Observation of hot-electron pressure in the vibration dynamics of metal nanoparticles

We investigate the vibration dynamics of ellipsoidal silver nanoparticles, using time-resolved optical pump-probe spectroscopy. When excited with femt osecond laser pulses, the particles execute anisotropic shape oscillations. We show that these vibrations are triggered by the thermal expansion of th e optically heated particles. The time dependence of the vibrations indicat es that this expansion is caused by two mechanisms: The lattice anharmonici ty and the extremely large pressure of the hot conduction electrons.