ON T H E VISIBLE DECOMPOSITIOK O F SILVER HALIDE GRAINS BY LIGHT* B Y A. P. H. TRIVELLI AND S. E . SHEPPARD
The photochemical decomposition of the silver halides is a phenomenon of considerable complexity; the interpretation of its many aspects is by no means complete nor conclusive. However, recent studies upon this subject have greatly increased our knowledge of the character of the decomposition, as a chemical reaction. The observations of Schwam and Gross1 on the decomposition of silver bromide and chloride in light have completely established the fact of the release of halogen as the result of the reaction, although showing that the rate of reaction (sensitivity) is greatly affected by the mode of preparation, the adsorption of salts, and the presence of moisture. Still more conclusive in some respects are the results of E. J. Hartung2. Using a sensitive micro-balance, this investigator has shown that the photochemical decomposition of silver bromide in vacuo in presence of bromide acceptor^,^ can cause the loss of over 90 per cent of the bromine. This, together with the continuity of the regain of bromine by silver on bromination, may be taken as quantitatively comfirming the conclusion that the products of the visible decomposition of the silver halides by light are metallic silver and halogen. The relation of this to the energy consumed in absorption of light is less definitely settled. J. Eggert and R. Noddack4 have published results which they interpret as confirming the Einstein photochemical equivalence principle for the decomposition of' silver bromide in gelatino-silver bromide emulsions. For every quantum of light hv absorbed by the saluer bromide for wave-lengths 3 6 ; p p , 405pp and 436pp they deduce that one atom of silver is produced,-by the reduction of one equivalent of silver bromide. F. Weigertj has severely criticized these results. It appears certain that the absorptions of the silver bromide mere incorrectly calculated by Eggert and Xoddack, aad that these results fall considerably short of confirming the Einstein photo-chemical equivalence. On the other hand, for the decomposition of a mixture of silver chloride and silver salt of an organic acid (silver citrate), Weigert6 claims to have demonstrated that if the absorption of light is calculated for the salve?. metal formed (as photoproduct), on extrapolation to zero silver, the Einstein equivalence is confirmed. According to this, under these conditions a trace of colloid silver
* Communication No. 23j from the Research Laboratory of the Eastman Kodak Company. Z.anorg. Chem., 133,389 (1924). J. Chem. Soc. 125,2198 (1924). 3 In some cases, the glass walls of the reaction vessel. 4 Sitxungsber. Akad. Berlin. 1923, I 16. 5 Z. Physik, 18, 232 (1923). 6 Sitzungsber. Akad. Berlin., 1921, p. 646.
VISIBLE D E C O M P O S I T I O S O F SILVEK. HALIDE GHAINS
1569
(initially) is the true photochemical inductor, the organic silver salt being indirectly decomposed, Keigert’s conclusion is disputed by Luppo-Cramer: and by Eggert and Soddackz. Direct proof of the Einstein equivalence relation for the photolysis of silver selts is therefore not yet forthcoming, To this it may he added that by zssuming the validity of this, Fejans and Franl~enhurger~ have given a plausible explanation of their results on the influence of adsorbed silver ions, of bromide ions, and hydroxyl ions on the optical sensitivity of silver halides. Further, Toy and Egerton4 regard their results in the relative spectral absorptions of silver bromide in relation to latent image formation as in good agreement with Eggert and Noddack’s conclusions if supposed to hold for the formation of the latent developable image. Microscopic Observations of the Decomposition of Individual Crystal Grains The observations made in the course of the present investigation were partly qualitative, on special individual crystals, partly quantitative (statistical) on the grains of emulsions. They were made in the microscope, and in so far as possible, recorded by photomicrography. These observations lead to a tentative theory of the mode of photochemical decomposition of the silver halides which is not without interest for the theories of photographic sensitivity and image formation. For the study of the decomposition in individual grains, regularly formed crystals of silver bromide were specially prepared from ammoniacal solution, as described in a monograph on the silver bromide grain5. As noted there, such crystals showed black dots of reduced silver scattered over the surface6 unless prepared and photographed in red light. On continued exposure these scattered centers increased in number and size unci1 the whole crystal Itas decomposed and deformed, Recently Luppo-Cramer7 has reported the same appearance of visibly isolated reduction centers, apparently haphazardly distributed, in the unusually large and regularly formed crystals of a specially prepared emulsion. Rut subsequent to the publication of the monograph, we have observed and describeda a photochemical decomposition of a quite definitely vectorial character, although still disperse or discrete in nature. We have further amplified ‘and confirmed these observations, and present here some illustrations of this interesting orientation of the photochemical effect in crystals. The crystals of silver bromide were exposcd in the focus of the Phot. Icorrespondenz, “Festnummw”, 1922, 49 2. Physik. 18, 299 (1923). 2. Elektrochem., 28, 499 (1922). * Phil. Mag., 48, 947 (1924). 5 A . P. H. Trivelli and S. E. Sheppard. “The Silver Bromide Grain of Photographic Emulsions”, (Monographs on the Theory of Photography, S o . I . Eastman Xodak Company, 1921). Ibid, p. 83. 7 Camera, 3. 89 (1924). * 4 P. H. Trivelli and S. E Sheppard: Phot. J., 63, 334 (1923)
BLUE 2 A
I,,%
,
3' P
2
I ? i
1
4,
,
I>,,..
