Optical radiation accompanying metal nanoparticles coagulation in superfluid helium

Abstract The spectra of visible radiation from small clusters of tungsten and nickel self-heated under their mutual fusion in the bulk of superfluid helium (He II) have been investigated. The emissions were shown to be close to the black-body emission. At the initial stages of condensation, the nanoparticles are heated to temperatures significantly higher than the metal melting temperature (T melt ), even for refractory tungsten. That would be so if the process of coagulation of clusters with similar sizes (not the deposition of atoms and small particles into a heavier center) to be the prevailing process. The condensation takes place only for particles trapped in the core of 1D quantized vortices existed in He II and it explains why the nanowires are the main product of metals condensation there. At further stages of condensation the color temperature of the thermal emission is almost constant and is close to the metal T melt . Along with exponential decay in the glow intensity with time, it fits scenario of nanowires growth by fusion of whiskers freely moving along the cores of vortices. The method just developed has good prospects for studying the radiative cooling of highly heated metallic and semiconductor nanoparticles.