Atomic composition analysis of iron-based particles via single-shot LIBS and spectral fitting under fluctuating plasma conditions

Abstract This study introduces a novel data evaluation framework for picosecond (ps)-excited laser-induced breakdown spectroscopy (LIBS) for single-shot measurement of the atomic surface composition of reactive iron (Fe) micro particles during thermochemical oxidation, which is relevant to metal-based energy storage. The temporal and spatial resolution of the LIBS diagnostics enables high-resolution analysis of surface processes during thermochemical oxidation. However, the boundary conditions of reactive micro particles in a gaseous environment pose challenges for the diagnosis and evaluation of the data. Single-particle LIBS in free-jets is strongly affected by the particle position within the probe volume, which alters laser pulse-particle energy coupling and therefore changes plasma excitation temperature and free electron number density on a shot-to-shot basis. Simultaneous diffuse backlight illumination measurement to the LIBS measurement, allowing the laser pulse-particle interactions to be investigated and enabling interpretation of shot-to-shot variability. In a reactive atmosphere, oxygen (O) is present in both the surrounding gas phase and the particle itself. The partial transfer of the gas phase interferes with the O plasma signal from the particle phase. These boundary conditions complicate the evaluation of LIBS data and require a novel approach. Therefore, this study presents a method based on the spectral fitting of temperature-dependent synthetic stick spectra. The study shows that it is possible to reliably differentiate between Fe and Fe 2 O 3 particles by determining their particle O to Fe ratio for plasma excitation temperatures above 13 370 K. The ideal delay time for this system with ps laser excitation is 150 ns. Robustness criteria based on sensitivity analyses are identified to define reproducible windows for the accurate in-situ LIBS diagnostics of reactive metal particles, a topic to be investigated in future.