Correlation spectroscopy based on non-linear response of silver colloids (including SEHRS)

The non-linear response (second harmonic generation, SHG, hyper-Rayleigh scattering, HRS, surface-enhanced hyper-Raman scattering, SEHRS, and continuum generation) of two different types of silver colloids is compared by stationary and correlation spectroscopy. Employing a poly-disperse colloid prepared after the protocol of Lee and Meisel we found that the efficiency of all types of non-linear response is greatly enhanced if the colloid is 'activated' by addition of chloride ions. This activation is also necessary to observe SEHRS with both the Lee-Meisel and the mono-disperse colloid prepared by hydrazine reduction. The correlation curves of both types of colloid show one step (r(1/2) similar to 10 ms) which can be associated with lateral diffusion of the individual particles. Its r(1/2)-value is larger for the poly-disperse colloid, which contains larger particles. In addition, we find a second step, its relative amplitude being dependent on experimental parameters, whose r(1/2)-value is, however. essentially the same for all samples investigated (r'(1/2) similar to 50 mus). We assign this correlation time to processes that lead to a restructuring of the surface and the formation and destruction of so-called 'hot spots'. Under optimum condition, the efficiency for all non-linear process's connected with one such 'hot spot' is extremely high. 'Hot particles' contain at least one hot spot and can therefore dominate the non-linear signal without the need of aggregation (field enhancement in the gap between particles). (C) 2004 Elsevier B.V. All rights reserved.