Spectroscopic characterization of uranium atoms in uranium-dioxide laser-produced plasma in a high-vacuum environment†

Abstract

The expansion dynamics of target particles are crucial for rapid isotope analysis using resonance ionization mass spectrometry (RIMS) combined with laser ablation as an atomization source. The spatial and temporal spectral signatures in laser-produced plasma (LPP) provide insights for applying RIMS to understand the expansion behavior of target particles. In this study, diode laser absorption spectroscopy was employed to observe U atoms in UO₂ LPP in a high-vacuum environment (3.0 × 10⁻⁴ Pa). Results showed the remarkable differences in the expansion dynamics of U atoms compared to Al atoms. Specifically, the UO₂ ablation plume had a double layer due to charged particles, and the spectra of the U atoms do not exhibit clear spectral splitting. Moreover, the U atoms in the ⁵L₆ ground state and low-lying ⁵K₅ meta-stable state (notably, >99% of U atoms generally exist in both energy states) are probed to examine their distributions between the two energy states. The results reveal that approximately 28–38% of the U atoms are distributed in the ⁵K₅ meta-stable state because of thermal excitation. These quantitative insights provide a clear pathway toward advancing the applications of laser ablation.