Radiolysis of Potassium Perchlorate.

EPR Spectra and Optical Spectra

A. A. Vasil'ev, Yu. N. Safonov, Yu. A. Zakharov, L. V. Serikov

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

In works [1, 2], the products of the radiation-chemical decomposition of potassium perchlorate after dissolution of the irradiated salt in water were studied.

Chlorate, chlorite, molecular oxygen, chloride, hypochlorite, chlorine dioxide were detected, and, in addition, the possible presence of \( \mathrm{ClO_3} \) \((\mathrm{Cl_2O_6})\) and potassium oxides was indicated. Similar results were obtained also in earlier works [3, 4].

T. Koulsi [5, 6] measured the EPR spectra of irradiated potassium perchlorate at room temperature, and the radicals \( \mathrm{ClO_2} \) and \( \mathrm{ClO_3} \) were found.

A detailed analysis of the EPR spectrum of chlorine dioxide was carried out in [7]. In [8] and [9], the paramagnetic absorption of \( \mathrm{KClO_4} \) irradiated at low temperature was studied. The spectra obtained were assigned to the radical \( \mathrm{ClO_4} \), and in [9] an assumption is made about the existence of the peroxide radical \( \mathrm{O_2Cl-O-O} \). In the same work, at room temperature, the formation under the action of ultraviolet light in irradiated samples of a paramagnetic center identified as \( \mathrm{Cl-O-O} \) was established.

An attempt was also made there to study the optical reflection spectra of irradiated polycrystalline samples of \( \mathrm{KClO_4} \), and absorption bands with maxima at 360 nm and 385 nm were noted, assigned to chlorine dioxide and the peroxide radical \( \mathrm{Cl-O-O} \), respectively.

In work [10], at low temperatures a paramagnetic center was found, identified as the doubly charged ion \(ClO_4^{2-}\).

In the present work we undertook efforts to determine the products of radiolysis of potassium perchlorate and to investigate their secondary reactions using the methods of optical spectrophotometry and electron paramagnetic resonance.

EXPERIMENTAL PROCEDURE

The work used both polycrystalline potassium perchlorate samples, obtained by repeated recrystallization of “ch” grade salt followed by drying and grinding (particle sizes did not exceed 200 μm), and single crystals grown by the methods of slow evaporation or slow cooling of aqueous solutions purified by repeated recrystallization of \(KClO_4\).

EPR spectra were recorded on an RE-1301 spectrometer and an ELA-2 spectrometer coupled to an ESG-2.5 electron accelerator, which made it possible to record spectra during irradiation. The sample temperature in this case was regulated by blowing gaseous nitrogen through the resonator.

Optical absorption spectra were obtained on an SF-4a spectrophotometer. Low-temperature optical spectra were measured on an SFD-2 spectrophotometer using a cryostat that made it possible to vary the sample temperature from \(-196^\circ\mathrm{C}\) to \(+20^\circ\mathrm{C}\) [18].

The radiation sources used were an ESG-2.5 electron accelerator (\(E = 1.6\) MeV) and a 3-BTM-200 X-ray tube operating at a voltage of 200 kV and a current of 20 mA.

Copper perchlorate and silver perchlorate, synthesized according to the procedures described in [11], were introduced into [[unclear: β-mark]] \(KClO_4\) by cocrystallization from aqueous solutions. The concentration of add-

The dose was determined by polarographic analysis of solutions of dosed potassium perchlorate.

To determine radical concentrations, double integration of the lines in the EPR spectra was carried out using a nomogram.

EXPERIMENTAL RESULTS

Irradiation causes the appearance of a group of optical absorption bands in the wavelength region 230–700 nm and the formation of a number of paramagnetic products. The form of the optical spectra and of the EPR spectra depends on the experimental conditions.

EPR spectra. The form of the EPR spectrum appearing upon electron irradiation of single crystals of $KClO_4$ at $T = -196^\circ C$ is shown in Fig. 1.

Three groups of lines, designated by the numerals 1, 2, and 3, belong to three types of paramagnetic centers.

In [8], groups of lines 1 and 2 are assigned to the $ClO_4^{2-}$ radical, located in the $KClO_4$ lattice in two magnetically inequivalent positions. Line 3 in [10] is attributed to the $ClO_4^{2-}$ center. The accumulation curves of $ClO_4^{2-}$ and $ClO_4^{2-}$ (the total concentration in the two positions) are shown in Fig. 2.

