ON THE POSSIBILITY OF STUDYING THE PHOTOLYSIS OF SOLID COMPOUNDS AND THE SPECTRAL SENSITIZATION OF THIS PROCESS

G. A. Kislin, Yu. A. Zakharov

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

In the photochemistry of solid inorganic compounds there are practically no works on the study of the kinetic regularities of the photolysis of solid salts upon irradiation with monochromatic light and on the influence of various kinds of additives on this process.

The present work describes an apparatus for studying the kinetics of photolysis, as well as the optical and chemical sensitization of solid compounds, and, as an illustration, presents results on the photolysis of several systems based on heavy-metal azides. A diagram of the apparatus is shown in Fig. 1. The sample under investigation is placed in a

Fig. 1. Diagram of the apparatus.
1 — reaction cell; 2 — getter-ion pump; 3 — diffusion pump; 4 — omegatron of the IPDO-1 mass spectrometer (when combined); 5 — LM-2 manometer; 6 — freezing trap; 7 — electromagnetic shutter; 8 — fore-vacuum pump; 9 — LG-2 manometer

quartz reaction cell (11), which is connected by a standard quartz—molybdenum transition to the pressure gauge (5) and the evacuation system. The sample is illuminated through an optical-quartz window with monochromatized light (SF-4 spectrophotometer) from a DRSh-1000 lamp. The reaction cell is evacuated by a combination of a titanium-

Fig. 2. Rate of photolysis of the system AgN₃ + 5% Cr₂O₃ in the absorption region of the additive.
1 — \(\lambda = 500\ \mathrm{m}\mu\); 2 — \(\lambda = 600\ \mathrm{m}\mu\); 3 — \(\lambda = 546\ \mathrm{m}\mu\); 4 — \(\lambda = 579\ \mathrm{m}\mu\).

of sorption, diffusion, and fore-vacuum pumps to a pressure of about \(10^{-7}\) torr.

In the variant described above, we investigated the kinetic regularities of the photolysis of silver azide and the influence on it of color-

Fig. 3. Mass spectrum of the photolysis of lead azide
(the background is shown by the dashed line)

inorganic semiconductor oxides. It has been established that pure silver azide, when illuminated with light of \(\lambda<400\,m\mu\), practically does not decompose. The addition of five weight percent of colored chromium oxide \(\mathrm{Cr_2O_3}\) (a mechanical mixture) makes silver azide sensitive to longer-wavelength light in the absorption region of the additive (Fig. 2), which apparently permits one to speak of spectral sensitization of photolysis.

We have established that combining the described setup with an IPDO-1 omegatron mass spectrometer makes it possible to analyze small amounts of gaseous photolysis products and to determine clearly the spectral boundary of the onset of photochemical decomposition. As an example, one may cite the determination of the edge of photoactive light for the decomposition of lead azide. The principal product of photolysis is nitrogen.

Comparison of the mass spectra before and during illumination with monochromatic light makes it possible to determine the boundary of photoactive light for \(\mathrm{PbN_6}\) from the increase in the current of \(\mathrm{N_2^+}\) ions relative to the background. Figure 4 gives the magnitudes of the peaks of the \(\mathrm{N_2^+}\) ion current before illumination and during illumination by different regions of the spectrum.

Fig. 4. Dependence of the magnitude of the peak current of \(\mathrm{N_2^+}\) ions on wavelength.
1 — background; 2 — \(\lambda=436\,m\mu\); 3 — \(\lambda=405\,m\mu\); 4 — \(\lambda=366\,m\mu\).

It is evident from the figure that photolysis of lead azide occurs when it is irradiated with light of \(\lambda<405\,m\mu\). The established position of the edge of photoactive light coincides, for \(\mathrm{PbN_6}\), with the position of the edge of the optical absorption band and with the maximum in the spectral distribution of the photocurrent.

ERRATA NOTED