Electron Paramagnetic Resonance in AgBr Photoemulsion Crystals

Yu. A. Breslav, V. A. Uksusova, B. G. Varshaver, A. G. Kogoe

(Presented by the scientific seminar of the Department of Radiation Chemistry)

When radiation energy is absorbed by crystals of silver halides, holes and conduction electrons are released; they diffuse throughout the volume of the crystal until they are trapped at occupied and free surface energy levels in the forbidden band of \(AgHal^{(1)}\). Since important photochemical processes occur at the defects that give rise to these surface levels and lead to the formation of the latent photographic image, knowledge of the nature of such defects, of the mechanism and conditions of their formation in the course of crystal preparation, of the optimal amount of these defects, etc., becomes essential.

However, the nature of the defects is determined by the biography of the crystals and differs for hydrosols of silver halides, single crystals, large plate-like crystals and macrocrystals prepared in aqueous solution, on the one hand, and for microcrystals of silver-halide photographic gelatin emulsions, on the other.

To solve certain practical problems of photographic chemistry, it is necessary to study the properties of real photoemulsion crystals, which are investigated in the present work by means of the EPR method.

Experimental Part

A silver-bromide ammoniacal photoemulsion with instantaneous emulsification was investigated. During the course of chemical ripening, every 30 minutes a portion of the photoemulsion was sampled and placed in quartz or glass (“Luch” glass) ampoules for recording EPR spectra. In these ampoules the emulsion was centrifuged at a temperature of \(42^\circ\mathrm{C}\), and then decanted. Special experiments established that, with this method of concentrating the emulsion microcrystals, their photographic properties remain unchanged. In parallel with the sampling of specimens for recording EPR spectra, sensitometric studies of the photoemulsion were carried out.

The precipitated silver bromide microcrystals had no definite faceting (Fig. I).

Fig. I. Electron micrograph of emulsion crystals. Magnification \(10000\times\).

The average size of the emulsion grains was \(0.7\mu\). Samples that had not undergone vacuum treatment, or samples vacuum-treated at a pressure of \(10^{-5}\) torr and at temperatures of \(20^\circ\) and \(100^\circ\mathrm{C}\), were then irradiated with \(\gamma\)-rays from \(^{60}\mathrm{Co}\) (\(y = 3000\ \mathrm{rad/sec}\)) at liquid-nitrogen temperature. This type of irradiation was chosen because \(\gamma\)-quanta have a considerably lower density of energy transfer than light quanta, and, unlike the latter, the energy of \(\gamma\)-quanta is distributed substantially more uniformly throughout the volume of the crystals. Therefore, at small irradiation doses, the \(\gamma\)-radiolysis makes it possible to study the primary processes of interaction of radiation with AgHal throughout the entire volume of the crystals and to avoid the superposition upon them of secondary processes that occur during photolysis.

The EPR spectra were recorded on an RE-1301 radiospectrometer. The method for recording EPR spectra is described in (2). The experiments were carried out under nonactinic illumination.

\(\gamma\)-irradiation of silver bromide microcrystals causes the formation of paramagnetic particles. Figure 2 (a, b) presents the EPR signal belonging to these particles. The figure shows that the EPR signal of \(\gamma\)-irradiated microcrystals changes upon vacuum treatment of the samples; in this case, the intensity of the spectral components denoted by the indices “C” and “A” decreases appreciably, while at the same time the intensity of the components with indices “B\(_1\)” and “B\(_2\)” increases.

Fig. 2. EPR spectra of $\gamma$-irradiated AgBr microcrystals.

a. Non-evacuated samples were irradiated in air at $-196^\circ\mathrm{C}$;

b. The samples were kept in vacuum for 1 hour at room temperature and irradiated at $-196^\circ\mathrm{C}$.

In non-evacuated samples of emulsion crystals, the intensity of component “F” depends on the storage time of the samples in air after preparation. With increasing storage time, the intensity of “F” increases up to a certain limit.

The amplitudes of components “B$_1$” and “B$_2$” are dependent on the time of chemical ripening of the photoemulsion. This dependence is presented in Table No. 1.

For identification of the EPR spectra entering into the superposition (Fig. 2 “a”), one can use the difference in the thermal stability of the paramagnetic particles to which this signal belongs. The EPR spectrum recorded in $\gamma$-irradiated at $-196^\circ\mathrm{C}$

Table 1. Dependence of the concentration of paramagnetic particles on the time of chemical ripening of the emulsion. Irradiation dose: 1 krad. Temperature of irradiation and recording of the EPR spectra: 77 K.

$A_{B_1}$ and $A_{B_2}$ are the ratios of the amplitudes of the EPR signals with indices “$B_1$” and “$B_2$,” respectively, to the weight of the sample and to the concentration of paramagnetic particles in the standard $(\mathrm{CuCl}_2 \cdot 2\mathrm{H}_2\mathrm{O})$.

in emulsion crystals change when they are heated to \(-120^\circ\mathrm{C}\). These changes consist in a decrease in the amplitudes of the signal components with indices “\(B_1\),” “\(B_2\),” and “\(F\),” and their complete disappearance at a temperature of \(-120^\circ\mathrm{C}\) (Fig. 3).

