Halides surfaces

We have performed ionic mobility studies on mica and in alkali halide surfaces. Here we shall describe some results obtained on mica with different surface ions. Alkali halides will be discussed in detail in the next section. 

Large domains of oriented single crystals of poly(TCDU) and poly(DMDA) were produced on the alkali halide surface. Figures 1 and 2 show the typical elongated platelet morphology. Selected... 

The Ag electrodes were subjected to potential sweep ORCs at 10 mV s"1 in either 0.1 M KC1 or 0.1 M KBr from an initial potential of -0.30 V to more positive potentials. After ca. 30 mC cm-2 of anodic charge was passed, the direction of potential sweep was reversed to reduce the Ag halide surface to Ag metal. The roughened electrode was then removed under potential control at -0.30 V and immersed in the Pb2+-containing test solution for SERS studies. 

On comparing the Pseudocyanine spectra of Figures 1, 2, 3, and 5 with those obtained in concentrated aqueous dye solutions (12, 56, 57, 58, 59), obvious similarities of the resulting /-states will be noted. Present and earlier data make it apparent that electronic coupling between adjacent dye molecules can produce similar /-states, regardless of whether appropriate orientation and proximity of the molecules is caused by Coulombic attraction, as in salt formation with polymeric ions, or whether it is induced by van der Waals forces. The latter are primarily involved in the formation of dye aggregates in water as well as in the charge-independent adsorption of dye monolayers at silver halide surfaces (23, 46). 

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. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

Modification of Vibrational Spectra of Diatomic Molecules Induced by the Adsorption on Oxide and Halide surfaces A method for Probing the Structures of the Adsorption Sites and the Surface Morphologies of Sintered Materials... 

F. Acidic and Basic Properties of Oxide and Halide Surfaces... 

In neither case was it possible to propose definitive mechanisms due to the complexity of the systems in the 7-alumina study, it is suggested that adsorption-desorption processes are slow relative to rapid dismutation between two adsorbed species [105], while from the chromia study mono-molecular halogen exchange reactions with metal halide surface sites are indicated [38], The latter mechanism is reminiscent of the halogen exchange model proposed [95] for C2 CFCs on fluorinated chromia. 

Hydroquinone and A -methyl-/j-aminophenol (Metol) form a superadditive mixture which was shown by Tausch and Levenson [47] to involve the consumption primarily of hydroquinone with the preservation of Metol. This led to the regeneration theory proposed by Levenson, that Metol was acting as the developing agent at the silver halide surface and that oxidized Metol was reduced back to Metol by hydroquinone as outlined in Eqs. (30)-(33). 

Superadditivity has been observed in both chemical and physical development [52] thus the presence of silver halide is not a necessary condition for its occurrence but is likely to modify its detailed course. This would appear to rule out the charge-barrier theory in its original form, although charge effects might also occur at silver as well as at silver halide surfaces. 

Another method used involved the casting of a thin film of N P Cl from decane on a freshly cleaved alkali halide surface at room temperature, followed by heating to 130 C. The molten trimer film was then slow cooled to 110 C (4 C below the bulk melting temperature) and held at that temperature for 1 hour. 

Figure 6. A flat band representation of electron (left) and hole (right) injection from an excited dye molecule on a silver halide surface. The first step in both processes is excitation of the dye molecule. The second step is the transfer of the electron (left) that was excited up to the LUMO into the conduction band before radiative or radiationless decay. In the case of hole injection (right), the hole remaining in the HOMO after excitation is transferred to the valence band (equivalently an electron is transferred to the HOMO from the valence band).

The photon-induced dissociation and desorption of methyl bromide from a LiF surface was monitored [113]. Discuss the evidence that the photon energy was absorbed directly by the molecule adsorbed on the alkali halide surface. Would you expect the same photon-induced dissociation behavior if methyl bromide was chemisorbed on a transition metal surface ... 

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