Size dependent properties of Au particles: Coherent excitation and dephasing of acoustic vibrational modes

Ultrafast laser spectroscopy has been used to characterize the low frequenc y acoustic breathing modes of Au particles, with diameters between 8 and 12 0 nm. It is shown that these modes are impulsively excited by the rapid hea ting of the particle lattice that occurs after laser excitation. This excit ation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to t he lattice via electron-phonon coupling. The measured frequencies of the ac oustic modes are inversely proportional to the particle radius; a fit to th e data for the different sized particles yields <(nu)over bar>(R) = 0.47c(l )/Rc, where R is the particle radius, c(l) is the longitudinal speed of sou nd in Au, and c is the speed of light. This functional relationship exactly matches the prediction of classical mechanics calculations for the lowest frequency radial (breathing) mode of a free, spherical particle. The invers e dependence of the frequency on the radius means that the modulations are damped for polydisperse samples. Analysis of our data shows that this inhom ogeneous decay dominates the damping, even for our high quality samples (8% -10% dispersion in the size distribution). The size dependence of the elect ron-phonon coupling constant was also examined for these particles. The res ults show that, to within the signal to noise of our measurements, the elec tron-phonon coupling constant does not vary with size for particles with di ameters between 4 and 120 nm. Furthermore, the value obtained is the same a s that measured for bulk gold. (C) 1999 American Institute of Physics. [S00 21-9606(99)70542-8].