M. R. V. Sohyun Imaging Research Laboratory 3M Company st. Paul, Minnesoto 55101
N0n-conventional Photochemical Imaging Processes
By non-conventional photochemical imaging processes we mean those techniques of photography which depend on chemical processes other than the well-known photolysis and develo~mentof silver halide microcwstals. While this definition includes a multitude of imaging systems, past and present, some generalizations can be made about those systems which are of commercial and scientific importance. The non-conventional imaging processes so far developed haue not proued capable of the same levels of photographic sensitiuity achieved by siluer halide photography. The light sensitive coating used in conventional photography consists of an array of silver halide microcrystals dispersed in a suitable hinder: each of these microcrystals has the ability to integrate the photochemical eve& occwrine within it-four or more auanta incident anywhere on the crystal may render t h d entire crystal, of -lo9 atoms, developable. On the other hand, many of the nonconventional systems are based on molecular photochemical processes; one incident quantum affects only the molecule which absorbs it. Figure 1 shows the relative radiant energy requirements for recording on several important imaging materials. While the molecular nature of many non-conuentional imaging materials limits their photographic sensitivity. it enhances their information storage capacity. The information storage capacity of an imaging material is, among other things, . inversely related to the size of the storage element-a silver halide microcrystal, composed of -lo9 atoms. in the case of a conventional photographic film or plate, or a single molecule in the case of the non-conventional material. For many contemporary imaging applications, e.g. microfilm, holography, or optically accessed computer memories, the information storage capacity is a critical aspect in determining the suitability of a given imaging material. However, the availability of practical high energy light sources, e.g. lasers or xenon flash lamps, allows the use of imaging materials less sensitive t h a n those of conventional photography in many of these applications. The process of using an imaging material normally involves at least three steps (1) image recording (2) image revelation (3) desensitization of the material
With conventional photographic materials, the last two steps involve wet chemical treatments. In general, these processing steps are either greatly simplified or eliminated with the non-conventional materials. A considerable saving in time and labor thus results in working- with the non-conventional materials, via-a-vis conventional photoeraohv. Real-time imaeine " - is thus a ~ossihilitvin nonconventional systems, also. Of growing importance, however, is the effect that elimination of some or all of the wet processing chemistry, characteristic of conventional photography, greatly reduces the environmental impact of imaging technology, to which the effluents from the wet orocessine -stens . contribute sienificantlv. Another aspect of non-conventional imaging systems which contributes to their growing commercial impor-
V
.
"
88 /Journal of Chemical Education
Figure 1 .
Relative radiant
energy
requirements of various photographic
sy5tems.
tance is the variety of image revelation techniques auailable among them, i.e. the different kinds of images which can be made with them. Systems are available which yield light absorptive (dye) images, much like conventional photography. Other systems are read out by means of light interference effects, as a result of image-wise variation in refractive index of the recording medium. This type of "phase imaging" is especially desirable for holography; light absorption in a hologram, of course, reduces the reconstruction efficiency, i.e. the brightness, of the reconstructed image. The chemistry of exemplary systems giving rise to each of these types of images will be described in detail below. The images recorded on some non-conventional imaging materials can also be revealed in terms of a solubility differential; an appropriate light sensitive layer can he removed from its support, imagewise, ,by solvent action to leave on that support a "resist" or "relief image." For example, note the possible difference in water solubility between the diazonium salt in eqn. (1) and its phenolic photoproduct. This type of imaging is of importance both in the printing industry for the production of photolithographic plates and in various proofing systems, and also in the fabrication of printed circuits (see below) (1). Ar-N=N-X
+
H20
hr +
ArOH
+
HX
4-
N2,(1)
History Photographic imaging dates back to 1826 when Nihpce -exposed a plate coated with bitumen in a primitive camera for 12 hr to insoluhilize the light-struck areas. The unexposed areas could then he dissolved away selectively to leave a negative relief image of the mask. Bitumen is a complex mixture of aromatic and other unsaturated hydrocarbons; its photoinsoluhilization may, perhaps, involve photodimerization reactions such as are characteristic of anthracene or acenaphthalene (eqns. (2) and (3)).
