OPTICAL ABSORPTION IN IRRADIATED AMMONIUM PERCHLORATE CRYSTALS

V. A. NEVOSTRUEV, Yu. A. ZAKHAROV, I. F. KOLESNIKOVA

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

To elucidate the mechanism of radiolysis of solid salts, it is necessary to know the nature and composition of both unstable intermediate and final products of radiolysis. In a number of cases, the information needed in this direction can be obtained by studying EPR spectra and optical absorption in irradiated materials.

Electron paramagnetic resonance in irradiated ammonium perchlorate has by now been studied in considerable detail [1—4]. There are, however, no data in the literature on the optical absorption spectra in this material.

On this basis, and also in order to obtain results on light absorption in NH$_4$ClO$_4$ crystals under conditions comparable with those used for recording EPR spectra, we studied the optical spectra in perchlorate irradiated by various radiation sources. Some of the data obtained in this direction are presented in the present work.

Experimental Part

For recording the optical absorption spectra of ammonium perchlorate, the most perfect single crystals grown from aqueous solutions of NH$_4$ClO$_4$ were selected.

In experiments carried out at 77°K, irradiation and recording of the absorption spectra of the crystals were performed in a metal cryostat equipped with quartz windows for recording spectra and having a ground joint for moving the crystals after irradiation into the recording position. A detailed description of the cryostat can be found in [5]. When a vacuum of about $10^{-4}$ mm Hg was reached in the cryostat, liquid nitrogen was poured into the Dewar vessel of the cryostat, and the temperature of liquid nitrogen was maintained both during recording of the optical absorption spectra and during irradiation of the crystals. Irradiation was carried out with a narrow beam of X-rays from a ZBPM-200 X-ray tube operating at 200 kV and 20 mA. Optical absorption spectra were recorded on an SF-4 spectrophotometer before irradiation of the crystal and after its irradiation. To obtain identical conditions, recording of the optical absorption spectra at room temperature was also carried out in the cryostat and in vacuum.

When proton radiation was used, irradiation was carried out at atmospheric pressure and room temperature. However, the resulting

measured in this way differed in no respect from that obtained in the cryostat.

Results and Discussion

The optical absorption spectra obtained at various temperatures for single crystals of ammonium perchlorate irradiated with X-rays at room temperature and at 77°K to a dose of \(\sim 10^{18}\) eV/g are presented in Figs. 1 and 2, respectively.

Fig. 1. Absorption spectrum of ammonium perchlorate. Irradiation with X-rays and recording of the spectrum at room temperature. Absorbed dose \(\sim 10^{18}\) eV/g.

In the ultraviolet region of both spectra, a band with a maximum at 270 and 280 m\(\mu\) (at room temperature) is clearly resolved; at low temperatures it is shifted somewhat toward the short-wavelength region (maximum at 260 m\(\mu\)). Apparently, this band is even more pronounced for crystals irradiated with protons at a current of 0.1 \(\mu\)A for 1 min at room temperature (Fig. 3). However, in the latter case, excessively large overall absorption of the crystal in the wavelength region below 300 m\(\mu\) prevents more definite conclusions from being drawn about the relative magnitude of this absorption band.

Among the possible products of radiolysis of the perchlorate ion, gaseous ClO [6] and \(\mathrm{ClO_2^-}\) in aqueous solution [7] have an absorption maximum at this wavelength. Since ClO, as a paramagnetic particle, should have been detected by the EPR method but was not detected in [1–4], we are inclined to ascribe the absorption band at 270 m\(\mu\) to \(\mathrm{ClO_2^-}\) ions.

For perchlorate irradiated with X-radiation, a small maximum in the region around 300 m\(\mu\) is characteristic; it is somewhat more clearly resolved at low temperatures. In [7] it was estab-

Fig. 2. Absorption spectrum of ammonium perchlorate at 77°K. Irradiation with X-rays at 77°K with a dose of \(\sim 10^{18}\) eV/g.

Fig. 3. Absorption spectrum of ammonium perchlorate at room temperature. Irradiation with protons at room temperature \(\sim 7 \cdot 10^{20}\) eV/g.

It has been established that among the chloroxy products the characteristic absorption band is the one with a maximum at 300 mμ, inherent in $\mathrm{ClO^-}$ ions in aqueous solution. On the basis of these data, and also of the results of the analytical determination of $\mathrm{ClO^-}$ in irradiated $\mathrm{NH_4ClO_4}$ [9], we assign the band at $\sim 300$ mμ to hypochlorite ions. The presence of this band in perchlorate irradiated with protons is not obvious and requires further confirmation.

