While reading up on SHSG this morning I ran across a paragraphy in BJelkhagen's Silver Halide Recording Materials pg 262 about increasing the sensitivity of DCG by adding fine silver-halide grains to the gelatin by bathing a prepared plate in a silver-nitrate solution.
I must say this doesn't make a lot of sense to me. I'm assuming that when he says that the "gelatin emulsion" is sensitised, he must mean a DCG emulsion, not just gelatin. He then presumably adds Silver Halide to a DCG emulsion and an "optical sensitizer" whatever that is - a dye? If the gelatin is pure gelatin, then he seems to be just making a fine-grained emulsion in situ. However, the exposure of 1 J/cm^2 sounds odd. The sensitivity of 8E75 was about 50 microJ/cm^2 and Slavich seems to be about 200 microJ/cm^2. DCG has a sensitivity anywhere from about 25 mJ/cm^2 to 200 mJ/cm^2. 1 J/cm^2 is 20,000 times more insensitive than Slavich and 20 times more insensitive than DCG. How this is supposed to be "...a method for increasing the sensitivity of regular dichromated gelatin..." is beyond me. Clearly I'm not seeing something.
Dinesh: "I must say this doesn't make a lot of sense to me. I'm assuming that when he says that the "gelatin emulsion" is sensitised, he must mean a DCG emulsion, not just gelatin. He then presumably adds Silver Halide to a DCG emulsion and an "optical sensitizer" whatever that is - a dye?"
Actually, it's the other way around: The Bulgarian group (Mazakova et al, "Dichromated gelatin for volume holographic recording with high sensitivity", 1982)) first produced a very (low silver) silver halide emulsion which is then sensitized in an ammonium dichromate solution (to which an optional dye for longer wavelength sensitization can be added). The following mechanism is involved:
"If small quantities of silver halide are present in the layer, besides the photoinduced dichromate reduction on exposure, silver halide photolysis takes place. If the resulting photolytic silver is oxidized by the dichromate, leaving Cr3, this could lead to more effective reduction and hence to better hardening of the illuminated areas of the light-sensitive layer. (...)
Experiments on the increase in sensitivity of the dichromated gelatin were conducted on layers produced by coating gelatin solution containing Cr(Ac)3 hardener, in a concentration of 1g in 100cm3 gelatin, on to a glass plate. A certain amount of silver nitrate is introduced into the gelatin layer before the coating, or in the dry layer by bathing. After washing with distilled water the layer is bathed in alkali halide solution. In this way superfine grains of silver halide are formed. After washing away the residual halide the layer is sensitized by bathing in a solution of dichromate and optical sensitizer of silver halide. A 1.2-dimethyl-3/3-methylpromedinidene-kerylidine sensitizer was employed." (Mazakova et al)
Dinesh: "If the gelatin is pure gelatin, then he seems to be just making a fine-grained emulsion in situ. However, the exposure of 1 J/cm^2 sounds odd."
The 1 J/cm^2 exposure time simply refers to illustrate how little speed/density the silver halide emulsion (without dichromate sensitization) yields.
For DCG speed is much better with this system: "The measured diffraction efficiency of the transmission and reflection holograms is greater than 50% for energies greater than or equal to 6 mJ" (at 488 nm).
Got it. Thanks. This raises an interesting question:
"If the resulting photolytic silver is oxidized by the dichromate, leaving Cr3, this could lead to more effective reduction and hence to better hardening of the illuminated areas of the light-sensitive layer. (...)"
Presumably the Dichromate is also photolytically reduced at the same time during exposure. Presumably also not all the Dichromate is reduced photolytically since otherwise the chemical reduction of the Dichromate with reduced Silver could not take place at the exposed sites. This also may not be 100%. So how what proportion of the Dichromate is reduced photolytically, what proportion is reduced by chemical action with Silver and what proportion remains? Can the remaining portion be further reduced by chemical action?
"If the resulting photolytic silver is oxidized by the dichromate, leaving Cr3, this could lead to more effective reduction and hence to better hardening of the illuminated areas of the light-sensitive layer. (...)"
"
Presumably the Dichromate is also photolytically reduced at the same time during exposure. Presumably also not all the Dichromate is reduced photolytically since otherwise the chemical reduction of the Dichromate with reduced Silver could not take place at the exposed sites. This also may not be 100%."
Seems true, they say that silver halide crystals must be superfine and in a quantity such as not to deteriorate by scattering of the recorded interference pattern...
This is a direct sensitization, the small reduced Ag formed (with the help of sensitizer) will recuce the dichromate in situ, no silver halide development.