Since neaative-positive photomanhv - . . usine silver halides as the light sensitive medium was not demonstrated by Fox Talhot until 15 years later, non-conventional ~ h o t o e raphy actually antedates the silver halide proc&s with which we are more familiar. Various diazotype imaging processes, as they are called, based on the photochemistry of eqn. (I), the blueprint prncess and imaging processes based on the bleaching of dyes have also been with us since the last century. The history and chemistry of non-conventional imaging processes disclosed through 1964 have heen reviewed by Kosar ( I ) , in an excellent monograph. It is the intent of the present article, therefore, to illustrate more recent developments in the area of non-conventional imaging, particularly those which have achieved or promise to achieve some commercial importance, and which are illustrative of the chemistry encountered in modern non-conventional imaging. In addition to the whole area of electrophotography, which is outside the scope of this article, the current technical and patent literature indicates that the greatest current interest centers on non-conventional processes based on photochemical formation of dyes ohotochromism (3) bhotopolymerization (4) physical development of latent images
(1)
(2)
The remainder of this article will he devoted to the consideration of specific examples, taken from the recent literature, of imaging processes falling into these categories. Photochemical Formation of Dyes
image results. Alternatively, the information may he recorded a t X2 to desensitize the material image-wise, and the image revealed by overall exposure to light of XI; a positive image then results. The photographic sensitivity of such an imaging system is, however, rather low, comparable to that of the diazotype materials (see Figure 1). Significant increases in sensitivity, reportedly by factors of up to IF,can be obtained through amplification of the initial photoeffect, by means of photodevelopment. In one system in which this phenomenon has been described, (4, 5), the radicals which oxidize the leuco dye (eqn. (4)) are formed by the radical chain decomposition of carbon tetrahromide or a similar halocarbon. This photoreaction is sensitized by the leuco dye (DH), which can form a charge transfer complex, sensitive to blue light, with the carbon tetrahromide. The blue dye (D+) formed in eqn. (4) can also sensitize the decomposition of the carbon tetrabromide, presumahly by energy transfer from its triplet state to the charge transfer complex. Thus any further irradiation of the system with red light (absorbed by D + ) leads to formation of more dye, and the effect of an image-wise information recording exposure of such an imaging material to blue light can be amplified by overall exposure of the material to red light. This clever scheme is illustrated, in terms of the energy levels involved, by means of a Jablonski diagram in Figure 2, and by equations in Figure 3. Some materials of this type can be desensitized by heat, which leads to decomposition of the leuco dye-halocarbon complex. Photochromism Photochemical formation of a dye, or other light absorbing species, is also involved in photochromism. In true photochromism, however, the photoproduct dye can also be bleached by the light i t absorhs, with reformation of its precursor. In theory, such a system can be cycled between
Formation of a dye by the free radical oxidation of its leuco counterpart (eqn. ( 4 ) ) appears to be the method of choice in stable, dry-working imaging materials.
D+ It is easy to generate radicals efficiently photochemically, for example by the dissociation of a he~aarylhiimidazole,~ sensitive to XI (eqn. .(4)). In combination with the chemistry of eqn. (41, a color can be formed directly upon ultra-violet irradiation
. . , .. ,
. ,,,
. , - n . .
,,
. . .. . ,,
- ".-" .
