Electronic and vibrational dynamics have been studied in noble metal nanoparticles with high sensitivity femtosecond pump-probe techniques using a Ti :Sapphire oscillator. The electron-lattice thermalization has fi rst been studied. The evolution from a weak excitation to a strong excitation regime has been analyzed performing measurement as a function of the pump pulse energy. In the former case, the measured characteristic times are independent of the pump power and decrease with the particle size for diameters smaller than 10 nm. This acceleration of the energy transfer from the electron gas to the lattice has been ascribed to the reduction of the screening of the coulomb interaction close to the surface. The acoustic vibrations of metal nanoparticles have then been studied. Contribution of higher order modes to the time-domain signal has been demonstrated. Optical control of the acoustic motion of the particles has permitted selective investigation of the n = 1 radial mode and direct determination of its characteristics, period and damping time. Finally, a new optical method to detect individual nanoparticles and quantitatively measure their extinction cross section has been developed. This method consists in periodically modulating the position of a sample made of nanoparticles deposited on a glass substrate with low density. Absorption by a single nanoparticle of a tightly focused laser beam leads to a modulation of the transmitted energy, that is detected by a lockin ampli fier. The extinction cross section of single gold nanoparticles of diameters down to 5 nm has thus been determined. The obtained values are in good agreement with those calculated from the Mie theory using an e ffective dielectric constant approach.
