SPECTROSCOPY AND DYNAMICS OF NANOMETER-SIZED NOBLE-METAL PARTICLES

Electron-phonon coupling in 11 +/- nm diameter Au particles and 10 +/- 3 nm and 50 +/- 10 nm Ag particles has been examined by ultrafast pum p-probe spectroscopy. The observed relaxation times are strongly depen dent 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 particle s, 1.1 +/- 0.1 ps for the 10 nm Ag particles, and 1.0 +/- 0.1 ps for t he 50 nm Ag particles. The measured relaxation times are similar to th ose for bulk metals, which implies that there are no size-dependent ef fects in the dynamics for particles in this size region. The transient absorption/ bleach recovery signals for the particles were modeled us ing the theory developed by Rosei et al. (Surf. Sci. 1973, 37, 689). T hese calculations yield the transient absorption spectrum as a functio n of the temperature of the electron distribution. The time dependence of the electronic temperature after pump laser excitation was calcula ted using the two-temperature model for electron-phonon coupling. The experimental signal versus time traces at selected wavelengths were th en simulated by combining the two calculations. The results from the s imulations are in semiquantitative agreement with the experimental res ults. In particular, the low-power relaxation times are correctly pred icted by the model calculations. At very high pump laser power(>5 mJ/c m(2)) the transient bleach signal for Ag shows an unusual 10 ps growth . This growth is attributed to either a change in the dielectric const ant of the surrounding medium due to heat transfer from the particles or thermally induced dissociation of adsorbed molecules.