We report the light emission decay in bare gold nanoparticles by using the fluorescence up-conversion technique. The emission decay, of which the time constants increase consistently with excitation intensity, closely follows the electron thermalization dynamics, which has been independently observed from the bleach recovery of the surface plasmon band under the identical experimental conditions. These results indicate that a weak light emission from relatively large gold nanoparticles is attributable to the interactions between surface plasmons and incident photons, which occur simultaneously with the thermal electron relaxation.

We report the femtosecond transient absorption dynamics of the gold and gold-polypyrrole nanoparticles by photoexcitation at various wavelengths. The bleach recovery dynamics of the surface plasmon band for the gold and gold-polypyrrole nanoparticles exhibit different responses to the pump beam energy and intensity in thermal energy transfer from the gold nanoparticles to the surrounding medium or embedding material. This indicates that directly attached polypyrroles provide fast thermal energy transfer pathways for the core gold nanoparticles.