When the crystal is illuminated with ultraviolet light (DRSh-1000 lamp) or with daylight, line 3 rapidly decreases in intensity and disappears.

Heating to $T = -160^\circ C$ also leads to changes in the spectrum. The lines of group 1 disappear completely, and the intensity of lines 2 increases by a factor of two.

In the temperature interval from $-160^\circ C$ to $-100^\circ C$, the form of the spectrum does not change qualitatively; however, all lines in the spectrum broaden, and at $T = -120^\circ C$ and $-100^\circ C$ the lines 2, in connection with this, are not observed.

By cooling the sample again to temperatures below $-160^\circ C$, it is possible to obtain the spectrum in its former form, without any changes in the intensities of the individual lines.

In the temperature range from \(-100^\circ\mathrm{C}\) to \(-80^\circ\mathrm{C}\), all the described spectral lines disappear. This process is irreversible and is not accompanied by the simultaneous appearance of new lines that could be attributed to any chlorine-containing paramagnetic centers. Further heating of the samples to room temperature leads to the appearance of eight lines belonging to \(\mathrm{ClO_2}\) radicals, and of groups of overlapping lines belonging, apparently, to centers of three types that do not contain chlorine nuclei. For their unambiguous identification the experimental data are insufficient. One may only suppose that these centers may be \(\mathrm{O_3^-}\), \(\mathrm{O_2^-}\), and \(\mathrm{O_2}\).

All chlorine-containing paramagnetic centers have anisotropic spectra. Rotation of single crystals in a magnetic field leads to splitting of each line into two components, corresponding to two magnetically equivalent positions of the radicals in the \(\mathrm{KClO_4}\) lattice. For the \(\mathrm{ClO_2}\) radical this phenomenon was first observed by T. Cole [6].

The EPR spectrum at \(T = +20^\circ\mathrm{C}\) includes the spectra of the radicals \(\mathrm{ClO_2}\), \(\mathrm{ClO_3}\), as well as a group of lines from paramagnetic centers not containing chlorine nuclei. In [10], among these products the ozonide ion and oxygen were identified.

Post-radiation changes in the concentrations of \(\mathrm{ClO_2}\) and \(\mathrm{ClO_3}\) are reported in [17]. The concentration of \(\mathrm{ClO_2}\) increases over 20–24 hours, whereas the concentration of \(\mathrm{ClO_3}\) decreases. As a result of these changes, certain values of \([\mathrm{ClO_2}]\) and \([\mathrm{ClO_3}]\) become established. The radiation-chemical yield of chlorine dioxide, measured 24 hours after irradiation, is 0.45 molecule per 100 eV.

Optical absorption spectra and reflection spectra

Figure 3 (curve I) shows the absorption spectrum of a \(\mathrm{KClO_4}\) crystal irradiated with X-rays.

radicals in \(KClO_4\) at \(T = +20^\circ C\) is shown in Figs. 5 and 6. In the case of additional doping of \(KClO_4\) with iodine ions, single-crystal samples were used and the concentrations of the radicals \(ClO_2\) and \(ClO_3\) were measured.

We were unable to obtain potassium perchlorate single crystals with additions of silver ions suitable for measurements, and therefore polycrystalline samples were used. In this case, the total concentration of paramagnetic centers was measured.

The measurements were carried out one day after irradiation in order to reduce the error possible because of the course of post-radiation processes, which were observed in the study of pure potassium perchlorate. Thus, the established concentrations of radicals were measured.

The values of these concentrations decreased upon the introduction of iodine and increased upon the introduction of silver ions.

The diffuse-reflection spectra also changed upon the introduction of additives. Additions of iodine decrease the intensity of the spectrum over the entire wavelength range, most strongly in the region of the maxima at 360 nm and 255 nm. Additions of silver ions weaken the bands at 255 nm, 280 nm, and 440 nm, while simultaneously increasing the optical density in the region of 300–390 nm.

DISCUSSION OF RESULTS

The study of the results obtained at low temperatures confirms the correctness of the assumption made in [8] concerning the existence of \(ClO_4\) in two magnetically nonequivalent positions with different spectral characteristics (line groups 1 and 2).

First, the changes in the EPR spectrum corresponding to the transition of \(ClO_4\) from one state to another are not accompa-

at liquid-nitrogen temperature with a dose of 2 Mrad.

Curves 2 and 3 in this figure are the spectra of the same sample after heating to \(T = -100^\circ\mathrm{C}\) and \(T = -20^\circ\mathrm{C}\), respectively.

