ON THE MECHANISM OF PHOTOLYSIS OF LEAD AZIDE

G. G. Savel’ev, Yu. V. Gavrishchenko, V. L. Shcherinskii, S. I. Rukodeev

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

It was previously suggested that the photolysis of heavy-metal azides in the solid state proceeds through the formation and subsequent recombination of \(N_3^0\) radicals [1]

\[ \mathrm{N}_3^- + h\nu \rightleftarrows \mathrm{N}_3^0 + e \tag{1} \]

\[ 2\mathrm{N}_3^0 \to 3\mathrm{N}_2 \tag{2} \]

In reaction (1), a free electron \(e\) and a hole \(\mathrm{N}_3^0\) are thereby formed; these may subsequently participate in the transfer of electric current, recombine according to the reverse of reaction (1), and be consumed in the chemical reaction (2) or in the growth of metallic nuclei \((\mathrm{Me}_n)\)

\[ e + \mathrm{Me}_n^+ \to \mathrm{Me}_n \tag{3} \]

If the photolysis mechanism 1—3 is operative, then a correlation should be observed between its characteristics and the photoelectric properties.

If, however, photolysis proceeds through excited states, bypassing the ionization stage, as the authors of work [2] suppose, then there will be no such connection.

Interesting results in this direction can be obtained by studying, in parallel, thermally stimulated currents (TSC) and gas evolution under the same conditions, i.e., in the course of thawing a sample previously irradiated with light from the region of its intrinsic absorption. In this case it should be expected that, if the mechanism described by reactions of type 1—3 is valid, the TSC peaks and gas evolution \((\mathrm{N}_2)\) will coincide. If such a coincidence is not present, then a mechanism excluding the ionization stage (1) is more probable.

Indeed, charge carriers formed by reaction (1) at low temperatures may be trapped in shallow traps and will not participate in decomposition and conduction. As the temperature is raised, both processes should begin at approximately the same temperatures.

We carried out such studies on lead azide, which was obtained in the form of a finely dispersed powder by “jet-in-jet” mixing of 0.2 N solutions of lead nitrate (“chemically pure”) and sodium azide (“pure,” twice recrystallized from bidistilled water).

Gas evolution was measured on a high-vacuum apparatus that had a sensitivity of \(10^{-8}\) torr at a pressure in the vessel of \(10^{-5}\)–\(10^{-6}\) torr. In this work the method of compensating the collector current in the circuit of a VIT-1 vacuum gauge with an LM-2 sensor was used [3].

TSC was measured with a TR-1051 electrometer, whose readings were recorded by an EPP-09 self-recording instrument. A sample in the form of an aqueous suspension was applied to a quartz substrate on which aluminum electrodes had first been deposited in such a way that the gap between them was in the form of a “snake” \(100\) mm long, with a distance between electrodes of \(0.1\) mm [4]. After the suspension had been applied, the plate was dried and placed in a cell, which was evacuated to \(10^{-3}\) torr and cooled to the temperature of liquid nitrogen. It was then illuminated for 1–2 minutes with a DRSh-1000 lamp, and subsequently heated by an electric furnace to \(\pm 50^\circ\text{C}\). During heating, the current through the sample was recorded.

The operations of preparing the sample, exciting it with light, and freezing in the gas-evolution and TSC experiments were analogous. The heating rate in both cases was \(0.1^\circ\text{C}/\text{s}\).

Figure 1 shows a typical gas-evolution curve during warming. Owing to the small volume of the cell for pressure measurement (about \(0.3\) l) and the very small gas evolution, evacuation of the manometer

Fig. 1. Gas evolution from lead azide excited at \(77^\circ\text{K}\) by light during heating

is quite appreciable; therefore the curve has maxima and minima. In all, the curve has 3 gas-evolution peaks: at \(-36\), \(13\), and \(25^\circ\text{C}\). A control experiment carried out under the same conditions, but without preliminary illumination, showed that at \(-36^\circ\text{C}\) the peak remains the same, while at \(13\) and \(25^\circ\text{C}\) there are only small bends.

This behavior of the peaks indicates that the peaks at \(13\) and \(25^\circ\text{C}\)

are caused by the action of light, while the peak at \(-36^\circ\)C is due to adsorption of residual gases and is not connected with illumination of the specimen.

Figure 2 shows the TSC curve. It has one maximum in the region of \(25^\circ\)C. At temperatures of \(0 \div 15^\circ\)C, only small inflections sometimes appear; these do not resemble a TSC signal and, apparently, are associated with the thawing of water and other residual gases.

Fig. 2. TSC in lead azide excited by light at \(77^\circ\)K

The coincidence of the TSC and gas-evolution peaks at \(25^\circ\)C may serve as good confirmation of the assumption that charge carriers—in this case holes [5]—participate in the decomposition of the azide.

It is clear from Fig. 1 that the amount of gas evolved from azide at \(25^\circ\)C is considerably greater than at \(13^\circ\)C. Therefore, the absence of a TSC peak at this temperature may be associated with a low concentration of charge carriers, which cannot be registered by the method we used.

In addition to the conclusion drawn above concerning the participation of holes in the photolysis of lead azide, the data obtained make it possible to draw one further conclusion: since the processes of electronic (hole) conductivity correlate with photodecomposition, ionic processes probably do not limit it.

REFERENCES

1. F. Bowden, A. Yoffe, Fast Reactions in Solids. Moscow, IL, 1963.
2. V. R. Pai Verneker, A. C. Forsyth, J. Phys. Chem. 71, 3736 (1967).
3. Yu. V. Gavrishchenko, G. G. Savel’ev, Izmeritel’naya tekhnika, 1968 (in press).
4. I. A. Akimov, A. M. Meshkov, PTE No. 4, 3, 1963.
5. G. G. Savel’ev, Yu. V. Gavrishchenko, Yu. A. Zakharov, Izv. Vysshikh uchebnykh zavedenii, “Fizika,” No. 7, 71 (1968).