MECHANISMS OF SPECTRAL SENSITIZATION OF SILVER-HALIDES - ROLE OF SENSITIZING DYE COMPLEXATION

We have observed concurrent processes of photoexcited dye deactivation and silver(0) cluster formation in real time under conditions of laser flash photolysis of a merocyanine dye adsorbed to an AgBr nanosol. To our Knowledge this is the first time such a comprehensive view of the process of spectral sensitization has been obtained experimentally. Spectral sensitization of AgBr is apparently biphotonic under our conditions; this result, along with observed kinetics of dye ground state repopulation and of silver(0) cluster growth, is consistent with Mitchell's mechanism of spectral sensitization, but not with single-electron transfer or radical pair mechanisms. Control experiments revealed a modicum of photolytic reactivity for undyed nanosol with 2.35 eV photons and suggested operation of an Auger mechanism of photoelectron generation. In this case amplified stimulated emission was observed from photogenerated silver clusters, Ago,, (or a byproduct) at photon energies comparable to those that produce the Herschel effect in conventional photography. We speculatively estimate nuclearity of the silver clusters produced under conditions of our experiments as n = ca. 12.