ELECTRON DYNAMICS OF PASSIVATED GOLD NANOCRYSTALS PROBED BY SUBPICOSECOND TRANSIENT ABSORPTION-SPECTROSCOPY

The electronic dynamics of gold nanocrystals, passivated by a monolaye r of alkylthiol(ate) groups, were studied by transient spectroscopy af ter excitation with subpicosecond laser pulses. Three solution-phase g old samples with average particle size of 1.9, 2.6, and 3.2 nm with si ze distribution less than 10% were used. The photoexcitation in the in traband (surface plasmon region) leads to the heating of the conductio n electron gas and its subsequent thermalization through electron-elec tron and electron-phonon interaction. The results are analyzed in term s of the contribution of the equilibrated ''hot'' electrons to the sur face plasmon resonance of gold. A different spectral response was obse rved for different sizes of gold nanoparticles. The results were compa red to the dynamics of the large (30 nm diameter) gold nanocrystals co lloidal solution. The size-dependent spectral changes are attributed t o the reduction of the density of states for small nanoparticles. The observed variation in the kinetics of the cooling process in gold nano particles with changing the laser intensity is attributed to the tempe rature dependence of the heat capacity of the electron gas.