Enhancement of the second harmonic response by adsorbates on gold colloids: The effect of aggregation

The extinction profiles as well as the hyper Rayleigh scattering signal of a highly monodisperse 22 nm diameter gold colloidal sol in the presence of pyridine have been measured. A strong enhancement of the HRS signal is obse rved in the presence of small amounts of pyridine. The relationship between the extinction spectra, the distribution of aggregated particles, and the second harmonic response has been investigated by evaluating the enhancemen t factor of the electromagnetic field in the colloids as they slowly aggreg ate. It is also shown that at larger pyridine concentrations, the hyper Ray leigh scattering signal levels off owing to the formation of large aggregat es.