Dose rate effect on size of CdS clusters induced by irradiation

Size-controlled CdS clusters, with subnanometric radii, have been produced by gamma-irradiation of aqueous solution containing Cd2+ and thiol. Absorption and fluorescence spectra show that the mean CdS cluster size increases in the distribution with the dose or the post-irradiation time. (On the contrary, at a given dose, the effect of the dose rate has been clearly demonstrated to lower the cluster size. The mechanism of CdS cluster growth is discussed via coalescence reactions. A numerical simulation taking into account the coalescence reactions between monomers produced by irradiation and then between clusters of any size is presented. Calculations show how the final nuclearity depends on the dose rate effect. The observation of isosbestic points during the slow coalescence (100 M-1 s(-1)) allowed us to derive the absorption spectra of oligomeric CdS clusters, successively formed with maxima at 270, 282 and 320 nm, respectively. At high dose rate and low dose, only the smallest clusters of CdS absorbing at 270 nm land never observed previously) are formed. (C) 2000 Elsevier Science Ltd. All rights reserved.