Speaker
Description
Optical spectroscopy in conjunction with laser-produced plasma (LPP) is a very promising tool for in-field and non-contact isotopic analysis of solid materials. Both emission and fluorescence spectroscopy of laser ablation plumes can be used for isotopic analysis. However, the reported isotopic shifts of U I and U II transitions in the visible spectral regime are in the range ~ 1-25 pm which necessitate the requirement of an extremely high-resolution spectrograph with a resolution ⪖60000 for using emission based diagnostic tools (eg. laser-induced breakdown spectroscopy) for isotopic analysis. In addition to this, the emission spectral analysis requires thermal excitation by electrons which happen at early times of plasma evolution when the lines are broader due to various line broadening mechanisms (Stark, Doppler etc.). Laser-absorption/ laser-induced fluorescence spectroscopy (LAS/LIFS) can be used to marginalize the effect of instrumental broadening. LAS and LIFS probe the ground state atoms existing in the plasma when it is cooler, which inherently provides narrower lineshapes. We recently reported the linewidths of U transitions using LAS/LIFS of laser-produced plasmas are ~ 1 pm which is significantly lower than the average isotopic shift of U atoms/ions (~ 9 pm). In addition to isotopic splitting, the hyperfine structures (hfs) may influence the lineshape of a transition. Hyperfine splitting’s are usually small; however, in certain cases, they can be larger than isotope splitting. In that scenario, isotope shifts of atoms and molecules can be entangled with hyperfine structure. Here we report the isotopic shifts between U-238 and U-235 transitions and hyperfine structures of U-235 using laser-induced fluorescence (LIF) of laser-ablation plumes. We used a collinear laser geometry for isotopic detection, where both the plasma generation beam and LIF excitation beams propagate near normal to the target, which is a prerequisite for any standoff analytical detection tool.
