Optical Spectra of Irradiated \(NaClO_3\) and \(KClO_3\)

D. A. Zakharov, D. N. Safonov, L. V. Serikov

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

At present, the occurrence, during the radiolysis of alkali-metal chlorates, of the ionization stage of the anion as one of the primary elementary processes may be regarded as experimentally established [1–2]:

\[ ClO_3^- \sim\!\!\!\to ClO_3 + \bar{e}. \tag{1} \]

The subsequent fate of the free electrons in chlorates is unknown; one may expect, however, that some fraction of them is stabilized in traps with the formation of electron color centers. The formation of series of such centers during photolysis and radiolysis has been established and studied in a number of salts of simple composition—for example, in halides and azides of alkali metals. For these systems, the important role of electron–ion processes in the formation of the final products of radiolysis has been established.

In the present work, data are presented on the low-temperature optical spectra of irradiated single crystals of \(NaClO_3\) and \(KClO_3\), from the analysis of which it may be concluded that one of the intermediate products of chlorate radiolysis is an electron stabilized in a trap. Previously, the optical spectra at low temperatures in irradiated chlorates had not been considered in sufficient detail; electron stabilization in crystalline chlorate matrices has been observed for the first time.

EXPERIMENTAL METHOD

Irradiation was carried out with X-radiation on a RUP-200-20 unit \((200\,\text{kV};\ 20\,\text{mA})\). The dose absorbed by the samples—

...doses varied within the range \(0.5 \div 1\) Mrad.

Single crystals of \(NaClO_3\) and \(KClO_3\) were grown by the method of slow evaporation of an aqueous solution of chemically pure (“ch”) grade salt, recrystallized three times from bidistillate.

The obtained \(NaClO_3\) single crystals had the form of rectangular parallelepipeds with dimensions \(6 \times 6 \times 2\) mm. The \(KClO_3\) single crystals were grown in the form of thin plates with dimensions \(8 \times 10 \times 0.5\) mm and required no additional treatment.

Low-temperature optical spectra were measured on an SFD-2 spectrophotometer using a cryostat that made it possible to carry out irradiation and measurement of spectra at \(-196^\circ C\). The design of the cryostat is shown in Fig. 1. The main parts of the cryostat are made of stainless steel. In the base (5) there are 2 through windows (1) made of optical quartz for spectrophotometric measurements; a third (4), for irradiation, located above one of the quartz windows, is covered with metallic foil \(0.1\) mm thick. The movement of the sample holder (2), fixed on the liquid-nitrogen reservoir (8), into the irradiation position is accomplished by means of a brass bellows (6) and a tension nut (9), moved along a thread cut on the stop cylinder (7). For rapid warming of the sample, an ordinary heater (3) is used. The lead wires and the copper-constantan thermocouple are brought out to a connector through a vacuum feedthrough (12). Pumping is performed through a fitting (13). The flanges (10, 11) are joined by means of a rubber gasket (14) and a bolted connection. During operation of the cryostat, a vacuum of the order of \(10^{-3} \div 10^{-4}\) torr is used. The practical nitrogen consumption is 250 ml per hour. This cryostat ensures trouble-free and economical operation.

EXPERIMENTAL RESULTS AND THEIR DISCUSSION

In Figs. 2a,b the optical spectra of irradiated \(NaClO_3\) and \(KClO_3\) single crystals are presented. Low-temperature spectra...

Fig. 1. Cryostat for low-temperature studies.

1. Irradiated at \(-196^\circ\mathrm{C}\) (1 Mrad).
2. \(-140^\circ\mathrm{C}\).
3. \(-100^\circ\mathrm{C}\).
4. \(-40^\circ\mathrm{C}\).
5. Room temperature.
6. Background of an unirradiated crystal.

a

same conditions

b

Fig. 2. Low-temperature optical spectra of irradiated single crystals: a) \(\mathrm{NaClO_3}\), b) \(\mathrm{KClO_3}\).

their compounds consist of a considerable number of mutually overlapping absorption bands. The absorption bands at 230–235 nm, 265–270 nm, 390 nm, and 560–580 nm in \(NaClO_3\), and at 225–230 nm, 260–270 nm, and 560–580 nm in \(KClO_3\), are clearly manifested. To determine the temperature stability of the identified bands, an experiment was carried out involving the slow thawing of single crystals. The method for measuring optical spectra during thawing is similar to that described in [4].

Single crystals of \(NaClO_3\) and \(KClO_3\) irradiated at \(-196^\circ C\) have a violet-blue coloration. A noticeable decrease in the intensity of the absorption bands in the spectra was observed only above \(-160^\circ C\). In the region of \(-100^\circ C\), both in \(NaClO_3\) and in \(KClO_3\), the broad absorption band at 560–580 nm completely vanishes, with the simultaneous disappearance of the violet-blue coloration of the crystals. Further heating to \(-40^\circ C\) leads to a sharp decrease in the intensity of the absorption maxima of the remaining spectral bands. The crystals thereby acquire a yellowish-brown coloration. The disappearance of the intensity of the band at 390 nm in the spectrum of \(NaClO_3\) occurs in the temperature region above \(-80^\circ C\). In \(KClO_3\), upon thawing to \(-40^\circ C\), a clearly noticeable increase in the intensity of the band with a maximum at 450 nm is observed experimentally. A similar effect is absent in \(NaClO_3\). When the single crystals are heated to temperatures close to room temperature, all the bands present in the spectra of \(NaClO_3\) and \(KClO_3\) irradiated at \(-196^\circ C\) are thermally bleached, and the spectra assume an appearance similar to the spectra of crystals irradiated at room temperature. In the spectra, the absorption bands with maxima at 420 nm for \(NaClO_3\) and at 450 nm for \(KClO_3\) then appear distinctly.