IMAGE RECORDING
no separate image revelation step is required. However, the system is not desensitized to the effects of ambient light; to accomplish this, a diazonium salt, sensitive to longer wavelength light (Az) than the hexaarylbiimidazole, is used to form a phenolic radical trap, according to eqn. (1) (3). The complete system, composed of the leuco dye, the radical progenitor and the diazonium salt, all in solid solution in a polymer matrix, may he imaged a t XI and desensitized with light of i n , in which case a negative
PHOTODEVELOPMENT
Figure 3. Chemistry of photodevelopment (after Fotland, ref. ( 4 ) )
Volume 50, Number 2,February 1973 / 89
its two (or more) differently absorbing forms ad infinitum, althoueh in nractice side reactions lead to deterioration of -~~ the system, a process known as "fatigue." The tendency for nhotochromie systems to fatime - has been one of the principal factors limiting their application. An excellent treatment of the chemistry of photochromic materials is available in the volume edited by Brown (6). The most extensively studied class of photochromic compounds are the spiropyrans, e.g. the benzoindolinos.~ i r .o ~ v r a nand s their merocvanine counterparts. The former, colorless form is converted by ultraviolet radiation to the latter, intensely colored form, which may he reconverted to the spiropyran form by the visible light it ahsorbs. ~
k spiropyran
R rnerocyanine
These forms are also thermally interconvertable; and the position of the thermal equilibrium depends on the suhstituents in the aromatic nuclei. With some compounds of this series, especially those with strongly electron withdrawing suhstituents, e.g., nitro, in the-oxygen-substituted ring, it is possible to bleach the merocyanine form photochemically and regenerate i t thermally (7). The study of the photochemistry of these compounds has proven to he an especially fruitful one for the application of modem photochemical theory. The whole problem of elucidating the mechanism of this chemistry is, however, complicated by the possible existence of eight geometrical isomers of the merocyanine dye derived from benzoindolinospiropyran. The four cis isomers are unstable owing to steric hindrance, hut on the other hand are, of necessity, intermediate in the cyclization (eqn. (5), right-to-left). Since only trans isomers are available for photohleaching, it is believed that this reaction involves a trans-to-cis photoisomerization, followed by a WoodwardHoffman allowed thermal cyclization of the cis-intermediate to spiropyran. The observed temperature dependance for the photohleaching, both in liquid and solid solution, substantiates this hypothesis (8, 9). The currently accepted reaction scheme for the spiropyran-merocyanine photochromism is illustrated in Fieure 4. Note that onlv two of the four possihle trans iso6ers (collectively deiignated T,) vield . cis-nhotonroduct from which cvclization is oossihle; the other pair of trans isomers are, therefore, not photoactive insofar as the bleaching reaction is concerned. This fact has been of use in identifying the various visible absorption hands observed for the merocyanine species with specific trans-isomers (9, 10). Note also that the cismerocyanine isomer may he formed in the photocoloration reaction initially in an excited state (11). It now appears that spiropyran-merocyanine photochromism occurs principally through triplet excited states (12). Since triplets are readily quenched by oxygen, and since imaging materials are normally expected to function in the ambient atmosphere, photographic coatings of these photochromic compounds, in solid solution in polymer matrices, function rather inefficiently. Microsecond duration flash exposure of imaging materials based on this photochemistry, so that the triplet excited states are populated and can react faster than oxygen can diffuse through the polymer host to quench them, results in significant (5- to 6-fold) apparent increase in the photosensitivity of these materials (9). A perhaps more familiar example of a photochromic imaging material is to he found in the photochromic
-.
90 / Journaiof Chemical Education
glasses, which have found application in eyeglasses and light control windows, as well as for transient storaae of information (13). Unlike the organic photochromic materials, described above, the sensitive component in the photochromic glasses, a silver halide, is usually dispersed in the host glass as microcrystallites. Unlike the microcrystals of silver halide used for conventional photography, which are usually between 0.01 and 2 pm in diameter, the silver halide microcrystallites of the photochromic glasses are of the order of 100 A in diameter. The information storage capacity of these materials is thus not expected to be extraordinarily different from that of materials in which the sensitive component is molecularly dispersed. The photolysis of the silver halides proceeds according to eqn. (7).