VISIBLE DE COMPOSITIOIV O F SILVER HALIDE GRAINS
I575
it being understood that a grain getting one quota of “blackening” is classed as “blackened.” Then the probability of a grain getting no quota, or remaining unblackened is
- act _ p = l - e whence the percentage number of blackened grains of a given class-size a is given by the expression p = I O O ( 1 - e-“) EASTMAN
40
Percentage Groins Darkenedby(Direc4) Li&t -Grain Size
Treatad with KM,O+
FIG. 8
where a is the class-size (area) of the grains, and c is a constant; evidently c = log
IO0
- log
(100
- p)
a This expression is found to represent the results of the classification over a considerable range :
TABLE a
Area in P2
P
percentage blackened
c
5
.074
30 45
.444 .37I
.9
57
.407
1.1
63 69
.392 ,391 .3 60
.3P2
‘5 ’7
1.3
1.5 2.1
71 76
.302
This relation is similar to one found to hold for the probability of spontaneous fogging (developability) of grains in an over-ripened emulsion’. The question as to a relation between these two phenomena will be noted later in discussing 1
Phot. J., March, (1925).
A . P. H. TRIVCLLI AKD S. E . SHEPPARD
1576
a tentative theory of the photochemical decomposition. Meanwhile it will
suffice to point out that these results indicate that the photochemical (visible) sensitivity is discrete in character, limited in magnitude, and behaves to a considerable extent as thougb alloted to the grains in definite quotas, the chance of a grain securing a quota being proportional to its size. The developable sensitivity of high speed emulsion is greatly reduced by treatment with oxidizing agents, such as chromic acid, permanganate, etc. The curves in Fig. 9, from a previous publication1 show the great effect of this treatment. The statistical survey of visibly darkening grains was repeated with grains exposed after treatment with permanganate. I t will be I
SEEDGRAFLEX
seen by comparison of Fig. 8 with Fig. 9 that the effect of desensitizing is very much less on the chance of visible darkening than it is on latent image formation. There is some tendency for the effect t o be greater for the smaller grains than for the larger, but the exrerimental errors are too large to regard this as established for this visible decomposition. The results may be equally interpreted as increasing the tendency of the curve to show an inflexion. A comparison of visible and developable sensitivity discloses a certain general parallelism with many particular deviations. I n each case grain size appears to be a factor statistically increasing sensitivity, if this be defined as the chance of blackening, or developability, for a given exposure-naturally of entirely different magnitude in the two cases. Again this factor may be outweighed in either case by others, since emulsions may have every similar grain size but differ greatly in developable sensitivity and visible sensitivity. The relatively small effect of oxidizing desensitizers on visible sensitivity, compared with its effect on developable sensitivity, is a point of difference. Brooksbank expresses disbelief in the existence of the inherently dark grains mentioned in the first Kodak monograph and remarks, “They (that is emulsion grains) are probably all transparent to the greater part of the visible spectrum in the unexposed plate, and it is only on exposure to light that some grains become visibly darkened.” We cannot agree with this. While differ1
J. Franklin Inst., November and December (1923).
VISIBLE DECOMPOSITION O F SILVER HALIDE GRAINS
I57i
entia1 focusing makes the average grain either translucent or dark there are some that cannot be made translucent even though only deep red light has been used, which is the condition he employed for obtaining his pictures of “unexposed” grains. These grains are undoubtedly thicker, and hence have greater absorption or have surfaces producing total reflections. In either case, the exposure would be more photochemically efficient and we would expect them to be more sensitive.
