Thermionic emission in gold nanoparticles under femtosecond laser irradiation observed with photoemission electron microscopy

This study reports that thermionic emission can dominate over multi-photon ionization in gold nanoparticles under femtosecond laser irradiation. The brightness of individual particles observed with photoemission electron microscopy increased steeply for laser power above 200 mW, which is reproduced well by the Richardson–Dushman equation. The temperature estimated by fitting the equation is about 1 eV. This high temperature suggests that the laser is absorbed by defects. The difference in brightness among the nanoparticles as large as a few orders of magnitude is attributed to variation in the defect density. The particles having a similar temperature despite exhibiting largely varying brightness levels can be explained by slow energy transfer from the defects to the ambient electron gas. Discussions for interpreting the observed results suggest that thermionic emission can be a useful diagnostic tool for characterizing nanoparticles.