This work presents the first quantitative analysis of time-resolved laser-induced incandescence (TiRe-LII) measurements on aerosolized nickel nanoparticles in several gases and over a range of laser fluences. A measurement model composed of spectroscopic and heat transfer submodels is used to recover the particle size distribution parameters and the thermal accommodation coefficient (TAC). A qualitative analysis of the results reveals evidence of nonincandescent laser-induced emission temporally aligned with the laser pulse, and more laser energy is absorbed than can be accounted for from the modeled spectral absorption cross section of the nanoparticles. The TiRe-LII inferred particle size parameters were generally consistent with values found from ex situ transmission electron microscopy (TEM) analysis. The TACs for nickel nanoparticles in polyatomic gases were larger than those in monoatomic gases, which may indicate chemisorption.

In conventional time-resolved laser-induced incandescence (TiRe-LII) measurements, a laser pulse heats the nanoparticles within a probe volume of aerosol, and the particle size distribution and other characteristics are inferred from the observed incandescence decay rate, which is connected to the change in sensible energy through a spectroscopic model. There is strong evidence, however, that for some aerosol systems, the incandescence signal is contaminated with other non-incandescent emission sources. Recent TiRe-LII measurements on polydisperse aerosolized silver and gold nanoparticles energized with a 1064 nm laser pulse exhibit broadband emission that is temporally aligned with the temporal profile of the laser pulse, suggesting that the signal is due to non-thermal emission. One candidate for this emission phenomenon is multiphoton-induced upconversion luminescence, in which the conduction-band electron gas is heated up to an effective lattice temperature, resulting in luminescence due to high-energy intraband transitions.

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This work examines the excessive absorption and anomalous cooling phenomena reported in laserinduced incandescence measurements on metal nanoparticles by considering the effects of aggregate structure and sintering. Experimental investigations are conducted on iron and molybdenum aerosols, which have different melting points and thus respond differently to the laser pulse. Although aggregation enhances the absorption cross-section of the nanoparticles and allows for higher peak temperatures, this enhancement does not fully explain the observed excessive absorption. Furthermore, as the aggregates of refractory metals such as molybdenum cool, they may sinter through gradual grain boundary diffusion; this change in structure alters their absorption cross-section, manifesting as a rapid drop in the pyrometric temperature, which could explain the anomalous cooling reported for this metal.

This work examines the excessive absorption and anomalous cooling phenomena reported in laser-induced incandescence measurements on metal nanoparticles by considering the effects of aggregate structure and sintering. Experimental investigations are conducted on iron and molybdenum aerosols, which have different melting points and thus respond differently to the laser pulse. Although aggregation enhances the absorption cross-section of the nanoparticles and allows for higher peak temperatures, this enhancement does not fully explain the observed excessive absorption. Furthermore, as the aggregates of refractory metals such as molybdenum cool, they may sinter through gradual grain boundary diffusion; this change in structure alters their absorption cross-section, manifesting as a rapid drop in the pyrometric temperature, which could explain the anomalous cooling reported for this metal.