Femtosecond transient-absorption dynamics of colloidal gold nanorods: Shape independence of the electron-phonon relaxation time

We studied the femtosecond dynamics of colloidal gold nanorods encapsulated in micelles after excitation with 400 nm pulses of 100 fs duration. It is found that the laser heating of the electron gas of gold nanorods with an a verage aspect ratio of 3.8 leads to the bleaching of both the transverse an d longitudinal mode of the surface plasmon oscillation at 520 and 750 nm. T he bleach recovers with the same time constant for both the transverse and longitudinal oscillation, for gold nanodots prepared by photothermal reshap ing of the rods as well as for nanodots synthesized chemically by citrate r eduction (and known to have twin boundaries and surface defects). Since the bleach recovery on the 3 ps time scale is assigned to electron-phonon rela xation processes, these results suggest that phonon dependent relaxation pr ocesses in gold nanoparticles are independent of the shape, size, type of t he surfaces, or the mode of the surface plasmon, oscillation excited. The f act that the mean free path of the electron in metallic gold is in the nano meter length scale (similar to 50 nm) raised the question of the importance of surface scattering to the electron-phonon relaxation process in gold na noparticles. Our previous studies showed little dependence of the relaxatio n rate of the size of gold nanodots (from 9 to 48 nm). In the present study , the electron-phonon relaxation is measured in gold nanorods, which have d ifferent facets from those of gold nanodots.