The absorption maxima are located near 255 nm, 440 nm, 530 nm, and 610 nm.

Heating is not accompanied by changes in the spectrum up to \(T = -160^\circ\mathrm{C}\).

At \(T = -100^\circ\mathrm{C}\) and above, weakening of the absorption bands at 610 nm and 530 nm occurs, along with some increase in optical density in the 440 nm band.

The absorption spectra of single crystals obtained after irradiation at room temperature are shown in Fig. 4. Here curve 1 is the spectrum observed two hours after irradiation. Curves 2 and 3 were obtained after illumination of the irradiated crystals with ultraviolet light and their heating to \(T = +60^\circ\mathrm{C}\), respectively.

The absorption bands in the spectrum have maxima at 255, 280, 360, and 440 nm.

The overall weakening of absorption that occurs upon heating is more pronounced in the region of the short-wavelength maxima.

Ultraviolet illumination (both with unfiltered light from a DRSh-1000 lamp and with a band-pass light filter at 310 nm), along with weakening of the 255, 360, and 440 nm bands, increases absorption in the region of 275 nm + 350 nm.

The diffuse-reflection spectra of irradiated potassium perchlorate powder differ from the absorption spectra of single crystals in the relative values of the optical density in different parts of the spectrum and in the distinctness of the individual absorption maxima. In this case the maximum at 360 nm is not manifested.

Effect of additions of copper and silver ions

The effect of additions of copper perchlorate and silver perchlorate, introduced by co-crystallization, on the values of the concentrations of ra-

...are not accompanied by any changes in the optical spectra. Conversely, under ultraviolet illumination of the samples one can observe the reverse transition, although again without quantitative agreement.

The high photosensitivity of the product giving line 3 (Fig. 1) in the EPR spectrum at \(-196^\circ\mathrm{C}\) does not contradict its identification as \(\mathrm{ClO_4^{2-}}\).

The broadening of the spectral lines with increasing temperature is evidently explained by the freezing-out of rotational degrees of freedom and the averaging of strongly anisotropic spectra.

The absorption bands observed in the ultraviolet and visible regions of the optical spectrum at different temperatures can be assigned to individual radiolysis products on the basis of the considerations given below.

The band with a maximum at 255 nm may be assigned to chlorite ions.

Chlorite is one of the radiolysis products of \(\mathrm{KClO_4}\). The absorption maximum of \(\mathrm{ClO_2^-}\), both in aqueous solutions and in solid chlorite matrices [12], lies near 255 nm.

Absorption in the region of 280 nm is apparently due to the existence in irradiated \(\mathrm{KClO_4}\) of hypochlorite ions, which have a similar absorption band both in irradiated chlorates and in aqueous solution. Part of the absorption in this region is caused by chlorite ions, whose band has a shoulder near 280 nm [12,13].

The band at 360 nm belongs to chlorine dioxide. The following grounds support this assertion: the spectrum of \(\mathrm{ClO_2}\) in the gas phase, in solutions, and in solid matrices has an absorption maximum at about 355–360 nm; heating, ultraviolet illumination, and the introduction of additives have the same effect on the intensity of this line and on the EPR spectrum of \(\mathrm{ClO_2}\).

The 440 nm band, close in position to the absorption band (maximum 450–460 nm) observed by many authors in irradiated \(\mathrm{KClO_3}\) [12–14], is assigned either to ozo...

the iodide ion \([13]\), or chlorine interstitials \([12]\), or an electron color center analogous to the \(F\)-center in alkali-halide crystals \([14]\). However, in potassium perchlorate the 440 nm band cannot be assigned unambiguously to \(O_3\) or \(Cl_2O_6\).

Chlorine interstitials are formed from chlorine vacancies, but in \(KClO_4\) samples irradiated at \(-196^\circ C\), noticeable amounts of \(ClO_3\) are not observed in the EPR spectra, whereas there is strong absorption at 440 nm. It would be difficult to explain the formation of a large amount of \(Cl_2O_6\) in this case.

The ozonide ion is a paramagnetic particle, and a change in the optical density in its absorption band should have been accompanied by corresponding changes in the EPR spectrum upon heating and ultraviolet illumination. But no such correspondence is observed.

Thus, assignment of this band to absorption by electron color centers appears the most probable. This is also confirmed by the fact that, when the temperature is lowered, the maximum shifts to the region of shorter wavelengths, which is characteristic of bands of \(F\)-centers in alkali-halide crystals.