One of the most distinct bands in $\mathrm{NH_4ClO_4}$ irradiated at room temperature has an absorption maximum at 360 mμ, which shifts to 340–350 mμ when the crystal is cooled. Low-temperature measurements, as well as measurements on crystals irradiated with protons, give grounds for assuming that this band consists of two closely spaced absorption bands (Figs. 2, 3). The broadening of this band observed for low-temperature X-ray and proton irradiation may be associated with the accumulation under these conditions of products possessing absorption in the short-wavelength band that is part of the complex 340–380 mμ band.

Absorption corresponding to these wavelengths is characteristic of chlorine dioxide in solutions [7–8]. However, there are certain objections to assigning the absorption band near 360 mμ to the $\mathrm{ClO_2}$ molecule. First, chlorine dioxide was not found among the radiolysis products of $\mathrm{NH_4ClO_4}$ [9–10]; second, in the EPR spectrum of irradiated ammonium perchlorate, no signal due to the $\mathrm{ClO_2}$ molecule has been observed by anyone [1–4]. These circumstances compelled us to seek other causes for the appearance of the absorption band in the region $\lambda \simeq 360$ mμ, observed under all the irradiation and spectrum-recording conditions tested (Figs. 1–3). On the one hand, chemical analysis of irradiated ammonium perchlorate shows the presence of molecular chlorine [9, 10]; at the same time, in irradiated crystals of alkali-metal chlorides there is an absorption band with a maximum at $\lambda = 355$ mμ, assigned to a $V_k$ center, whose chemical nature is identified with the $\mathrm{Cl_2^-}$ ion [11]. For the reasons indicated, the most probable cause of the appearance of the absorption band under consideration is the presence of $\mathrm{Cl_2^-}$ in irradiated $\mathrm{NH_4ClO_4}$.

The relative intensity of the band at 420 mμ under proton irradiation is much smaller than under X-ray irradiation at room temperature (Figs. 1–3). Since the proton-irradiation dose is approximately 2 orders of magnitude higher than the X-ray dose, this may be due either to the magnitude of the absorbed dose or to intratrack phenomena. It is known from the literature that radicals $\mathrm{ClO_3}$ have an absorption maximum at this wavelength [12]. These radicals were detected by EPR in ammonium perchlorate irradiated both with X-rays at room temperature [1] and, at lower temperatures, with accelerated electrons [2, 3]. However, we were unable to detect them in $\mathrm{NH_4ClO_4}$ irradiated with protons at room temperature. Confirmation that this band belongs to $\mathrm{ClO_3}$ radicals is apparently also provided by the lower relative intensity of this peak for $\mathrm{NH_4ClO_4}$ irradiated with protons, since, owing to the high temperatures in the tracks, $\mathrm{ClO_3}$ radicals should in this case be unstable.

As for the other two absorption bands—at 460 and 520–540 mμ—which are clearly visible only under X-ray irradiation (Figs. 1, 2), it is not yet possible to say anything definite about their nature. In this direction one can make only certain assumptions. Thus, for example, the absorption maximum

absorption at 460 mµ appears upon irradiation of KClO₄ at 300°K and is due to the presence of the ion-radical O₃⁻ [13].

However, in [14] it was shown that ammonium ozonide is stable only below 180°K. The stability of O₃⁻ in the NH₄ClO₄ lattice and, consequently, the assignment of the 460 mµ band to ozonide therefore requires special investigation.

There is still greater uncertainty in the interpretation of the absorption band at 520 mµ, since the radiolysis products of the perchlorate ion in this region have no absorption bands [12].

The appearance of this band upon irradiation of frozen NaClO₄ solutions at 77°K was attributed by Bugaenko and Belevskii to electrons stabilized at defects [12, 15]. The appearance of electronic color centers in NH₄ClO₄ irradiated at room temperatures, however, has not been reported in studies on perchlorate radiolysis and requires further clarification. The broad singlet in the EPR spectra of NH₄ClO₄, which may be assigned to a stabilized electron, is stable only in the low-temperature region. It should be noted that preliminary experiments on the thermal stability of the products responsible for the absorption bands to some extent confirm the acceptability of the above interpretation of the NH₄ClO₄ absorption spectrum. Thus, for example, when single crystals irradiated at room temperature with X-rays are heated for 4 hours at 340°K, the absorption bands at 420 and 520 mµ disappear, while the 360 mµ band remains unchanged under these conditions.

The identification given above of some absorption bands in irradiated ammonium perchlorate should not be regarded as definitive, in particular because the possible absorption of intermediate products formed during oxidation of the ammonium ion was not taken into account, owing to the absence of information about them. For a more detailed study and identification of the radiolysis products of NH₄ClO₄ responsible for these or other absorption bands, we intend in the future to study the kinetics of growth of the band intensities upon irradiation of NH₄ClO₄ and the behavior of the bands under the action of heat and light on the irradiated crystals.

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