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% 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.