The colloidal silver (Ago) deposited in the host glass as a result of this reaction makes it opaque. It is important to note that the reaction is reversible, i.e., the system is truly photochromic, either thermally or photochemically; the photolysis of the silver halides is effected, of course, by ultraviolet and (in the case of the bromide and iodide) blue light, while its regeneration is effected by red light (14). The hack photoreaction is attributed to the effects of light absorbed hv the colloidal silver formed nhotolvtical" ly, i.e., it is not the result of photolysis of the photogenerated haloeen. In conventional ~hotoeraohicmaterials. the gelatin present as hinder anh vario& added r e d h g agents remove the photogenerated halogen by reaction, giving reaction (7) a modicum of irreversibility. In the photochromic glasses, however, no halogen-reactive species are present, however and the halogen is trapped in the glass matrix, where it is available for either dark, thermal regeneration of silver halide or the photohleaching reaction (15). The silver halide photnchromic glasses have found commercial application more readily than the oreanic materials. such as described above. because thev are more fatigue-resistant, It is reported that a typical silver halide ~hotochromicglass can he cvcled between its transparentand opaque states up to 3 X lo5 times (16). Photopolymerization
Photoinitiated polymerization has received much attention in recent years as the basis for imaging systems, owing to the potential amplification of the initial photoeffect offered by this chemistry. That is to say, one ahsorbed quantum of actinic radiation can trigger the reaction of perhaps ,104 monomer molecules, thereby giving
Figure 4. spiropyran phatochromlsm (from ref. (91, reproduced courtesy
of the Society of Photographic Scientists and Engineers).
rise to an inherently more sensitive imaging system than one in which one quantum effects the reaction of only one molecule, e.g., the photochromics. The data of Figure 1 indicate that in practice the full potential for enhanced photosensitivity through this sort of amplification of the photoeffect has not been realized. Earlier efforts to exploit photopolymerization for imaging application centered on systems in which the polymerization reaction converted a soluble monomer to polymer (in 40-70% yield) with consequent insoluhilization of the entire exposed mass; unexposed portions of the imaging material could then be dissolved away to yield a relief image, as illustrated in Figure 5. (Since the sensitive material remaining after imagewise exposure is removed in the image revelation step in such systems, they require no further desensitization). Details of this sort of chemistry have been reviewed (17); it is of obvious application in the formation of printing plates, e.g., for use in letterpress printing, in place of metal type, and also for offset printing. More recently, photopolymer processes have been of interest for the formation of light scattering images, obtained when the photoproduced polymer separates from the monomer with formation of a separate phase, and of so-called phase images, which res& from the altered index of reaction of the light sensitive material as a result of photoreaction. These image-wise variations in refractive index give rise to interference patterns detectable, for example, by means of Schlieren optics, and are especially desirable for holography (18). Generally speaking, the photoinitiated polymerization reactions of utility for imaging applications involve radical chain polymerization of ethylenically unsaturated monomers, especially acrylic acid derivatives (see eqns. (8) and (9)1.
CO-R'
Various systems described in the literature have used a number of different radical generating photoreactions, e.g., eqn. (4), as means of initiating the polymerization. One system which has been described in detail (19) involves the use of photooxidation of an organic sulfinate by means of a reducible dye, e.g., methylene blue; the re-
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"
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." -
A'.
"Z'
.
., ""'.
,
'.
'""
'
" - '"
Figure 6. steps in photolnltlatea polymerlzatlon (atter srauli er a!.. re,. 1161 1.
