Spectroscopy and Dynamics of Nanometer-Sized Noble Metal Particles

Abstract

Electron−phonon coupling in 11 ± 2 nm diameter Au particles and 10 ± 3 nm and 50 ± 10 nm Ag particles has been examined by ultrafast pump−probe spectroscopy. The observed relaxation times are strongly dependent on the pump laser power. At the lowest pump powers used, the time constants for relaxation are 0.8 ± 0.1 ps for the 11 nm Au particles, 1.1 ± 0.1 ps for the 10 nm Ag particles, and 1.0 ± 0.1 ps for the 50 nm Ag particles. The measured relaxation times are similar to those for bulk metals, which implies that there are no size-dependent effects in the dynamics for particles in this size region. The transient absorption/bleach recovery signals for the particles were modeled using the theory developed by Rosei et al. (Surf. Sci. 1973, 37, 689). These calculations yield the transient absorption spectrum as a function of the temperature of the electron distribution. The time dependence of the electronic temperature after pump laser excitation was calculated using the two-temperature model for electron−phonon coupling. The experimental signal versus time traces at selected wavelengths were then simulated by combining the two calculations. The results from the simulations are in semiquantitative agreement with the experimental results. In particular, the low-power relaxation times are correctly predicted by the model calculations. At very high pump laser power (>5 mJ/cm2) the transient bleach signal for Ag shows an unusual 10 ps growth. This growth is attributed to either a change in the dielectric constant of the surrounding medium due to heat transfer from the particles or thermally induced dissociation of adsorbed molecules.