Spectroscopic studies of silver(0) centres formed radiolytically in water-ethanol solvents at 4 and 77 K

Exposure of dilute solutions of silver perchlorate in water-ethanol systems to 60Co γ-rays or X-rays at 4 and 77K gave Ag(II) and Ag(O) centres, identified by e.s.r. spectroscopy. The isotropic hyperfine coupling is ca. 90% of the atomic value in these media. This sudden change is attributed to a phase transition. On annealing the solutions to 77 K, or on irradiating at 77K, a range of different species were formed. Formation of the atomic species competes with formation of Ag2+ and Ag43+ centres, which tend to dominate in ethanol-rich systems. Concurrent optical studies of the systems irradiated at 77K show that there is a marked shift in the first absorption band to low frequencies for the major primary centres relative to the atomic value. This is greatest for the aqueous centres, and there was a clear shift towards the pure atom value as the ethanol concentration was increased. Similarly, on annealing the solutions above 77K the band narrowed and shifted towards the gas-phase frequency. These results show that even when the values for Aiso(Ag) decrease, the centres nevertheless become more atom-like in their optical spectra. This apparent contradiction is discussed in terms of loss of solvation of the parent Agsolv+ ions. For strongly aqueous systems it is suggested that on annealing, only one water molecule is partially or completely lost, leaving an asymmetric centre. For more ethanolic, glassy systems, there is a general loosening of bonding as the solvent molecules move away from silver, the process of solvent loss being catastrophic once it is thermally initiated. The resulting atoms are either trapped or move through the glass and react with Ag+ ions to give Ag2+ units. (author)