sulting radical species initiate the polymerization of metal acrylates, e.g., barium acrylate, in aqueous solution. The product poly-metal acrylate crystallizes out of solution, and a light scattering image is thus formed directly by irradiation, for which a HeNe laser is suitable.1 In practice, the monomer solution is held in a cell composed of two glass plates -0.2 mm apart, or thickened with gelatin and coated on a photographic film base. (Note that no separate image revelation step is required by this system.) The initiation chemistry is uniquely useful since on heating the imaging mixture, as for example, to dry the gelatin-thickened coating after exposure, unreacted sulfinate reacts with unreacted monomer to form a photochemically inert sulfone. This chemistry is summarized in Figure 6. As mentioned above, for holography images refractive index variations in the recording medium may be desired. T o this end, a different monomer or monomer mixture, e.g., acrylamide, can be substituted for the barium acrylate in the previously descrihed mixture. For recording of holograms, the photopolymer solution is kept between glass plates only 25 pm apart. The solution can be desensitized after hologram recording by exposure to ultraviolet radiation, if an aromatic nitro compound, e.g., sodium p-nitrophenyl acetate, is included; it forms an acinitro anion photochemically, which is capable of reducing the dye (D+),according to eqns. (10) through (13) (19).
PHOTOPOLYMERIZABLE MIXTURE SUPPORT EXPOSE ( U V ) REMOVE NEGATIVE
IMAGE- WISE POLYMERIZED LAYER
4
WASHOUT UNREACTED MONOMER
FINISHED PRINTING PLATE ktgure 5 . rnotopolymer prlnttng plaie (alter walner el al.. re]. 11 ! I J
'The 633 nm emission of this laser corresponds directly in wavelength with the principal absorption hand of methylene blue; the dye in its excited state is, itself, the oxidizing species, and the photooxidation in this ease requires no oxygen. Volume 50, Number 2, February 1973 / 91
An alternative holographic photopolymer system, in which the monomer and initiator are in solid solution in a host polymer, has also been described (18, 20). In this case, the variation in refractive index of the material results from an image-wise variation in concentration of photoproduced polymer, generated as follows: first, the material is exposed to the light interference pattern to he recorded; then it is given an overall light exposure. The first exposure leads to imaae-wise of the .polvmerization monomer; unreacted monomer subsequently diffuses into the exposed areas of the material from unexposed areas to equalize the monomer concentration throughout the medium. The second, overall exposure then ~olvmerizesthe remaining monomer. The distribution o f photoproduced polvmer in the material is thus modulated according to t h e interference pattern of the first exposure; the exposed regions contain more polymer than those not exposed at the first stage. The initiation and chain propagation reactions of this system are analogous to those illustrated in Fimre - -- ~ ~6. -~ - Rather than generate a polymer chain photochemically, it is possible t o photo-crossli~kpreformed polymer molkcules. In this case, only a few crosslinks, produced bv a few quanta, are necessary to convert soluble polymer chains of, say, lo3 repeat units into insoluble material. Comparable q u a n t u m amplification and, hence, photographic sensitivity relative to photoinitiated polymerization are thus obtained. This chemistry has found particular application in the formation of photoresists, as required in the production of printed circuits. This process involves the following steps, which are illustrated in Figure 7. (1) application
of the photosensitive polymer over a layer of copper, laminated to a suitable support (2) irradiating the photosensitive polymer in those areas where it is desired to retain conductivity in the finished circuit board (3) dissolving away the unexposed polymer to expose the copoer surface .~~ ~~
~~~~
(4) treatment of the assemblage with ferric chloride soiution to dissolve (by oxidation) the copper in all areas where the
protective polymer coat has been removed (51 removal of the remaining polymer with a stronger solvent to leave the desired conductive copper elements of the circuit on the insulatingsuppart
Figure 7. Steps in circuit board fabrication (after Walker et a!., ref (171 I .
92 / Journalof Chemical Education
As the photosensitive material, it has been customary to utilize polymers substituted with cinnamate ester side chains, which crossline by photodimerization of the cinnamate ester moieties (eqn. (14) ). This chemistry has been extensively reviewed (21, 22) and will not he considered further here.
