Silver halides recombination

In summary, there is evidence that the multitalented sulfur sensitization product can trap electrons, can trap holes to reduce recombination, can stabilize photolytic silver atoms, and can accelerate reduction sensitization. Conceivably, each of these properties could be of importance for latent image formation under at least some conditions. The silver sulfide centers are not of uniform size, they probably are not uniformly related energetically to the silver halide matrix, and they may differ in chemical consititution. 

Photoinduced electron injection is by no means a new development. This process has already been applied in areas such as silver halide photography. In this discussion, only sensitized TiC>2 surfaces will be considered. Many experiments have shown that the charge injection into the semiconductor surface is very fast. In order to study these processes, fast spectroscopic techniques are preferred. Whether or not charge injection takes place can be studied conveniently on the nanosecond time-scale by using transient absorption spectroscopy. However, to address the injection process directly, experiments are carried out on the femtosecond time-scale, while recombination and charge separation require the nanosecond to microsecond range. 

Another important contribution of radiation chemistry in photography was the enhancement of the sensitivity of photographic emulsions. The primary effect of photon absorption by silver halides is the formation of an electron-hole pair. However, because of the very fast and efficient electron-hole recombination and oxidation by hole of the newly formed silver atoms, the conversion yield of light is very low. The analogy with HO oxidation processes occurring in irradiated solutions led to the use of the same scavenging method to inhibit the electron-hole pair recombination and the oxidation by the... 

Donors and acceptors exist in silver halides both as intrinsic and extrinsic centers. Ionized donors have been identified as interstitial silver ions and substitutional uncompensated divalent cation impurities from their IR spectra [77-79]. Ionized acceptors are probably halide ions surrounding silver ion vacancies [74,80,81] and possibly some incompletely compensated substitutional divalent anion impurities such as sulfide or selenide. Carriers trapped at donors and acceptors can undergo radiative recombination by tunneling if the spatial separation of the donor-acceptor pair is not too large [6,82], The emission energy of a donor-acceptor (D-A) pair separated by a distance r in an isotropic medium is given by... 

In the silver halides Mott and Gurney suggested a mechanism for the formation of colloidal Ag [167]. A conduction-band electron produced by irradiation is first trapped at a lattice imperfection which may be a silver atom or ion, a chemical impurity, or a trapping site along a dislocation. The trapped electron then attracts a Ag interstitial ion to form a Ag atom. Following this, electrons and Ag " interstitials are trapped at the site in proper sequence to cause the buildup of a colloidal silver particle. This mechanism requires the presence and mobility of silver ions, and it is further required that the hole motion be sufficiently small that trapped electrons are not annihilated by electron-hole recombinations. 

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