Photoemission from alkali-metal nanoparticles reveals a size-dependent melting transition

The melting behavior of nanoparticles deviates significantly from that of bulk materials. One unexplored aspect of this behavior is the relationship between nanoparticle ionization energy (IE) and melting. As it happens, the connection between melting and the work function of bulk materials also remains underexplored, and the study of isolated nanoparticles offers a route to the study of this effect in a contamination-free environment. By carrying out a high-precision photoionization measurement of the ionization thresholds of alkali nanoparticles in a beam as a function of their temperature (T), we observed a transition in the slope of the IE(T) line, marking the melting point of the nanoparticle and indicating a change in the electronic structure associated with the change of phase. The large downward shift of the nanoparticle melting temperature is in good agreement with the Gibbs–Thomson equation which describes the effect of surface curvature on finite-size phase transitions. These findings highlight the interplay between electronic and structural dynamics in both nanoscale and bulk systems.