Analysis of the low-temperature spectra shows the absence in them of the absorption band attributed to the \(ClO_3\) radical. Thawing of the single crystals to temperatures of \(-160 \div -150^\circ C\) does not produce any transformation of the optical spectra, whereas at the same time

as in work [10] showed that the radical \(\mathrm{ClO_3}\) disappears completely at \(-160^\circ\mathrm{C}\) in \(\mathrm{NaClO_3}\) and at \(-150^\circ\mathrm{C}\) in \(\mathrm{KClO_3}\). The absence of \(\mathrm{ClO_3}\) in the optical spectra can be explained by its thermal instability even at \(-196^\circ\mathrm{C}\). The \(\mathrm{ClO_3}\) signal in the EPR spectra [10] disappears completely in the light within 20 minutes. Irradiation with X-rays was carried out over 2–3 hours. Therefore, it is evident that \(\mathrm{ClO_3}\) disappears already in the course of irradiation of the single crystals. In Patrikeev’s work [4], the violet-blue coloration of single crystals irradiated at \(-196^\circ\mathrm{C}\) is attributed to the formation of \(\mathrm{ClO_3}\) radicals; however, as was noted in [10], such coloration is preserved even after complete disappearance of \(\mathrm{ClO_3}\).

In the present work this external coloration is attributed to the presence in the spectra of a band at \(560\text{–}580\ \mathrm{nm}\). The disappearance of this absorption maximum occurs in the region of \(-100^\circ\mathrm{C}\), simultaneously with the disappearance of the violet-blue coloration of the crystals. In this region of the spectra there lies none of the absorption maxima of the radiolysis products of alkali-metal chlorates that we have identified. The ease of thermal annealing of the bands in the low-temperature optical spectra of chlorates, the different coloration of the crystals, and its smooth transition from one to another indicate the presence of centers of an electronic nature. Therefore, by analogy with works [5] carried out on frozen alkaline matrices, we attributed this absorption band to a stabilized electron. The intense absorption band at \(390\ \mathrm{nm}\) in \(\mathrm{NaClO_3}\) irradiated at \(-196^\circ\mathrm{C}\) was also tentatively attributed by us to an electron, evidently stabilized at a deeper trap, since the thermal disappearance of this center occurs at a temperature above \(-100^\circ\mathrm{C}\), while the absorption lies in a shorter-wavelength region of the spectrum.

The thermally unstable band at \(260\text{–}270\ \mathrm{nm}\), whose final disappearance occurs at a temperature of \(-30^\circ\mathrm{C} \div 0^\circ\mathrm{C}\), was attributed by us to the \(\mathrm{ClO}\) radical, pol-

increase of light emission, which was observed in the work of Norman and Porter [6] at 270 nm.

Particular attention should be paid to the identification of the absorption bands at 420 nm in \(NaClO_3\) and at 450 nm in \(KClO_3\), which appear when the crystals are thawed to room temperature. In the literature the question of the nature of this band has been discussed. Hill [8] believes that absorption in the 420–450 nm region is due to \(Cl_2O_6\), which is formed upon dimerization of \(ClO_3\). The authors of [7] suggest the simultaneous presence of \(Cl_2O_6\) and \(O_3^{-}\). Boyd [3], however, showed the absence of \(Cl_2O_6\) in \(KClO_3\) irradiated at room temperature, and named the ozonide ion \(O_3^{-}\) as the only product absorbing in this region. In contrast to these opinions, we believe that the bands under discussion are electronic in nature and belong to color centers. Indeed, the formation and thermal stability of the ozonide ion were studied by us [4]. It turned out that \(O_3^{-}\) in the chlorate crystal matrix is destroyed completely at temperatures below room temperature, which does not correspond to the thermal stability of the 420–450 nm bands. The position of the absorption maxima changes along the series of chlorates (Table I), correlating in this case with the change in the lattice energy of the salts.

Table I.

The nature of the observed shifts of the absorption maxima of such bands is typical for electronic color centers. For molecular absorption bands, however, one should not expect the noted dependences; thus, according to the data of [9], absorption in the series of alkali-metal ozonides undergoes no shift and is located at 450 nm.

Thus, the reported data made it possible to assume the existence, in irradiated alkali-metal chlorates,

of an entire series of color centers. Further work will be devoted to establishing their nature, identifying them by independent methods, and [[unclear: studying their manifestations]].

REFERENCES

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3. H. Heel, Canad. J. Chem., 37, 5; 979–987, 1959.
4. L. V. Serikov, Yu. N. Safonov, Yu. A. Zakharov, this collection.
5. J. Jortner, B. Scharf, J. Chem. Phys., 37, 2506, 1962.
6. I. Norman, G. Porter, Proc. Roy. Soc., A230, 399, 1955.
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8. G. E. Boyd, J. Phys. Chem., 73, 369, 1969.
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10. L. V. Serikov, A. A. Vesnyanov, Yu. A. Zakharov, Izv. TPI, vol. 199, 1969, p. 76.