  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 strong 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. The
size-dependence of conduction electron scattering rates is also discussed.