Recently, other photochemical reactions have been used to effect the photochemical crosslinking of polymer chains. In some sensitive materials, the polymer chain is substituted with azide (23) or sulfonyl azide (24) functional groups which photolyze to form reactive nitrene intermediates. Nitrenes characteristically react by insertion into C-H bonds (eqn. (15) )
and crosslinks between polymer chains can be formed in this manner. Visible images can also be formed hy this chemistry; one approach is to use polymers substitued with carhoxyl functional groups, which can mordant dyes applied from solution, after the unexposed polymer is dissolved away. A potentially more important application of this chemistry involves the photochemical modification of surfaces (24). For example, polyolefins and polyesters are characterized by highly hydrophobic surfaces. A layer of polymer, suhstituted with both sulfonyl azide and polar, e.g., carboxyl and sulfonic acid, functional groups is photochemically bonded to the polyester or polyolefin by means of the reaction of eqn. (15). thereby providing a hydrophilic surface suitable, for example, for the application of subsequent coatings from polar solvents. A layer of the sulfonyl azide substituted polymer can also he used as a photoactivated "glue" to bond dissimilar materials, both of which contain nitrene-reactive moieties. Physical Development It should be apparent from the above discussions of both free radical and photopolymer imaging systems that one emphasis of recent research in non-conventional imaging has been means of obtaining increased photographic sensitivity relative to the processes of historical importance, e.g., diazonium salt photolysis. One way in which the necessary amplification of a photoeffect can he achieved is by means of a process known as physical development. I t is possible to formulate solutions of a metal ion (M+"), usually complexed, and a reducing agent (R), capable of reducing i t to the metallic state (MO). Such solutions, although thermodynamically unstable, may he practically stable owing to the very low velocity of the spontaneous reaction (eqn. (16)).
In the presence of nuclei of the metal M, or of a metal below it in the electromotive series, the reaction proceeds raoidlv. of as analogous to . " The nrocess mav be tboueht " the seeding of a supersaturated solution in order to induce crystallization of the solute (though this analogy must not be pushed too far in trying to understand the mechanism of ohvsical develo~mentl.If the nuclei are distributed . . image-wise on some support, they may he enlarged by this
Figure 8. U s e f u l physical d e v e l o p m e n t r e a c t i o n s (from d a t a of J o n k e r et a,.. ref. I251 ).
process of metal deposition to amplify the photoeffects which originally led to the image-wise distribution of nuclei. This technique has been known in conventional photography since the last century, and is known as physical development. In terms of non-conventional imaging, a number of metals can be deposited by physical development, among them silver, copper, lead, tin, nickel and cobalt (25). With the exception of silver, the metals are usually present in the physical developing solution as complexes of their dipositive ions. Typical redox pairs which are of practical use in physical development imaging, and which illustrate eqn. (141,are given in Figure 8. In many cases the metal image which is deposited is conductive; in the case of tin deposition, formation of a superconductive deposit has been reported (25). Physical development of photogenerated nuclei is thus a viable alternative to the method described in the preceeding section for fabrication of printed circuits; in this case the conductive element is deposited on its support, rather than the unwanted metal etched away. One reaction which has been utilized successfully to photogenerate the desired nuclei is the photolysis of titanium dioxide; the titanium dioxide is "activated" either before or after exposure with, for example, silver nitrate which converts the latent image in the titanium dioxide to physically developable silver nuclei (26). While the photochemistry going on in titanium dioxide has not yet been elucidated, it is known that zinc oxide behaves similarly, and in this case the intermediacy of hydrogen peroxide, formed the zinc oxide photoreduction of atmospheric oxygen, has been implicated (see eqns. (17)through (19)) (27).
~n'
+
H02-
0,
+
+
H20
-
A ~ +d
~n'+
+
HO;
+
HO;
+
~ g 'etc.
OH-
PbI?
