Conditions for the Formation of Two Types of Spectra in Irradiated CaF₂ Crystals

E. K. Zavadovskaya, L. A. Lisitsyna, V. M. Lisitsyn

At present, the literature is discussing the question of the reasons for the appearance, under photochemical and additive coloration of CaF₂ crystals, of two types of spectra: the Smakula spectrum, consisting of four bands with maxima at 225, 335, 400, and 580 mμ, and the Mollow-type spectrum, whose largest bands are those with maxima at 380 and 525–560 mμ. Luty and Fong [1, 2] ascribe the appearance of the different types of spectra to the influence of thermal treatment of the crystal during additive coloration.

A number of authors [3, 4] explain the appearance of the Mollow spectrum by the presence of oxygen in the crystal. Bontinck [5] considers that the form of the spectrum depends on the radiation energy; Przibram [6], on the irradiation dose; and Ratan [7], on the irradiation temperature. Clarifying the reasons for the appearance of two types of spectra is of fundamental importance in solving the question of the formation of color centers in CaF₂ crystals.

In the present communication, results are given from investigations of the conditions for the formation of two types of spectra in colored CaF₂ crystals.

The following experiments were carried out. A CaF₂ crystal, transparent in the range 200–1000 mμ, was heated in a quartz tube at a temperature of 700°C, in a vacuum of \(4 \times 10^{-2}\) mm Hg, for 4 hours, and then was rapidly cooled to room temperature. It was found that in the spectrum of such a crystal a band appears with a maximum at 200 mμ. Subsequent irradiation of this crystal with x rays, electrons, or protons causes the appearance in the spectrum of additional bands with maxima at 320, 380, 470, and 560 mμ (Fig. 1). The spectrum of a crystal that, before irradiation, did not have the band

Fig. 1. Spectra of a CaF₂ crystal subjected to thermal treatment. \(d = 0.6\) cm.
1 — spectrum of the crystal before irradiation.
2 — crystal irradiated with x rays for 0.5 hour (70 kV, 10 mA)

in the ultraviolet, has the form of Smakula’s spectrum and consists of bands with maxima at 225, 335, 400, and 580 mµ (Fig. 2). It has been noted that the sensitivity to coloration of a CaF₂ crystal having a band in the ultraviolet increases sharply in comparison with a crystal transparent in this region of the spectrum.

The study of spectra of doped crystals made it possible to establish the following fact: introduction into a CaF₂ crystal of a sodium impurity which, when incorporated in the place of a cation, brings with it an additional number of anion vacancies, causes the appearance in the spectrum of the unirradiated crystal of a band with a maximum at 210 mµ. The spectrum of such a crystal, weakly irradiated with protons of energy 2 MeV, consists of bands with maxima at 206, 320, 380, and a broad band at 560 mµ (Fig. 3). It is necessary

Fig. 2. Spectrum of a CaF₂ crystal not subjected to heat treatment. \(d = 0.6\) cm.
The crystal was irradiated with x-rays for 4.5 hours (70 kV, 10 mA).

Fig. 3. Spectra of a CaF₂—Na crystal.
1 — crystal before irradiation,
2 — crystal irradiated with protons, dose \(1.2 \cdot 10^{13}\) protons/cm²,
3 — CaF₂ crystal before irradiation

to note that a spectrum similar to the spectrum of an irradiated CaF₂—Na crystal is also observed in irradiated synthesized CaF₂ crystals, grown

produced in our laboratory from CaF₂ powder obtained by synthesizing calcium chloride with sodium fluoride at 1000°C in air.

Thus, CaF₂ crystals that, before irradiation, contain an absorption band in the ultraviolet, after irradiation have a spectrum different from that of a crystal that was transparent in the ultraviolet before irradiation. The appearance of the band may be caused either by oxygen entering the crystal as a result of heat treatment, or simultaneously with the introduction of the Na impurity into the crystal [8], or by anion vacancies introduced into the crystal by impurities of oxygen and sodium.

Apparently, the presence in the crystal of defects responsible for the band with a maximum at 205–210 mµ leads to a sharp increase in the efficiency of formation by radiation of color centers responsible for the bands with maxima at 380 and 560 mµ. The intensities of these bands are considerably greater than the intensities of the Smakula spectrum bands under isodose irradiation; therefore, they determine the type of spectrum of the crystal.

The authors express their gratitude to L. V. Zagumenova for assistance in carrying out the experiments.

REFERENCES

1. F. Luty. Z. Physik. 134, 596, (1953).
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3. J. H. Schulman, R. J. Ginther, R. D. Kirk. J. Chem. Phys., 20, 1966 (1952).
4. D. Messner, A. Smakula. Phys. Rev. 120, 1162 (1960).
5. W. Bontinck. Physica 24, 639 (1958).
6. K. Przibram. Z. Physik, 154, 111 (1959).
7. V. V. Ratnam. Phys. Stat. Sol. 16, 559 (1966).
8. W. J. Scouler, A. Smakula. Phys. Rev. 120, 1154 (1960).