Surface Collective Excitations in Ultrafast Pump–Probe Spectroscopy of Metal Nanoparticles

The role of surface collective excitations in the electron relaxation in small metal particles is studied. It is shown that the dynamically screened electron– electron interaction in a nanoparticle contains a size–dependent correction induced by the surface. This leads to new channels of quasiparticle scattering accompanied by the emission of surface collective excitations. In noble–metal particles, the dipole collective excitations (surface plasmons) mediate a resonant scattering of d–holes to the conduction band. The role of this effect in the ultrafast optical dynamics of small nanoparticles is studied. With decreasing nanoparticle size, it leads to a drastic change in the differential absorption lineshape and a strong frequency dependence of the relaxation near the surface plasmon resonance. The experimental implications of these results in ultrafast pump–probe spectroscopy are addressed. We also discuss the size–dependence of conduction electron scattering rates.