PICOSECOND DYNAMICS OF COLLOIDAL GOLD NANOPARTICLES

Colloidal gold nanoparticles with an average radius of 15 nm have a su rface plasmon absorption band at 530 nm. Excitation by laser pulses of 450 fs duration, and wavelength of 600 or 380 nm ''bleached'' the pla smon band and produced a transient absorption at the wings of the ''bl each'' spectrum. The transient absorption was found to have a similar temporal behavior at different wavelengths. Analysis of their temporal behavior showed two time constants: 2.5 ps, and a slower component of > 50 ps. Laser excitation close to the plasmon band at 600 nm leads t o the formation of ''hot'' non-Fermi electronic distribution within th e colloidal particles. Transient absorption from these ''hot'' electro ns led to different absorptions from that of the plasmon absorption of ''cold'' electrons. The ''hot'' electrons relax via electron-phonon c oupling in 2.5 ps, and the phonon-phonon relaxation of the lattice occ urs in > 50 ps. At 380 nm excitation, the amplitude of the blue wing b ecomes smaller, and the slow component becomes longer, which could be due to possible excitation of the d-band electrons. These results are discussed in terms of Mie theory and a two-temperature model (TTM), an d their consequences on the optical absorption spectrum.