Mechanism for laser-induced fluorescence signal generation in a nanoparticle-seeded flow for planar flame thermometry

The mechanism of atomic indium generation for laser-induced fuorescence (LIF) of indium from laser ablation seeding was investigated in a hydrogen/nitrogen non-premixed fame. The morphology and particle size distributions of the ablation products were examined with scanning electron microscopy and transmission electron microscopy. These investigations show that the ablation products comprise complex agglomerates of nano-sized primary particles of indium compounds and micron-sized spherical indium beads. Images of the atomic indium LIF, Mie scattering of ablation products and natural fuores-cence emission of indium in the fame were recorded to investigate the mechanism of fuorescence signal generation. The relative contribution of natural fuorescence emission of indium towards the total indium fuorescence signal was assessed by comparing these images. These images also reveal the evolution of ablation products through the fame structure and the correlation between LIF signal and ablation products. It is found that the LIF signal generation is associated with the vapourisation of indium nano-particles into the gas phase by thermal decomposition in the fame. A further mechanism for thermal decomposition of the nanoparticles was also identifed, that of heating the ablation products by in situ laser ablation. This was assessed by means of a second laser, introduced prior to the excitation laser, to reveal that the LIF signal can be enhanced by in situ laser ablation, particularly in the upstream regions of the fame. These fndings supersede the mechanism deduced previously by the authors that neutral atomic indium can survive a convection time of the order of tens of seconds and be directly seeded into reacting or non-reacting fows. The possible infuences of laser ablation seeding on the nonlinear two-line atomic fuorescence thermometry technique were also assessed.