The bands with \(\lambda_{\max} = 530\) nm and 610 nm, existing at \(T < -100^\circ C\), apparently belong to electron-excess centers. One of them may be the doubly charged ion \(ClO_4^{2-}\). The other, evidently, is an electron captured by a shallow trap.

A certain increase in the intensity of the 440 nm band during thermal destruction of the 610 nm and 530 nm bands can be explained by the transfer of part of the captured electrons from shallow traps into deeper ones.

Subtraction of the spectra at \(T = -196^\circ C\) and \(T = -100^\circ C\) shows the existence of absorption with a maximum near 260 nm of a thermally unstable product, which is possibly chlorine tetroxide.

As shown in \([15]\), an increase in optical density at a wavelength of 320 nm under the action of ultraviolet irradiation

is associated with the formation of \(\mathrm{Cl_2^-}\) ions. But absorption in the 300–340 nm region also exists when there are no dichloride ions; moreover, when silver ions are introduced its intensity increases. The possibility cannot be excluded that chlorine oxoanions are responsible for this absorption.

The results of the study of optical absorption and paramagnetic resonance, taking into account the interpretation carried out, can provide some information on the mechanism of the processes occurring in \(KClO_4\) upon irradiation.

There is a substantial difference between the optical spectra and the EPR spectra obtained at \(T=-196^\circ C\) and \(T=+20^\circ C\).

Heating irradiated samples from the temperature of liquid nitrogen to room temperature is accompanied by changes both in the optical spectra and in the EPR spectra in the direction of reducing this difference.

Taking into account that the primary processes of interaction of radiation quanta with molecules of the substance do not depend on temperature, it may be assumed that many of the products observed at \(T=+20^\circ C\) are formed as a result of secondary reactions.

The existence of competing pathways for the conversion of the primary radiolysis products into final ones may ultimately lead to different compositions of the final products in the case of irradiation at different temperatures. A change in the experimental conditions can shift the process toward one or another competing reaction, thereby changing the relative yields of the products.

The data available in [16] on the measurement of radiation-chemical yields of the principal chemically analyzed products of the radiolysis of \(NaClO_4\) at different irradiation temperatures show that the observed changes are small. This may

would be a consequence either of the existence of only a single analogous path of secondary transformations, or of the formation of identical final products of radiation reactions.

Chlorine trioxide, observed by the EPR method and formed with a considerable yield (at a dose rate of \(10^{4}\ \mathrm{rad}/\mathrm{min}\),

\[ [G_{\max}] = 0.8 \text{ molecules per } 100\ \mathrm{eV} \]

) at room temperature, is absent in \(KClO_{4}\) irradiated at \(-196^{\circ}C\). The yield of chlorine dioxide (the concentration was measured 24 hours after irradiation at \(T = +20^{\circ}C\)) decreases from 0.45 to 0.11 molecule per event when the irradiation temperature is lowered from \(+20^{\circ}C\) to \(-196^{\circ}C\). Consequently, first, \(ClO_{2}\) and \(ClO_{3}\) are not the primary products of radiolysis and, second, there exists a reaction (or group of reactions) that competes with the reactions of their formation and proceeds at low temperature. If the formation of \(ClO_{3}\) occurs by the reaction:

\[ ClO_{4} \longrightarrow ClO_{3} + O, \]

then the competing reaction will be:

\[ ClO_{4} + e^{-} \longrightarrow \text{products}. \]

The electron participating in this reaction is released upon destruction of color centers of the crystal or during the course of the reaction:

\[ ClO_{4}^{2-} \longrightarrow ClO_{4}^{-} + e^{-}. \]

During irradiation these may be thermalized electrons, formed as a result of the action of ionizing radiation. The participation of \(ClO_{4}\) in reactions with electrons leads to a nonlinear character of the accumulation curve for \(ClO_{4}\).

As the concentration of \(ClO_{4}\) increases, competition between all processes and the reaction of electron acceptance by anions becomes stronger, leading to a decrease in the yield of \(ClO_{4}^{2-}\).

Defects of the crystal structure, by trapping electrons, likewise affect these reactions at the initial stage of radiolysis, when their concentration is substantial.

