Silver halides electron trapping

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

The majority of inorganic systems reported to exhibit photochromism are solids, examples being alkali and alkaline earth halides and oxides, titanates, mercuric chloride and silver halides.184 185 The coloration is generally believed to result from the trapping of electrons or holes by crystal lattice defects. Alternatively, if the sample crystal is doped with an impurity capable of existing in variable oxidation states (i.e. iron or molybdenum), an electron transfer mechanism is possible. 

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. 

Rhodium, incorporated in the silver halide grains, decreases sensitivity and increases contrast. This action has been attributed to depression of latent image formation because of deep electron trapping by the trivalent rhodium ion (183-185). Eachus and Graves (184) showed that rhodium, probably as a complex, acts as a deep trap for electrons at room temperature. Weiss and associates (186) concluded that the rhodium salts introduce deep traps for both electrons and holes. Monte Carlo simulation showed that the photographic properties could be accounted for in this way over a wide range of exposure times. 

For an electron to be transferred from an excited dye molecule to the silver halide conduction band, the excited level should be either above the bottom of the conduction band or near enough to it for thermally assisted transfer to occur during the normal lifetime of the excited state of the dye. If the lowest vacant level of the unexcited dye is below the bottom of the conduction band, transfer of an electron from the conduction band to the unexcited dye molecule becomes possible. This "electron trapping" by the dye could be a factor in the desensitizing action of the dye. 

Some excited dye molecules can inject holes into the valence band of the silver halide. Photohole injection has been demonstrated experimentally by the photobleaching of chemically produced fog silver (248) to produce direct positive photographs. This action depends on the favorable location of the SQ ground state of the dye relative to the valence band of the silver halide. The injection is accomplished by the transfer of an electron from the valence band to the vacant SQ level of the excited dye molecule, leaving a mobile hole in the valence band which can oxidize a silver atom. Conversely, if the SQ level of a dye molecule is located sufficiently above the top of the valence band, a mobile hole in the silver halide can become trapped by the nonexcited dye. Hole-trapping by dyes has been detected by ESR signals (249,250). 

In addition to the question of reversibility, there remains the question of the extent to which the potentials measured in solution are modified by adsorption and aggregation at the silver halide surface. No measurements are available for the potentials of the adsorbed molecules or aggregates. The adsorbed reduced form of the dye molecule would correspond to the radical formed when an electron from the conduction band is trapped by the ground state dye, or when the excited dye molecule injects a hole into the valence band. In each, the dye molecule acquires an additional electron. 

For Class 1 dyes, sensitization occurs by transfer of the electron from the state of the excited dye to the conduction band of the silver halide. The hole created by the transfer remains in the dye molecule. If the hole is not too deeply trapped, it may eventually escape into the valence band with the aid of thermal energy. These dyes provide few, if any, electron traps and desensitization by oxygen/moisture in their presence would equal that for the undyed emulsion. 

Figures 49(c) and 49(d) show two other important processes which can occur when the silver halide is excited directly in the presence of adsorbed dyes. In these cases an electron is transferred from the VB to the CB upon excitation, and the holes in the VB may be filled by electron transfer from the HOMO of the adsorbed dye. The product of this process in Figure 49(c) is the same as that from the electron-injection dye sensitization in Figure 49(a), i.e., a dye radical cation and a conduction band electron which may be trapped and contribute to latent image formation. Illustrated in Figure 49(d) is the consequence of excitation of silver halide in the presence of a dye in which the energy of the LUMO is lower than that of the CB. In this case, direct excitation of the silver halide results in a conduction band electron which can be transferred to the LUMO of the dye. Subsequent electron transfer of an electron from the HOMO of what would then be a dye radical anion results in effective deactivation of the band-gap excitation, and overall reduced photographic sensitivity of the silver halide toward direct excitation due to the presence of the dye. This process is known as dye desensitization.

Figure 52. Hole-trapping mechanism for supersensitization. The super-sensitizer, SS, transfers an electron to the excited state of the dye to form the radical anion of the dye, before electron transfer occurs to the silver halide. The radical anion subsequently injects the electron to the CB [207].

Calculations have shed light on the mechanism of formation of silver particles by the action of light on silver halide. We find that surface defect sites present a particularly favorable trapping level for electrons. After this trapping step, silver ions diffuse to the electron, forming a deeper trap for additional electrons. 

Figure 1. Mechanism of latent-image growth in silver halide. Key e, electrons Ag, interstitial silver ion h , holes T, traps and S, special sites.

An electron in a solid behaves as if its mass [CGS units are used in this review the exception is for the tabulation of effective masses, which are scaled by the mass of an electron (m0), and lattice constants and radii associated with trapped charges, which are expressed in angstroms (1A = 10 8 cm)] were different from that of an electron in free space (m0). This effective mass is determined by the band structure. The concept of an effective mass comes from electrical transport measurements in solids. If an electron s motion is fast compared to the lattice vibrations or relaxation, then the important quantity is the band effective mass (mb[eff]). If the electron moves more slowly (most cases of interest) and carries with it lattice distortions, then the (Frohlich) polaron effective mass (tnp[eff]) is appropriate [11]. The known band effective and polaron effective masses for electrons in the silver halides are listed in Table 1. The polaron and band effective masses are related to a... 

When shallow Coulomb trapping centers are introduced into the bulk of a silver halide crystal, the electron is presumed to spend some time in the conduction band and some time in the shallow traps. If the electron is lost only out of the conduction band and no other loss processes are introduced, then the time before final trapping is lengthened by the time spent in the shallow traps. The number of trapping events (Z) during an electron s free time (<) in the conduction band is given by... 

Shallow traps for electrons were first detected experimentally in nominally pure silver halides by Brandt and Brown [77] using UV-induced IR absorption spectroscopy. This and subsequent optical and magnetic resonance studies demonstrated that, when filled, such traps are paramagnetic,... 

After relaxation to the Is ground state, the electron may still be thermalized. If this process is dominant, as it is for Pb2+ and Cd2+, the impurity then behaves as a shallow electron trap affecting both the rate at which electrons are made available for latent image formation and their location in the silver halide lattice. 

To illustrate ligand and lattice effects more clearly, a reasonably comprehensive set of kinetic data for a range of transition-metal complexes in the silver halides is presented in Table 11. It is clear from the results included for Ir3 + and Rh3 + that aquation leads to a significant increase in the lifetime of the trapped electron state. This must cause important changes in the photographic effects associated with the addition of a particular impurity ion. In the case of Ir3+, such differences in lifetime do not appear to result from a... 

TABLE 11 Kinetic Data for Various Transition-Metal Electron Traps in the Silver Halides Determined by Kinetic EPR Spectroscopy... 

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