Vibrational modes in metal core-shell nanoparticles*

We study the spectrum of lowest vibrational modes of bimetallic nanoparticles in dielectric surrounding. For solid spherical particles, the mode's period and decay time are determined by the ratio of particle radius to longitudinal sound velocities in metal and outside medium, respectively. In contrast, in bimetallic nanoparticles, the dependence of both period and damping time on aspect ratio is highly non-monotonic. In particular, for Au/Ag core-shell nanoparticles, in a wide range of aspect ratios, the fundamental mode period is lower than in both Au and Ag solid particles of the same overall size, while the damping time exhibits a minimum at aspect ratios around 0.5. The unique acoustical signature of complex nanostructures allows unambiguous determination of their composition from ultrafast pump-probe and Raman spectroscopy measurements.