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Silver ions are reduced in aqueous solutions that contain sodium polyacrylate or partly carboxylated polyacrylamide or glutaric acid. The reducing agent is the hydrated electron (generated γ-radiolytically) or hydrogen. Complexes of small silver clusters with the polyelectrolyte are formed in the early stages of reduction, the color of the solutions varying from rose to green or blue depending on the size of the complexed silver clusters. In the case of 10% carboxylated polyacrylamide, large colored complexes are not formed, which indicates that a high carboxylate density along the polymer chain is necessary for their formation. The complexes are stable toward heat (100°C) and oxygen. In the later stages of reduction, the long-wavelength absorptions of the silver-polyacrylate complexes disappear. The UV absorption bands of a magic cluster (probably Ag 9 + ) are present during a short time interval, until the plasmon absorption band of metallic nanoparticles develops at 380 nm. The colored clusters react with ammonia and cyanide to form large silver particles through the intermediate stage of the magic cluster.

Ag+ ions are reduced in aqueous solutions by a short pulse of high-energy radiation. At the low radiation dose applied, second-order reactions of the early intermediates are negligible. Three elementary processes occur during microseconds after the pulse, in each of which Ag+ is a reactant: eq-+ Ag+ -Ago (k = 4.8x 1O' O M-' s-l); Ago + Ag+ -Ag2+ (k = 8.5 x lo9 M-' s-l); Ag2+ -t Ag+ -Ag32+ ( k = 2.0 x lo9 M-' s-l). The reactions overlap temporally. Computer simulation is used to obtain the rate constants and the absorption spectra of the three species involved. Ag32+ has not yet been detected in water at ambient temperature; it was known to exist only in zeolite cages and frozen organic glasses.

Pulse radiolytic reduction of silver ions leads to an oligomeric cluster absorbing at 295 nm and most intensely and sharply at 325 nm. The cluster has a half-life of >5 min in scrupulously clean reaction vessels. During its decay, larger metallic particles absorbing around 380 nm are produced. The cluster is long-lived only in the presence of excess Ag+ ions. A second cluster absorbing at 345 nm is also formed, which lives for about 100 s. The 325-nm cluster is oxidized by O2, but larger particles are also formed during the oxidation. Tetranitromethane reacts with the cluster to form the nitroform anion, from which the absorption coefficient per Ag atom is calculated to be 2 X 104 M"1 cm-1. The formation of the cluster is accelerated by NaClOt he Bjerrum-Brdnstedt evaluation of this effect shows that a precursor of the cluster carries three elementary charges. The formation of larger metallic particles is even more strongly accelerated by NaCIO*. In the presence of 10~4 M Na2SC>4, the formation and decay of the cluster occur practically at the diffusion-controlled rate. From the rate of the buildup under these conditions, it is estimated that the cluster contains eight reduced silver atoms. The effects of ionic strength on the growth of the clusters and metallic particles are compared to the salt effects in slow and fast coagulation of colloids.