Fig. 3. EPR spectrum of AgBr microcrystals \(\gamma\)-irradiated at \(-196^\circ\mathrm{C}\). Irradiation dose: 1 Mrad. Spectrum recording temperature: \(-120^\circ\mathrm{C}\).

The EPR spectrum (Fig. 3) consists of three isotropic components of hyperfine structure; the total width of the spectrum, measured by the distance between the extrema of the outer lines of the HFS, is 120 Oe. The \(g\)-factor of the observed spectrum is close to the \(g\)-factor of the free electron. The radiospectroscopic characteristics and the form of the EPR spectrum presented in Fig. 3 are in complete agreement with the radiospectroscopic characteristics and the form of the well-known EPR spectrum of nitrogen dioxide in the gas-

the gas phase \(^{(3)}\) and in the adsorbed state \(^{(4)}\). Thus, the STS lines marked in Fig. 2 “a” with the index “A” belong to \(NO_2\) molecules.

The concentration of \(NO_2\) molecules increases with the dose of \(\gamma\)-irradiation of the emulsion crystals and, at an absorbed dose of 15 Mrad, is approximately \(10^{19}\) particles/g, which corresponds to \(\sim 10^5\) particles per microcrystal.

When \(\gamma\)-irradiated emulsion crystals are heated in vacuum at \(-196^\circ\mathrm{C}\) to a temperature of \(-150^\circ\mathrm{C}\), no noticeable changes are observed in the EPR spectra. However, when molecular oxygen (or air) is adsorbed by these microcrystals at \(-150^\circ\mathrm{C}\), substantial changes are observed in the EPR spectra. As the amount of adsorbed oxygen increases, the amplitude of components “\(B_1\)” and “\(B_2\)” decreases, and spectrum “b” (Fig. 2) gradually transforms into the spectrum presented in Fig. 4 “a”; with further heating to a temperature of \(-130^\circ\mathrm{C}\), it transforms into spectrum “b” (Fig. 4).

irradiation temperature. The temperature of irradiation and recording of the spectra was \(-196^\circ\mathrm{C}\).

a. EPR spectra recorded after adsorption of molecular oxygen by microcrystals at \(-150^\circ\mathrm{C}\), at \(\tau_s < 90\) minutes.

b. EPR spectra recorded after adsorption of molecular oxygen by microcrystals at \(-130^\circ\mathrm{C}\), at \(\tau_s > 90\) minutes.

In this case the “\(B_1\)” and “\(B_2\)” lines disappear completely for crystals with \(\tau_s \le 1.5\) hours and do not disappear completely for \(\tau_s > 1.5\) hours.

Graphical addition of the EPR spectra shown in Fig. 2 “b” and 4 “b,” taking into account the \(g\)-factors of the summed signals, gives a resultant spectrum analogous to that presented in Fig. 2 “a.” It is not difficult to see that the components of the “\(F\)” spectrum of EPR shown in Fig. 2 “a” belong to paramagnetic particles having the EPR spectrum shown in Fig. 4 “b.”

Discussion of Results

The results obtained allow one to conclude that the EPR spectra recorded after low-temperature radiolysis of emulsion microcrystals do not belong to free radicals of adsorbed gelatin, which, at the specified method of concentrating the emulsion crystals, constitutes approximately a monolayer (5). The observed spectra also cannot belong to radiation-induced paramagnetic centers of the quartz walls of the ampoules. Thus, the recorded spectra should be

attribute to paramagnetic particles that are formed in the bulk or on the surface of emulsion crystals of silver bromide.

Upon low-temperature absorption of \(\gamma\)-quanta by silver bromide crystals, a large number of holes and conduction electrons are formed, which:

1. can recombine with one another, and this process is not of interest for photographic chemistry and cannot be detected by the EPR method;

2. can be stabilized at structural and impurity defects with the formation of electron-excess and electron-deficient centers;

3. holes can be trapped on the surface of emulsion crystals in irreversible interaction with electron-donor particles.

During \(\gamma\)-radiolysis of silver bromide crystals, radicals \(\mathrm{NO_2}\) are observed in high concentrations. Nitrogen dioxide evidently arises as a result of the capture of holes or electrons by \(\mathrm{NO_3^-}\) ions adsorbed on the crystal surface according to the reactions:

\[ \mathrm{NO_3^- + e^+ \longrightarrow NO_3 \longrightarrow NO_2 + O} \]

and

\[ \mathrm{NO_3^- + e^- \longrightarrow NO_3^{--} \longrightarrow NO_2 + e^- + O^- .} \]

Such a process has been well studied and occurs during the radiolysis of various inorganic salts with nitrate impurities \(^{(6)}\). In addition, the EPR spectrum marked with the index “г” (Fig. 2a) is evidently due to the \(\mathrm{NO_3}\) radical. In our case this singlet

with a width of ~15 Oe and \(g = 2.0240\), which is in good agreement with the values previously found and described in the literature.