Molecules of chlorine trioxide can participate in several parallel reactions considered in [17]. The formation of chlorine dioxide by the reaction:

\[ ClO_{4} \longrightarrow ClO_{2} + O_{2} \]

is excluded, since the decay of \(\mathrm{ClO}_4\) radicals according to the EPR spectra is always observed considerably earlier than the formation of an appreciable amount of \(\mathrm{ClO}_2\). Post-radiation accumulation of chlorine dioxide may proceed according to the scheme:

\[ 2\mathrm{ClO}_3 \rightleftarrows \mathrm{Cl}_2\mathrm{O}_6,\qquad \mathrm{ClO}_3 \rightarrow \mathrm{ClO}_2 + \mathrm{O}. \]

In this way one may find an explanation for the fact that the observed increase in the concentration of \(\mathrm{ClO}_2\) is not equal to the decrease in the concentration of \(\mathrm{ClO}_3\).

Chlorate, chlorite, and hypochlorite are either products of reactions of electron recombination with the radicals \(\mathrm{ClO}_4\), \(\mathrm{ClO}_3\), \(\mathrm{ClO}_2\), or are formed in the decay of the excited anion \(\mathrm{ClO}_4^{-*}\), which, according to data [1–4], is one of the primary products of the interaction of radiation with potassium perchlorate.

The nature of the influence of additives introduced into the \(\mathrm{KClO}_4\) lattice on the composition and yields of radiolysis products may be determined by various processes. In different cases the most significant may be either the donor-acceptor properties of the additives, or the nature of changes in, or defectiveness of, the crystal lattice, or their ability to serve as centers of recombination of electrons with radicals, or some other properties.

The increase in the radiation-chemical yields of silver ions is explained by the fact that they are electron acceptors and, by their presence in the lattice, decrease the probability of electron recombination with radicals.

The general weakening of the diffuse-reflection spectra of irradiated \(\mathrm{KClO}_4\) and the decrease in the yields of \(\mathrm{ClO}_2\) and \(\mathrm{ClO}_3\) by \(\mathrm{Cu}^{2+}\) ions can be explained predominantly by their action as recombination centers, as well as by a possible decrease in the concentration of anion vacancies in doped samples. The influence of \(\mathrm{Cu}^{2+}\) ions, as electron acceptors, in the present case is compensated by predominant processes in the opposite direction. Accepting such an explanation, one should assume that the recombination reaction involving \(\mathrm{Cu}^{2+}\):

\[ \mathrm{ClO}_4 + \mathrm{Cu}^{2+} + \bar{e} \rightarrow \mathrm{ClO}_4 + \mathrm{Cu}^{+} \rightarrow \mathrm{ClO}_4^{-} + \mathrm{Cu}^{2+} \]

is not dissociative in character, in contrast to the reaction:

\[ \mathrm{ClO}_4 + \bar{e} \rightarrow \mathrm{ClO}_4^{-*} \rightarrow \text{products}. \]

Changes in the optical absorption spectrum under the action of ultraviolet light are due to a decrease in the concentration of color centers (\(\lambda_{\max}=440\) nm), chlorine dioxide (\(\lambda_{\max}=360\) nm), and chlorite (\(\lambda_{\max}=255\) nm). The recovery of the optical density at 320 nm, caused by the accumulation of \(\mathrm{Cl}_2\), masks a possible decrease in absorption in this region due to the destruction of chlorine trioxide. Simultaneously with the decomposition of chlorine dioxide, chlorine trioxide, and chlorite ions, as a result of the direct action of ultraviolet light, reactions of \(\mathrm{ClO}_2\) and \(\mathrm{ClO}_3\) with electrons released from traps take place. Dichloride ions are formed from chloride and atomic chlorine, which is a decomposition product of chlorine dioxide.

Judging from the optical spectra, heating leads to a strong decrease in the concentration of most of the observed products. This occurs for the following reasons: chlorate and hypochlorite are converted into chloride and oxygen; color centers decay, and the electrons released, recombining with \(\mathrm{ClO}_2\) and \(\mathrm{ClO}_3\) radicals, form products that are converted into chloride, oxygen, and chlorate.

