Vibrational wave packets in metal nanoparticles

Due to their small size, metal nanoparticles are expected to show confined vibrational modes with discrete frequencies. In fact, time-resolved optical pump-probe experiments on metal nanoparticles have revealed periodic signal modulations with frequencies consistent with the lowest (n=1) vibrational breathing mode. Higher (n>1) vibrational modes have not been observed, presumably due to their smaller vibration amplitudes. The vibrations in these experiments have been argued to be excited by the thermal expansion of the particle following the lattice heating by the pump laser pulse. We show that vibrational wave packets consisting of up to five vibrational breathing modes (n=1...5) are observed in femtosecond optical pump-probe experiments on spherical silver nanoparticles. The spectral distribution of the wave packets shows that lattice heating is not the only mechanism driving the particle expansion; there is also a considerable electronic contribution.