A Tentative Theory of the Photochemical Decomposition
It appears to us that both the resemblances and the dissimilarities between the developable sensitivity (latent image) and the visible sensitivity of silver bromide grains can be explained by the following considerations. The decomposition by light of the crystal is a t first only (auto) catalytically oriented, Le., affected as to locus and distribution, not magnitude per energy incident and absorbed. The energy effective is that absorbed by the silver halidel. Later, as the period of latent image formation passes into that of visible image formation, the decomposition is not merely autocatalytically oriented but accelerated and intensified, to some extent by spectral (auto)sensitization. Furthermore, the initial orientation of the photolysis is dominated by the presence of “sensitizing nuclei”-those destroyed by oxidizing desensitizers; these have little effect (not necessarily no effect) on the subsequent stages, when the photolysis is becoming considerable and producing visible decomposition. In this stage, the progress of the photolysis is determined by two principal factors, vix. (a) the structure of the grain, in particular, the numbers and direction of lines of growth, and (b) the thereby regulated autocatalytic orientation and sensitizing by the photoproduct itself. There ensues a synergy or antagonism between the pure original structural influence of the silver halide crystal on the photolysis, and the derivative (autocatalytic) effect of the products, upon further decomposition. In very regular and symmetrically developed crystals, these factors harmonize, so to speak, leading to definitely vectorial decomposition patterns, (q.v.) . But in less regularly developed crystals, when the directions of most rapid growth are deviated, multiplied, and repeatedly reoriented, the secondary autocatalysis becomes dominant, with the patchy and irregular effect observed in emulsion grains. The effect of size in grain is probably complex also. In the case of developable sensitivity, me have suggested, in recent paper with Loveland2 that the larger crystals contain, on an average, larger sensitizing nuclei, which therefore initiate the secondary condensation of photoproduct in a similar haphazard fashion. Reaching a size which induces developability earlier than smaller nuclei, (which have to concentrate a greater number of silver atoms about them) they therefore ensure greater apparent photo-sensitivity for ‘Cf.Toy and Egerton: Phil. Mag., 48, 947 (1925); Sheppard, Trivelli and Loveland: J. Franklin Inst., 200, 51 (1925). * J. Franklin Inst., 200, 5 1 (1925).
1578
A.
P. H.
TRIVELLI AND S. E. SHEPPARD
development. Howcver, the effect of szxe on relatzve sensitivity remains, if anything enhanced, when the nuclei are destroyed1 by oxidizing desensitizers (cf. Fig. 9 ) . To explain this, and other characteristics of latent image formation, we have proposed, in the paper referred toj2 the hypothesis that not only the original foreign sensitizing nuclei, but also photolytically formed silver nuclei, orzent the photodecomposition of the AgBr to occur immediately adjacent to them. Hence, 2 nucleus present, or formed, in a (large) grain ha,s a greater chance of growing, by orienting decomposition about itself, according as the number of coherent silver bromide particles exposed to light is greater. i. e., as the superficial area of the crystal is greater. According to this, it is not necessary to suppose that the sensitivity nuclei zncrease the amount of silver reduced by a given amount of light energy. It suffices if the energy decomposes silver bromide according to Einstein’s photochemical equivalence principle, or at even lower efficiency. It is only necessary that they concentrate the reduced silver atoms in groups or aggregates more rapidly than would occur in their absence. When we pass beyond the stage of latent image formation, where these aggregates or development centers may not number more than a few hundred silver atoms, to vzszble decomposition, the position may be changed. Here the znztzal effect of any foreign nuclei is less important-at least if they are small and few-and is rapidly overshadowed by that of the photoproduct itself. It is very possible that a t this stage not only a catalysis of orientation, zffecting the direction of the photolysis, but a true acceleration occurs. It is true that the sorting of blackened grains, as previously described, involves a large subjective factor of “visibility”, whereby concentration of the photoproduct gives the impression of a greater blackening than the s2me amount of material more dispersed. But it is difficult not to conclude that a real and objective increase of blackening, i.e. decomposition, existed in the larger grains. Decision on this must await chemical analysis on centrifugally fractionated grains of different sizes, Such a true autocatalysis may well occur, however, In the first place consider the reverse action of the liberated halogen on the silver formed. When a number of silver atoms are reduced close together, the probebility of escape of the halogen is greater, due to its greater concentration head. In some cases it may even tend to remove smaller aggregates of silver or single etoms by recombination. This holds also for halogen released znszde the grain affecting the surface layers as it diffuses out. For the visible decomposition the interior of the grain is much more important than it is for the developable (latent) decomposition. Moreover, it is possible that the colloid silver formed can act as an optzcal sensitizer for the decomposition, i.e., lower the hv limit dfective in reducing a silver atom. This possibility has been discussed by one of us in relation to oriented autocatalysis in iodide treated silver bromi+e emulsion3 and evidence for silver optical sensitizing exists. This effect is At least, as far as ascwtainable J. Franklin Inst , Ibid S. E. Sheppard: “The Action of Soluble Iodides and Cyanides on the Photographic Emulsion”, Phot. J., 62,88 ( 1 9 2 2 ) . 1
