Study of the Photolysis of Solids by Electrophysical Methods
Abstract Generated abstract
This study examines whether electrophysical measurements can clarify the mechanism of photolysis in solid ionic compounds, focusing on heavy-metal azides and their sensitization by organic dyes. The authors compare photochemical decomposition with photoconductivity and photopotential measurements in pure and dye-sensitized lead azide systems. The results indicate that sensitized photoconductivity is necessary but not sufficient for sensitized photolysis, which also depends on the presence of a blocking surface potential. Spectral measurements further suggest that surface recombination and the electrical state of the surface play an important role in controlling the photolysis rate.
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STUDY OF THE PHOTOLYSIS OF SOLIDS BY ELECTROPHYSICAL METHODS
Yu. V. Gavrishchenko, G. G. Savel’ev, Yu. L. Zakharov
(Presented by the scientific seminar of the Department of Radiation Chemistry, KhTF)
It has now been established that the reactions of the photochemical decomposition of ionic compounds include, as a primary and one of the most important elementary stages, the stage of anion ionization. Since this process creates free charge carriers in the solid, one may expect a correlation between the photoelectric properties and photolysis, and the possibility arises of studying the mechanism of photolysis by means of electrophysical measurements.
All previous studies of the mechanism of photolysis were based on searches for correlations between photoconductivity and photolysis. However, this correlation is often qualitative in character and is not always observed. An example of the absence of correlation is the sensitized photolysis and photoconductivity of inorganic compounds with organic dyes [1]. This can be explained by the fact that the appearance of photoconductivity is determined mainly by the formation of free charge carriers—electrons and holes—whereas the photochemical decomposition of solids is a more complex process that includes other stages as well.
In the decomposition of heavy-metal azides, the holes formed as a result of ionization, upon reaching the surface, recombine with one another with the formation of nitrogen. We have previously shown that lead and silver azides have a hole type of conductivity [4]. For this reason, an antiblocking surface potential should promote the motion of holes toward the surface and increase the rate of the bimolecular reaction that is the final stage of photochemical decomposition:
\[ 2N_3^{0}\to 3N_2, \]
whereas a blocking potential should decrease it.
To clarify the mechanism of sensitized photolysis, we carried out parallel studies of the photochemical decomposition and the internal photoeffect of pure heavy-metal azides and those sensitized with organic dyes.
The inorganic azide–organic dye system is more convenient for this type of investigation than the classical silver bromide–organic dye system, since the final products of azide photolysis—metal and nitrogen—are low-reactive and do not destroy organic dyes.
The rate of photochemical decomposition was determined by means of
vacuum technique using a compensating method for measuring pressure [2]. The methods for measuring photoconductivity and photopotential were similar to those described in [3].
The results of measuring sensitized photoconductivity, photopotential, and photolysis, given in the table, show that sensitization of photoconductivity is a necessary but not sufficient condition for sensitization of photolysis, since sensitization of photolysis also requires the presence of a blocking surface potential.
Table 1
Photosensitization of Lead Azide by Dyes
| Lead azide–organic dye system | Region of sensitization of photoconductivity (nm) in azide | Surface potential | Sensitization of photolysis |
|---|---|---|---|
| Erythrosine | 550–560 | blocking | sensitizes |
| Sodium eosin | 530–550 | blocking | sensitizes |
| Cryptocyanine | 620–710 | blocking | sensitizes |
| Trypaflavine | does not sensitize | blocking | does not sensitize |
| Thiocarbocyanine | 500–555 | barrier | does not sensitize |
| Indocarbocyanine | 520–560 | barrier | does not sensitize |
The influence of the surface potential on the photolysis of azides is most clearly manifested when the specimen is illuminated with light of a wavelength greater than the wavelength corresponding to the edge of intrinsic absorption of the azide. The point is that, when the specimen is illuminated with light from the region of intrinsic absorption, the motion of charges due to the concentration gradient (the Dember effect) always creates a blocking surface potential, independently of the sign of the dark surface potential [5].
A study of the spectral distribution of the photolysis rate showed that, as the wavelength of the incident light decreases in the intrinsic absorption region, the photolysis rate decreases, despite the increase in the energy of the light quanta (Fig. 1). These results are consistent with the results of measuring the spectral distribution of photoconductivity, where the photoconductivity decreases because of an increase in the rate of surface recombination with increasing absorption coefficient. It is possible that the decrease in the photolysis rate is also determined by a decrease in the concentration of \(\mathrm{N}_3\) radicals owing to an increase in the rate of their surface recombination with electrons.
The investigation carried out shows the important role of the electrical state of the surface in the photolysis of heavy-metal azides.
Fig. 1. Spectral distribution of the photochemical decomposition of lead azide.
REFERENCES
- I. A. Akimov. DAN SSSR, 164, (3) 533 (1965).
- Yu. V. Gavrishchenko, G. G. Savel'ev. “Izmer. tekhn.” (in press).
- A. M. Meshkov, I. A. Akimov. PTE, No. 3, 5 (1964).
- G. G. Savel'ev, Yu. V. Gavrishchenko, Yu. A. Zakharov. Izv. vuzov, “Fizika”, No. 7, (1968).
- I. A. Akimov. Optiko-mekh. prom., No. 5, 10 (1966).