"So how what proportion of the Dichromate is reduced photolytically, what proportion is reduced by chemical action with Silver and what proportion remains? Can the remaining portion be further reduced by chemical action?"
Not all dichromate is reduced by the silver but the signal to noise is very good, I think total red sensitization with silver halide (perhaps increasing molar concentration) is possible, One reference (Oster) claims that direct dichromate sensitization with dye is possible, but an attempt were unsuccessful.
Perhaps the diffusion method could be applyed here with a mixed silver DCG process even more simple than SHG.
My reading of this paragraph ("If the resulting photolytic silver is
oxidized by the dichromate, leaving Cr3, this could lead to more effective
reduction and hence to better hardening of the illuminated areas of the
light-sensitive layer") suggests the bleaching of the latent silver image. However, as far as I know dichromate does not destroy the latent image (as more potent oxidizers like persulfate would do). So one might wonder what's actually happening here. Another difficulty is the dubious nature of silver formed for the latent image. Obviously, its amount must be tremendously small. Hence the quantity of Cr3 thus directly generated, will be almost negligible. For that reason I would rather believe in a catalyzing effect by the silver.
" However, as far as I know dichromate does not destroy the latent image (as more potent oxidizers like persulfate would do)."
Here the latent image is direct "oxidized" by the dichromate helping the crosslink effect, because the dichromate solution and fixer used to develop is exactly the standard normal dichromate development, on other side the liquid dichromate solution may react with silver to reduce the first locally.
"So one might wonder what's actually happening here. Another difficulty is the dubious nature of silver formed for the latent image. Obviously, its amount must be tremendously small."
I agree, the ammount is small compared to necessary index modulation even after development..
"Hence the quantity of Cr3 thus directly generated, will be almost negligible. For that reason I would rather believe in a catalyzing effect by the silver."
In this case the silver is oxidized locally in the development stage(!) catalyzing the Cr6 reduction again locally, I wonder if this process could be optimized, by using only dichromate bath solution to crosslink gelatin.
"In this case the silver is oxidized locally in the development stage(!) catalyzing the Cr6 reduction again locally, I wonder if this process could be optimized, by using only dichromate bath solution to crosslink gelatin."
"So one might wonder what's actually happening here. Another difficulty is the dubious nature of silver formed for the latent image. Obviously, its amount must be tremendously small."
Do you believe that the grain is small? In other words, is it that the proportion of the area of the reduced silver speck is large relative to the area of the speck? I was wondering what made the reaction go since the redox potential seems too high for spontaneous reduction at room temps. Either the silver is acting as a catalyst or there is adsorption on the specks and the larger surface area of the silver relative to the speck is causing the reaction.
But the speck is formed only with those 4 silver atoms?Those silver atoms could not reduce spontaneously the dichromate? I think this is a delicate point because some I do not see if the developer(dichromate) could "develop" or reduce the silver atoms agregates or catalize some reduction reaction with the acid pH..
I was a little confused, the silver halide upon excitation by the light or energy from the dye is reduced with the Br ion to the silver atom, if it is correct the silver cluster is very small and must interact by collision effect to directly reduce the dichromate ion. I don't believe that these small coupled single atoms could do the job of gelatin crosslinking with efficiency directly compared to the Cr6 present.. so what in this porcess could work as developer (silver halide) agent? The Kodak fixing bath, after the dichromate bath could catalize the silver halide reduction on the rest of the tiny grain?
Dinesh: "Do you believe that the grain is small? In other words, is it that
the
proportion of the area of the reduced silver speck is large relative to the
area of the speck?"
Yes, I have come to think this system only works with ultra-small grains (5
nm or even smaller). The paper says:
"The silver halide crystals must be superfine and in a quantity such as not to deteriorate by scattering the contrast of the recorded interference pattern (...)."
Based on the diffusion method, at low silver
content ultra-small grains can be easily achieved, I believe. .
Dinesh: "Either the silver is acting as a catalyst or there is adsorption on
the
specks and the larger surface area of the silver relative to the speck is
causing the reaction."
Or, there is an additional effect from the excited dye...
Sergio: "I don't believe that these small coupled single atoms could do the
job of gelatin crosslinking with efficiency directly compared to the Cr6
present.. so what in this porcess could work as developer (silver halide)
agent?"
Yes, agreed, there must be some additonal process involved here.
The Kodak fixing bath, after the dichromate bath could catalize the silver
halide reduction on the rest of the tiny grain?"
I do not think so. It will dissolve all the silver grains and remove the chrome ions.
