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 fluorescence (LIF) of indium from laser ablation seeding was investigated in a hydrogen/nitrogen non-premixed flame. 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 fluorescence emission of indium in the flame were recorded to investigate the mechanism of fluorescence signal generation. The relative contribution of natural fluorescence emission of indium towards the total indium fluorescence signal was assessed by comparing these images. These images also reveal the evolution of ablation products through the flame 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 nanoparticles into the gas phase by thermal decomposition in the flame. A further mechanism for thermal decomposition of the nanoparticles was also identified, 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 flame. These findings 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 flows. The possible influences of laser ablation seeding on the nonlinear two-line atomic fluorescence thermometry technique were also assessed.