrated with 2.5 ms time resolution, improving to 100-400 ppb*m with 100 ms averaging.

Infrared laser absorption spectroscopy provides a powerful tool for probing physical and chemical properties of high-explosive detonations. A broadly tunable swept-wavelength external cavity quantum cascade laser operating in the mid-wave infrared (MWIR) spectral region is used to measure transmission through explosive fireballs generated from 14 g charges of 4 different explosive types detonated in an enclosed chamber. Analysis of time-resolved transmission and emission at a 2 μs sampling rate shows the evolution of fireball infrared opacity in the first 10 ms after detonation. Broadband high-resolution absorption spectra acquired over the spectral range of 2050–2300 cm⁻¹ (4.35–4.88 μm) at a 100 Hz rate are used to measure properties of fireball evolution over longer time scales out to 100 s. Path-integrated concentrations of combustion products CO, CO₂, H₂O, and N₂O are measured and show evolutions over multiple time scales and significant differences between explosive types. Spectral analysis is used to characterize gas temperature and to measure broadband attenuation from absorption and scattering of particulates. Analysis of the results provides information on the MWIR optical properties, gaseous detonation/combustion products, and particulates throughout the explosive process including initial detonation, fireball expansion and cooling, and diffusive mixing in the chamber.

We performed simultaneous measurement of absorption, emission, and laser-induced fluorescence spectroscopic signatures for determining nanosecond and femtosecond laser-produced plasma’s (LPP) physical properties throughout its lifecycle. Plasmas are produced by focusing either ∼6 ns, 1064 nm pulses from an Nd:YAG or ∼35 fs, ∼800 nm pulses from a Ti:sapphire laser on an Inconel target that contains Al as a minor alloying addition. A continuous-wave narrowband tunable laser was used for performing absorption and fluorescence spectroscopy while a fast-gated detection system was used for emission spectroscopy. The temporal evolution of emission, fluorescence, and absorbance of Al transitions are compared for both ns and fs LPPs. Time-resolved absorbance was also used for evaluating linewidth, lineshape, temperature, and column-averaged atomic number density at late times of ns and fs plasma evolution. Our results demonstrate that lower and excited-state populations of fs LPPs are short-lived in comparison to those in ns plasmas. The lower state population is observed to reach a maximum value earlier in time for the fs plasma versus the ns plasma, while the kinetic temperature for the ns plasma was higher than for the fs plasma at most times of the plasma evolution.