VISIBLE DECOMPOSITION O F SILVER HALIDE GRAINS
7579
definite in the case of plates exposed first to X-rays, then to ordinary light, and may well occur in ordinary light1. We propose to investigate this spectro-sensitometrically, however, and regard it at present as uncertain for the point a t issue. If an orientation, eventually autocatalysed by the photoproduct itself, of the decomposition in silver halide grains exists, it evidently requires itself an explanation. That such an orientation does not exist appears certain from the results with the specially prepared silver bromide crystals, and one specifically related to the directions of most rapid growth. Equally the orienting influence of the photoproduct appears in the cases of deviation from the vectorial decomposition. In explanation of this orientation, a modification and application of an idea suggested by F. Weigert seems worthy of thought. The application of Maupertuis’ principle2 t o photochemical reactions had been made in very generalized form by Shepparci3. As Weigert justly points out, the fruitfulness of the application necessarily lies in the installation of specific hypotheses as to its mechanism. The specific hypotheses he proposed4 was as follows: “On excitation of a system in which the electrons move in deformed orbits, by a frequency within the deformation interval, an alteration of the system takes place in the sense of most completely removing the deformation.” A deformed orbit we shall regard as synonymous with a perturbed orbit. Weigert considered the simplest response of a system to such an excitation to consist in a mutual repulsion of the parts from each other, in which connection he refers to Bohr’s conception of light absorption by increase of the size of orbits. Reigert’s interpretation of his idea in terms of a mechanical separation of neighboring particles “not yet aggregated to one molecule” may be regarded as less happy. In any case, it is very difficult to apply to atoms or ions forming part of a crystal, which may therefore be regarded as practically parts of one molecule. It may be equally argued that expansion of the orbit to infinity, Le., transfer of the electron to a hyperbolic orbit, would “most completely eliminate the perturbation.’’ This seems to us most consonant with both outer photoelectric effects and those inner photoelectric effects inferred in photochemical reaction. In “ideal” reactions, the optical coupling, resulting in perturbed orbits, is a minimum. Weigert points out that in concentrated solutions, and gases under pressure, the number of optically coupled molecules must be very considerable, and even in dilute systems “fluctuations” lead to local concentration spots, indicated by deviations from Beer’s law. “These complexes of optically coupled molecules are evidently in many cases the real seat of the photochemical reaction, for which the above mentioned specific application of the
* Cf. Luppo-Cramer:
Phot. Ind., 1924, 982. Sometimes termed the LeChatelier-Braun-van’t Hoff principle, but fundamentally the principle of least action. “Photochemistry”. (1914). Z. Elektrochem., 23, 366 (1917).
I 580
A . P.H. TRIVELLI A I i D S. E . SHEPPARD
LeChatelier principle holds. This implies the possibility of catalytic lightreactions.” It is evident that in the close packing of the crystalline state there exists the greatest possibility for the development of perturbed orbits. Although Weigert has subsequently considerably modified his views, and not continued to develop this hypothesis, there are many valuable features in it. The importance of perturbed orbits, and deformed ions, for reactivity has been recently extensively discussed by K. Fajansl. I t is considered in regard t o spectral emission and absorption, particularly with regard to the broadening of spectral lines, by Foote and Mohler2. “The quantum theory substitutes for impact damping the influence of the electrical fields of neighboring atoms upon the position and energy of an electron in a quantized orbit, Since the energy of any orbit is altered, the energy difference of two orbits between which an electron jump takes place may be changed, with a resulting modification of hv” The photochemical reduction of silver halide may be regarded as consisting fundamentally in the transfer of a valency electron from a halide ion to a silver ion of the crystal lattice3. It is generally considered a t present that the silver halides are distinctly polar, and that the lattices consist of silver ions and bromide ions held together by electrovalences4. The orbit whose perturbation would be of principle importance on this view is that of the valency electron of the bromide ion. X o w it seems