Using the results presented and the data of works [1–3, 10, 15, 17], one may conclude that the following reactions occur in the process of radiation-chemical decomposition of potassium perchlorate:

\[ \begin{aligned} 1.\quad & \mathrm{ClO}_4^- \rightarrow \mathrm{ClO}_4 + \bar e \\ 2.\quad & \mathrm{ClO}_4^- + \bar e \rightarrow \mathrm{ClO}_4^{2-} \\ 3.\quad & \mathrm{ClO}_4^{2-} \rightarrow \mathrm{ClO}_4^- + \bar e \end{aligned} \]

\[ 4.\quad \mathrm{ClO}_4^- + e \rightarrow \mathrm{ClO}_4^{-*} \begin{cases} \rightarrow \mathrm{ClO}_3^- + \mathrm{O},\\ \rightarrow \mathrm{ClO}^- + \mathrm{O}_2 + \mathrm{O},\\ \rightarrow \mathrm{ClO}_2^- + \mathrm{O}_2,\\ \rightarrow \mathrm{Cl}^- + 2\mathrm{O}_2,\\ \rightarrow \mathrm{ClO}_4^-. \end{cases} \]

\[ 5.\quad \mathrm{O}_2 + \bar e \rightarrow \mathrm{O}_2^-;\qquad \mathrm{O}_2^- + \mathrm{O} \rightarrow \mathrm{O}_3^- \]

\[ 6.\quad \mathrm{ClO}_4^- \rightarrow \mathrm{ClO}_3 + \mathrm{O} \]

\[ 7.\quad \mathrm{ClO}_3 + \bar e \rightarrow \mathrm{ClO}_3^{-*} \begin{cases} \rightarrow \mathrm{ClO}_3^-,\\ \rightarrow \mathrm{ClO}_2^- + \mathrm{O},\\ \rightarrow \mathrm{ClO}^- + \mathrm{O}_2,\\ \rightarrow \mathrm{Cl}^- + \mathrm{O}_2 + \mathrm{O}. \end{cases} \]

$$ \begin{aligned} 8.\quad &2ClO_3 \rightleftarrows Cl_2O_6\\ 9.\quad &ClO_3 \to ClO_2 + O\\ 10.\quad &ClO_2^- \to \begin{cases} ClO^- + O,\\ Cl^- + O_2 \end{cases}\\ 11.\quad &ClO^- \to Cl^- + O \end{aligned} $$

Lattice defects, by capturing electrons, form color centers.

Some products may apparently be formed during the decay of excited anions according to the reactions:

$$ 12.\quad ClO_4^- \leadsto ClO_4^{-*} \to \begin{cases} ClO_3^- + O,\\ ClO_2^- + O_2,\\ ClO^- + O_2 + O,\\ Cl^- + 2O_2. \end{cases} $$

The ultraviolet light initiates the following reactions in the irradiated \(KClO_4\):

$$ \begin{aligned} 13.\quad &ClO_2 \to Cl + O_2\\ 14.\quad &Cl + Cl^- \to Cl_2^-\\ 15.\quad &ClO_3 \to ClO_2 + O\\ 16.\quad &ClO_2^- \to ClO^- + O \end{aligned} $$

In addition, destruction of the color centers occurs with the participation of the released electrons according to reaction (7) and the reaction:

$$ 17.\quad ClO_2 + \bar{e} \to ClO_2^{-*} \to \begin{cases} ClO^- + O,\\ ClO_2^-,\\ Cl^- + O_2. \end{cases} $$

The reaction scheme presented does not contradict many experimental results; however, it cannot be regarded as exhaustive. It requires both additional experimental confirmations and the establishment of the actual values of the kinetic constants.

Fig. 1. EPR spectrum at \(T=-196^\circ\mathrm{C}\).

Fig. 2. Accumulation of \(\mathrm{ClO}_4\) (1) and \(\mathrm{ClO}_4^{2-}\) (2).
\(T=-196^\circ\mathrm{C}\).

Fig. 3. Optical absorption of \(\mathrm{KClO}_4\) irradiated at \(-196^\circ\mathrm{C}\).

Fig. 4. Optical absorption of irradiated \( \mathrm{KClO_4} \) at \( T = +20^\circ\mathrm{C} \).

Fig. 5. Accumulation of radicals in \( \mathrm{KClO_4} \) (1) and \( \mathrm{KClO_4} + 0.72\%\,\mathrm{AgClO_4} \) (2).

Fig. 6. Accumulation of \( \mathrm{ClO_2} \) (1, 2, 3) and \( \mathrm{ClO_3} \) (4, 5, 6) in \( \mathrm{KClO_4} \), \( \mathrm{KClO_4} + 0.16\%\,\mathrm{Cu(ClO_4)_2} \), and \( \mathrm{KClO_4} + 0.36\%\,\mathrm{Cu(ClO_4)_2} \).

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