Silver-centered excited state

The formation of color centers in the alkali halides, especially silver, has been studied extensively and in great detail, in an attempt to understand the photographic process. At least half a dozen color centers have been identified in these materials, of which the most widely studied is probably the F center, defined as an electron trapped at an anion vacancy. The name comes from the German word for color Farbe. In the case of KBr, the F center (Fig. 16.7) is believed to be an electron trapped at a bromine vacancy. The F center can be modeled by assuming the electron is trapped in a box of side d, which scales with the lattice parameter of the alkali halide. The F center transition is believed to be between the ground and first excited state of this particle in a box. This model, while crude, qualitatively explains the data for some of the alkali halide F center spectra. 

The assignment that the emissions arise from a LMCT [E M4] excited state with mixing of a metal-centered d sjd p) state has been substantiated by ab initio and Fenske-Hall molecular orbital calculations performed for the silver(I) clusters. These results revealed that the HOMOs of the clusters are principally Ag—E bonding orbitals, while the LUMOs are metal-localized orbitals with predominantly 55 and 5p character. Furthermore, the calculated HOMO-LUMO energy gaps decrease in the order (13a) > (13b) > (13c), consistent with the observed trend for the emission energies of the complexes. 

The idea of metal-metal bond establishment in the excited state of this complex has also stimulated the interest on the photophysical and photochemical investigations of related d -d complexes. In 1970, Dori and co-workers first reported the luminescence properties of phosphine complexes of d metal centers such as copper(I), silver(I), gold(I), nickel(0), palladium(0), and platinum(0) [12]. Later in 1985, Caspar reported the luminescence properties of polynuclear nickel(O), palladium(O), and platinum(O) complexes of phosphine, phosphite, and arsine [13]. Similar to the related d -d systems, the emissive states of [Pd2(dppm)3] and [Pd Cdpam),] have been suggested to be metal-centered (d-p) in nature modified by metal-metal interaction. 

To examine the role of the LDOS modification near a metal nanobody and to look for a rationale for single molecule detection by means of SERS, Raman scattering cross-sections have been calculated for a hypothetical molecule with polarizability 10 placed in a close vicinity near a silver prolate spheroid with the length of 80 nm and diameter of 50 nm and near a silver spherical particle with the same volume. Polarization of incident light has been chosen so as the electric field vector is parallel to the axis connecting a molecule and the center of the silver particle. Maximal enhancement has been found to occur for molecule dipole moment oriented along electric field vector of Incident light. The position of maximal values of Raman cross-section is approximately by the position of maximal absolute value of nanoparticle s polarizability. For selected silver nanoparticles it corresponds to 83.5 nm and 347.8 nm for spheroid, and 354.9 nm for sphere. To account for local incident field enhancement factor the approach described by M. Stockman in [4] has been applied. To account for the local density of states enhancement factor, the approach used for calculation of a radiative decay rate of an excited atom near a metal body [9] was used. We... 

Belous et al. [238] investigated the role of impurities as they affect luminescence and photolysis of silver azide. Subsequent to X-ray irradiation, luminescence appeared on UV excitation and was associated with the same interstitial Ag ions which influence the photolysis kinetics. By studying the growth of luminescence, it was concluded that the migration of energy to centers where decomposition occurs is via excitons, presumably excited anion states. 

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