-
Pb"
+
Conclusions One objective in the preparation of this article has been to illustrate some of the varied kinds of chemistry that have been of recent interest as the basis for novel imaging systems. It should be clear from the scope of this chemistry that the photographic scientist cannot l w k too narrowly at any one kind of chemistry in his search for technology applicable to a particular imaging situation. Another objective has been to highlight some of the more important newer imaging applications-holography, microfilm, high-density optically accessed computer memories, and the fabrication of printed circuits-which call for new imaging systems and new imaging chemistry, outside the scope of conventional photography. Finally though references to the original work, it has been my objective to introduce the reader of this article to the literature of imaging technology. Literature Cited (11 Kaaar, J.. "LightSenrifivoSystoms,"John Wileyand Sons,NeluYorh. 1965. (21 Ceseon, L. A.. U.S. Parent 3.445.234. (31 Caseon,L.A.,U. S.Pstent 3,445,233, (41 Fot1and.R.A.. J. Phofogr. S c i . lR.33(1970). (51 H a w A.L.. andPetm. V. P.. LoaerForua, 8 (2l.32(19721. (61 Brown G . L. "Phofoehmmism "John Wiley and Sons. NmYork. 1971. (7) ~ e r m a bE., . b. S. ~atont3.477.k50. (8)Bereovici. T . . Heiligmsn-Rim R., and Fisher. E., M o l Pholochemlafgv, ,r*i
(18)
.i,18.41~1970l. I . E. A . m "Theory or the Phofoglsphic Pmnss:
(Edi-
J a m n . T . H.1 Macmillan. NewYork, 1966.p. 155ff.
(19)
Alternatively, the radiolysis of lead salts, e.g., PbO or PhIz (28,29), can be used to generate lead nuclei r-irradiation
sort of polymeric hinder. Such layers are, of course, opaque, which may he undesirable in some applications, and are not characterized by as high information storage capacity as layers in which the sensitive material is molecularly dispersed (see above). An interesting scheme has been developed in which potassium ferrioxalate and a palladium(I1) salt are imbibed into a partially hydrolyzed cellulose triacetate support or into paper. Photolysis of the resulting photosensitive media leads to both ferrous and palladium (Pdo) latent images which can serve as nuclei for the deposition of nickel, cobalt or copper by physical development (31). The chemical changes which take place as a result of irradiation of the palladium doped ferrioxalate salts, which are believed to be present in the imaging media as microcrystallites, though of insufficient size to interfere with the transparency of the materials, are summarized in eqns. (21)-(23).
Brplt. R_ G., ?enmy, J. A.. Margerum. J. D.. Miller. L. 3.. and Rust. J . B.. '' ~ril/Mayl97ll. ium Ill. Unconventional Phofogrsphic Systema. Sariety ssndEngineem, Washington.D.C., 1971.
."
I,
(20)
This reaction provides a potentially important entry to novel X-ray sensitive imaging media, owing to the very high X-ray stopping power of the lead compounds. The greatly enhanced quantum efficiency of the reaction of eqn. (20) in the presence of moisture is a curious aspect of this photochemistry which has not as yet been satisfactorily explained (30). Imaging materials based on the photochemistry of titanium dioxide or lead salts generally involve coatings of microcrystals of the sensitive material dispersed in some
..
aineem. Warhinpton.DC.. 1971. (251 Jonker. H.. Mnlenasr. A . andDippel C . J . Phafogr Sei Eng.. 13.38 (1969). (26) Jonkor. H.. Jsnrren C. J. G . F. o i b l . k. J . Thijsans, T . P. G. W. and Paatma, L., ~hotogr.Qci. ~ n g . 13: , 45 (1969): ~ e i m a n E., , ~hotogr.sci. &g., 13, 51 (271 Ostel. G.. andYamamdo.M., J. Phys. Chem., 70.3033(1966). (28) Van Den Heuvd, W. A., Van Halat, J. E., and Brinekman. E. M.. Fmneh Paant 9 "9.2 ,,a -,"-",."".
(291 Van Den Heuvel, W. A,. Van Halsf. J. E.. and Brinekmsn. E. M.. French Paant 2,023,139, (SO) Van Hal% J. E., Brinkman. E. M., and Vsn Den Heuvpl. W. A,. French Patent 2,028,140. (311 Callaby,D.R..andBmtto. M.. J Photogr. S c i . I8.8(1970).
Volume 50, Number 2, February 1973
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