probable that in the case of crystals the orbital perturbations proper to the constituent atoms will be more or less symmetrically partitioned according to the homogeneity and symmetry of the crystal. In particular, there mill be definite traces of the directions of most rapid growth, reflected also in differential densities of packing. The modified Weigert principle, that the photochemical reaction will be in the sense of completest elimination of the perturbed orbits, is therefore in good accord with the vectorial patterns of decomposition observed in special crystals. But a factor of equal, in some cases greater importance in this matter, than the growth structure of the crystals, is the presence of foreign inclusions in the crysta,l. The “foreigness” of these may vary from that of atoms capable of entering into the lattice, but having different volume, and altering the lattice interval” to that of substances very slightly if at all congruent with the lattice. Certain substances, notably metallic silver, will induce marked perturbation of the orbits of electrons of adjacent silver halide. T h e nucleating e$ect of socalled “sensitivity centers”, a s also of photochemically reduced silver in the crystal i s probably due to this e$ect. In consequence of this, and helped by the much less symmetrical growth of the crystal grains of emulsions, we have in these latter photochemical decomposition much less affected by the vectorial Naturwibsenschaften, N o . IO, March 9, (1923). “The Origin of Spectra”, p. 92. 3 Cf. S. E. Sheppard and A. P. H. Trivelli: “On the Relationship between Sensitiveness and Size of Grain in Photographic Emulsions” Phot. J., 61, 403 (1921). Independently proposed by K. Fajans: 8. Elektrochem, 28, 499 (1922). 4 Cf. Wightman, Trivelli and Sheppard : Trans. Faraday SOC., 19, Pt. 2 , Oct., (1923). 6 Cf. Iodide Effects in Silver Bromide. On the X-ray Patterns, R. B. Wilsey: Phil. Mag., 46, 67 (1923). 1 2
T-ISIBLE DECOMPOSITION O F SILVER HALIDE GRAINS
I581
character of the crystal, much more by the orienting effect of the photoproduct formed. In sum, then, if a photochemical change can occur in a crystal, it is reasonable to suppose that this will occur preferentially at the boundary of foreign inclusions (particularly free metals and substances of high refractive index) eliminating the perturbed orbits adjacent to these boundaries. Taken in conjunction with the vectorial character of the crystal, this nucleation effect may well extend over a considerable region. In this way the greater sensitivity of larger silver bromide crystals is enhanced over and above that afforded by their greater chance of having larger original sensitivity nuclei present. This would remain effective, whether the light be incident in continuous waves or by quanta.l
It may be suggested that the pecularities of the distribution of visible blackening in silver halide crystals could be sufficiently explained by considering only the diffraction and internal reflection of light in the crystals. This might be assumed to produce local concentrations of energy, giving at least some of the effects observed. But, such an explanation, although having the merit of apparent simplicity, does not in our opinion satisfactorily account for the gradation of effects observable between specially prepared crystals and emulsion grains. Nor does it direcfly cohere with the explanation of the primary photochemical effect itself. In a paper on “The Dispersity of Silver Halides in Relation to Their Photographic Characteristics”2 one of the writers concluded, in discussing dispersity and sensitivity that “since the atom itself is now recognized to be a disperse system, we may have to follow through into the silver atom itself.”
If the importance of perturbed electronic orbits for sensitivity is sustained, this conclusion is justified. Summary The photochemical darkening of specially prepared silver bromide crystals has been studied microscopically. It is shown that the decomposition occurs in definitely oriented patterns depending upon the growth and structure of the crystal. I,
2. The visible darkening of the silver bromide grains of emulsions was studied statistically. In contrast to the specially prepared crystals, the decomposition is largely irregular. The visible sensitivity is found to increase with the size of grain in the same emulsion. The lack of parallelism between the developable sensitivity and the visible sensitivity of different emulsions is due t o the fact that the visible sensitivity is relatively independent of sensitivity nuclei.
1
Cf. J. Franklin Inst., loc. cit. S. E. Sheppard: “Colloid Symposium Monograph”, 1, (1923)
I 582
A. P. H. TRIVELLI AND S. E. SHEPPARD
3 . A tentative theory is proposed. It is suggested that the photochemical decomposition is oriented in the crystal according to the gradients of ionic deformation, or perturbation of electron orbits following certain directions of growth. Since sensitivity nuclei, as also the reduced silver, mill induce marked deformation in contiguous silver bromide, the decomposition becomes autocatalytically oriented. This explains the contrast in behavior between the special silver bromide crystals and emulsion grains, as well as the effect of size.
The writers wish to express their thanks to Mr. R. H. Loveland for assistance in the experimental work. Rochester, N . Y . .l.ilarch 20, 1926
