In situ imaging of oxidation dynamics within aluminum laser induced plasmas

Laser ablation (LA) methods underpin a wide class of analytical techniques, including laser-induced breakdown spectroscopy (LIBS), LA-ICP-MS, and laser ablation molecular spectrometry. Yet, interpretation of these signals remains limited by an incomplete understanding.

Here we integrate fast-gated emission spectroscopy with planar laser-induced fluorescence (PLIF) to map excited- and ground-state Al/AlO produced by nanosecond (1064 nm, ∼6 ns, 35 mJ) ablation of Al at 150 Torr with controlled oxygen mole fractions [Formula: see text] . Targeted transitions include Al I at 394/396 nm and the AlO B²Σ⁺→X²Σ⁺ system (440-530 nm); emission images used 1 nm FWHM bandpass filters (396 and 487 nm), and PLIF probed Al at 394 nm and AlO at 464 nm with detection at 396 nm and 487 nm, respectively. Time-resolved emission revealed that AlO production peaked at 25-33 μs depending on oxygen content, while PLIF identified ground-state shells that encapsulated excited species, driving distinct stages of plume evolution: expansion, collapse, and stagnant intermixing. In contrast to emission-only studies that ascribed AlO formation largely to post-shock-collapse chemistry, our direct probing of Al and AlO reveals a finite mixing layer at the contact surface and shows AlO encapsulating the emission-bright core during expansion and collapse, clarifying the spatial origin of oxidation within the plume. This joint diagnostic refines shock-mediated interpretations derived from spontaneous emission alone by resolving ground-state speciation and internal flow structures (including onset of vortex formation coincident with peak AlO), thereby establishing when and where oxidation proceeds in LA plumes used as surrogates for post-detonation fireballs. Quantitative trajectory analysis showed that early expansion followed Sedov-Taylor blast-wave scaling, with later descent and confinement captured by drag and semi-empirical models. Relative to emission-only LIBS, the joint modality improves selectivity against a continuum background and resolves ground-state speciation and internal flow structures (including a contact surface mixing layer and toroidal-vortex formation) that are not accessible by emission alone, clarifying when and where oxidation initiates and how reactive environments shape LIBS signal formation.

These results strengthen the analytical basis for LA across platforms, improving interpretation and quantitation in LIBS and informing the fidelity of LA surrogates for post-detonation fireballs, and provide method details (gating, spectral windows, replication) that enhance reproducibility and transfer to other LA matrices.