Sensitization Centers

Tani, T., and Murofushi, M. (1994) Silver microclusters on silver halide grains as latent image and reduction sensitization centers. /. Imag. Sci. Technol. 98, 1. 

Therefore an increase in conductivity upon illumination (photoconductivity) can be due to either an increase in carrier concentration and/or an increase in mobility. In general, it is believed that an increase in carrier (hole) concentration is the dominant cause for room-temperature photoconductivity for the lead chalco-genides and that an increase in mobility becomes increasingly important at low temperatures. The dark conductivity of films deposited with or without added oxidant were similar the difference in photoconductivity between them was ascribed to the formation of sensitizing centers (interband states) due to the oxidant. 

A stereotyped compulsive behavior is induced both in humans and in laboratory animals by amphetamines. This provided the basis for a method that has been used to measure the action of drugs on amphetamine-sensitive centers of the brain. A lesion in the nigrostriatal bundle on one side of a rat brain was made by injection of a neurotoxic compound such as 6-hydroxydopamine. This caused degeneration of dopamine-containing neurons on one side of the brain. When rats that had been injured in this way were given amphetamines, they developed a compulsive rotational behavior. Administration of chlorpromazine and several other antipsychotic drugs neutralized this behavior and in direct proportion to the efficacy in clinical use, an observation that also supports the theory that schizophrenia involves overactivity of dopamine neurons. 

There is no unambiguous evidence that at least the great majority of reduction sensitivity centers formed at low to moderate levels of sensitization act in the same way as latent subimage centers. Even at high levels of sensitization, only a minority of the centers may be able to trap electrons, and these may already be fog centers which would detract from rather than add sensitivity. 

After growth to a larger size they cause fog." Experiments by Roth and Simpson (137) and by Corbin and associates (138) likewise support a stepwise formation of the sensitivity centers. Mumaw (139) observed photoluminescence peaks which he attributed to silver sulfide islands on the grain surface. 

They reasoned that in the vicinity of a silver sulfide speck the conduction band is bent downward and photoelectrons could reach the surface of the crystal more easily, and hence more readily form surface image. However, the thickness of the silver sulfide layer in their experiments was about 4 pm, which far exceeds that of any sensitivity center on emulsion grains. The relevance of their observation to S-sensitization of emulsions is doubtful. Starbov (147) found no evidence of change in the surface potential when either the (111) or the (200) surface of an evaporated silver bromide layer was sulfur-sensitized. 

Pitt, Rachu, and Sahyun (167) proposed a multistep formation of sensitivity centers, according to which "the initial formation of gold silver sulfide. .. 

Sulfur sensitization does not change the number of latent image centers formed per grain for low irradiance of the mono-disperse fine-grain emulsions. Sulfur sensitivity centers cannot be deep enough to affect the chance establishment of a single stable latent subimage center. In coarse, polydisperse... 

Aggregates of other impurities, such as chemical sensitization centers. The addition of Ag atoms to them will also reduce the free energy of the whole system. 

In addition to determining the distances Rgft from lifetimes, the concentration [S] of the sensitizer centers can be obtained from the measured intensities and lifetimes. Denoting the illumination intensity as I nc at a steady state there is a balance between the rate of S+S excitation, n lincCs], where n is... 

Because the activator is formed in this system at the eiqpense of the sensitizer (the activator centers are oxidized sensitizer centers), its relative concentration is equal to 1 - [Sl/lS]. ... 

A second important decay mechanism for the iridium-associated trapped electron center involves the thermally assisted excitation of the trapped carrier to the conduction band. It is retrapped many times at other Ir3 + sites, so that the time before the electron is permanently annihilated by silver formation at an intrinsic sensitivity center or recombination is increased. When this decay mechanism predominates, the addition of Ir3 + reduces the emulsion s high intensity reciprocity failure (see Section I.B). 

Sharma, J., Forbes, J.W., Coffey, C.S., and Liddiard, T.P. (1987) The Physical and Chemical Nature of Sensitization Centers Left from Hot Spots Caused in Triaminotrinitrobenzene by Shock or Impact Journal of Physical Chemistry 91, 5139-5144. 

J. Sharma, J.W. Forbes, C.S. Coffey, J.P. Liddard, "The Nature of Reaction Sites and Sensitization Centers in TATB and TNT", in Shock Waves in Condensed Matter, Eds. S.C. Schmidt, N.C. Holmes, Elsevier Science Publishers, Amsterdam, 1987, pp 565—568. 

Sharma, J., and Beard, B.C., (1990) "XPS Study of Hot Spots and Sensitization Centers in Energetic Materials". This Volume... 

XPS STUDY OF HOT SPOTS AND SENSITIZATION CENTERS IN ENERGETIC MATERIALS... 

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