Thermovacuum treatment at \(100^\circ\mathrm{C}\) leads to desorption or destruction of \(NO_3^-\) ions, and paramagnetic \(NO_2\) molecules are not observed in the EPR spectra. It is known that at temperatures below \(150^\circ\mathrm{C}\) thermal destruction of the crystals does not occur; consequently, the results obtained are evidence that the \(NO_3^-\) ions are located on the surface, and not in the bulk of the emulsion crystals. Nitrate ions constitute approximately one tenth of a monolayer, if one proceeds from the assumption that all available \(NO_3^-\) ions are destroyed during radiolysis and all \(NO_2\) molecules formed in this process are registered in the EPR spectra. Evidently, \(NO_3^-\) ions reach the surface of the microcrystals during their synthesis from silver nitrate.

The EPR spectrum shown in Fig. 4 “b” consists of a single asymmetric line, having a maximum in high fields and a shoulder in low fields. The line shape is due to the axial asymmetry of the \(g\)-factor. It was found that \(g_1 = 2.0340\), \(g_2 = 2.0037\), and \(g_3 = 1.9950\), with \(g_1\) corresponding to the shoulder in low fields, and \(g_3\) to the maximum in high fields. The EPR spectrum under consideration is analogous to the EPR spectra of ion-radicals \(O_2^-\), recorded after capture of electrons from conduction bands or from electron-excess centers by adsorbed \(O_2\) molecules \((7)\). Analysis of the recorded spectrum and its comparison with the EPR spectra of the \(O_2^-\) radical allows one to assert that the EPR spectrum (Fig. 4 “b”) and component “F” of the spectr-

The EPR spectrum (Fig. 2 “a”) belongs to the ion-radical \(O_2^-\). The fact that spectrum “b” (Fig. 4) arises upon adsorption of molecular oxygen (or air) on crystals irradiated in vacuum and is not observed after preliminary vacuum treatment of the crystals and subsequent irradiation (Fig. 2 “b”) confirms this conclusion.

The concentration of \(O_2^-\) ion-radicals does not depend on the time of chemical ripening of the photoemulsion and is equal to \(\sim 10^{17}\) particles/g at an absorbed dose of 5 Mrad. Thus, in each emulsion microcrystal after its preparation there are no fewer than \(10^3\) molecules of adsorbed oxygen capable of capturing electrons, stabilized on the surface or injected during irradiation.

Also of practical interest is the fact that the concentration of \(O_2^-\) ion-radicals increases with the storage time of emulsion crystals in air by several times. This indicates that chemisorption of atmospheric oxygen proceeds over time, and the process reaches an equilibrium state only after the samples have been aged in air for approximately 20 days.

The results obtained make it possible to conclude that \(O_2\) adsorbed on the surface of emulsion crystals can compete for the capture of a free electron with certain electron-acceptor centers (which also include sensitivity centers) and can even capture electrons stabilized at these centers during post-effect adsorption of oxygen.

It is of interest to note the fact that, when oxygen is adsorbed on evacuated and irradiated samples, simultaneously with an increase in the concentration of the molecular ion \(O_2^{-}\), the concentration of paramagnetic particles responsible for lines “\(B_1\)” and “\(B_2\)” in the total EPR spectrum decreases (Fig. 2 “e”), and, conversely, removal of oxygen from the surface of emulsion crystals leads to an increase in the intensity of lines “\(B_1\)” and “\(B_2\)” (Fig. 2 “d”). Lines “\(B_1\)” and “\(B_2\)” do not belong to the same paramagnetic particle, since the ratio of the amplitudes of these lines changes significantly with the time of chemical ripening while the form of these lines remains unchanged (see the table). This would not be observed if these lines were components of one and the same EPR spectrum.

It may be assumed that lines “\(B_1\)” and “\(B_2\)” of the EPR spectrum (Fig. 2 “a”) belong to charge carriers localized on the surface of emulsion crystals.

In conclusion it should be noted that, in real photographic crystals, impurities located on their surface play a significant role in the electronic processes caused by irradiation.

Cited Literature

1. Mitchell J. W., Reports on Progress in Physics, 20, 433 (1957).

2. Breslav D. A., Cand. Sci. dissertation, Karpov Scientific Research Institute of Physics and Chemistry, Moscow, 1968.

3. Castle J. G., Beringer R., Phys. Rev., 80, 144 (1950).

4. Kotov A. G., Breslav D. A., Shekhotskii S. Ya., Theoretical and Experimental Chemistry 5, 367 (1969).

5. Zamkin V. A., Klyuchevich V. F., ZhPKh 5, 397 (1965); J. Phot. Sci., 16, 154 (1968).

6. Shekhotskii S. Ya., “The Mechanism of Radiation-Chemical Reactions,” Khimiya Publishing House, 1968, p. 268.

7. Mikheikin I. D., Maevskii A. N., Kazanskii V. B., Kinetics and Catalysis 8, 1563 (1967).

Physicochemical Institute named after L. Ya. Karpov

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