Significance of ambient conditions in uranium absorption and emission features of laser ablation plasmas

Abstract

This study employs laser ablation (LA) to investigate mechanisms for U optical signal variation under various environmental conditions during laser absorption spectroscopy (LAS) and optical emission spectroscopy (OES). Potential explored mechanisms for signal quenching related to ambient conditions include plasma chemistry (e.g., uranium oxide formation), ambient gas confinement effects, and other collisional interactions between plasma constituents and the ambient gas. LA-LAS studies show that the persistence of the U ground state population is significantly reduced in the presence of air ambient compared to nitrogen. LA-OES yields congested spectra from which the U I 356.18 nm transition is prominent and serves as the basis for signal tracking. LA-OES signal and persistence vary negligibly between the test gases (air and N 2 ), unlike the LA-LAS results. The plume hydrodynamic features and plume fundamental properties showed similar results in both air and nitrogen ambient. Investigation of U oxide formation in the laser-produced plasma suggests that low U concentration in a sample hinders consistent detection of UO molecular spectra.

Two-dimensional fluorescence spectroscopy of uranium isotopes in femtosecond laser ablation plumes

“…Prior work on optical spectroscopy of laser-ablated uranium has been performed using a variety of uranium-containing solids, including uranium oxides 12 – 15 , 22 , uranium-doped glasses 3 , 16 , 18 , 23 , 24 , uranium ore 22 , 25 , and uranium-containing soils 6 , 26 . These samples contain a range of other elements as part of their sample matrix, which complicates interpretation of the optical spectra.…”

Impact of oxygen chemistry on the emission and fluorescence spectroscopy of laser ablation plumes

“…Moreover, for any atomic spectroscopic tool, the persistence of the signal influences the analytical merits of the technique. For example, the cover gas used in the LIBS technique confines the plasma and hence maintains the temperature and density of the plasma system for a longer duration in comparison with LPP expansion under a rarefied atmosphere [26,27]. It is well-known that oxygen present in the ambient environment may inhibit LIBS emission through plasma-assisted chemical reactions [7].…”

“…1, the persistence of LIBS emission is determined by the time dependence of thermal excitation and number density of the atoms in the plasma plume. As the plume expands into an ambient environment, the plasma begins to cool and the measured LIBS emission intensity commensurately decreases.If reactive species are present in the ambient environment, plasma-assisted chemical reactions can occur, which will deplete the population density of excited and lower energy state atoms in the plasma plume[27,38,39]. In the present experiment, N 2 and O 2 are used (in varying relative concentrations for a total pressure of 700 Torr) as the buffer gases.…”

“…LIF was collected over a gate width of 100 s and gate delay of 50 s.Oxygen-induced fluorescent state deactivation of organic molecules is well known[29].However, limited studies exist on the role of oxygen in depletion of the total atomic number density and its effect on the measured LIF and LAS signals. Recently, Skrodzki et al[27] compared the absorption signals of U species in air and N 2 ambient and noticed significantly reduced ground state population persistence in air versus nitrogen environment. The recorded persistence for the lower energy state population of U I in N 2 and air were ~80 μs and ~10 μs at 760 Torr, respectively.…”

Standoff Detection of Uranium and its Isotopes by Femtosecond Filament Laser Ablation Molecular Isotopic Spectrometry

“… 24 , where rarefied atmospheric conditions were used. The molecules originate either from direct vaporization of the sample surface or through plasma-assisted chemical reactions of the vaporized sample with the ambient atmosphere 21 25 26 .…”