Anomalous behavior of optical density of iron nanoparticles heated behind shock waves

Nonmonotonous variation of the optical properties of iron nanoparticles with a temperature increase during heating behind reflected shock waves is discovered. Iron nanoparticles, within $12$ nm in size, were formed at $0.5$–$1\%\,\rm Fe(CO)_5$ pyrolysis in argon behind the incident shock waves. Using a laser extinction method, a variation of the volume fraction of the condensed phase was registered at the main wavelength of $633$ nm and, in several experiments, at the additional wavelengths of $405$, $520$, and $850$ nm. At the second heating of the produced nanoparticles behind the reflected shock waves within the temperature range $800$–$1500$ K, the function of the complex refractive index, $E(m)$, decreased at all the wavelengths. Within the temperature range of $1500$–$2250$ K, it increased with the temperature increase behind the reflected shock wave almost up to the values that we observed behind the incident shock wave. At the temperatures above $2250$ K, due to the essential evaporation of the iron nanoparticle material, the optical properties were not measured. The iron nanoparticle $E(m)$ variations within the temperature range $800$–$2250$ K are possibly related to their structure variations.