ON THE OSCILLATORY STRUCTURE IN THE SPECTRA OF “CANDOLUMINESCENCE” OF MAGNESIUM OXIDE

V. A. SOKOLOV, N. A. NAZIMOVA

(Presented by the Scientific Seminar on Dielectrics)

As early as Nicol [1], in the candoluminescence spectra of certain substances, including the spectrum of calcium oxide excited by a flame, noted the presence of an oscillatory structure in the form of faintly noticeable or, as he puts it, “submerged” maxima, separated by an approximately constant frequency interval.

V. M. Kudryavtseva [2], comparing the frequencies of the electron-vibrational bands of CaO photoluminescence observed by Yule with the frequencies of the candoluminescence bands of the same substance according to Nicol’s data, showed that they may be connected with one and the same electronic transition in combination with vibrational quanta of the lattice and described by a single general formula. V. M. Kudryavtseva and co-workers [2] also detected a faint oscillatory structure in the spectrum of thermally excited “candoluminescence” of zinc oxide and gave a formula for calculating the bands revealed. P. S. Litvinova [3], using this formula, was able to calculate the fine structure of the photoluminescence spectra of zinc oxide that she had discovered.

We have found an oscillatory structure in the spectra of “candoluminescence”¹) of magnesium oxide, obtained in one case by photographing the flame of burning magnesium, and in the other case by photographing incandescent magnesium oxide excited by an illuminating-gas flame.

In Fig. 1 are shown, above—the first, and below—the second of the spectra thus obtained for one and the same region of the visible range. It is easy to see that, on the one hand, the spectra repeat one another, and, on the other, an oscillatory structure is clearly outlined in the form of weak but regularly arranged maxima. The former indicates that in the present case the glow of the magnesium flame is due mainly to the emission of solid oxide particles formed as a result of the reaction, thermally excited by the high temperature of the flame; the latter testifies, in both cases, to the presence of a close interaction between the vibrations of the particles composing the lattice and the electronic transitions responsible for the radiation. Since the latter, in the cases cited, has been connected by some authors with the candoluminescence of magnesium oxide, in calculating the oscillatory structure discovered we attempted to relate the electronic transitions to the “ac-

¹) The so-called candoluminescence of MgO at high temperatures is, for the most part, merely the ordinary thermal radiation of this substance [6].

“activators,” that is, with excess magnesium atoms. Moreover, as a result of a prolonged selection of various combinations of vibrational transitions, it proved possible to obtain only a somewhat remote agreement with experiment.

However, it subsequently turned out that almost the entire system of weak maxima shown in Fig. 1 in this region of the magnesium oxide spectrum is accounted for in a natural way by using the known data [4] on the green system of electron-vibrational bands of the MgO molecule*). Moreover, subsequent experiments established that, upon

Fig. 1

changing the conditions of magnesium combustion—namely, in the case when this combustion proceeds explosively, in the form of a flash—the vibrational structure in the photographed spectrum appears no longer as a barely noticeable modulation of the oscillations against the background of continuous radiation, revealed only as a result of photometry, but as a banded structure quite distinctly visible in the photograph to the eye, with a violet shading and with a regular decrease in the intensity of the bands in the same direction.

In Fig. 2 are presented the results of photometry of such a spectrum. It turns out that in the spectrogram under consideration almost all the intense maxima of the green system of bands of the MgO molecule, well known from the literature data [4], were marked exceptionally clearly. However, in the spectra shown in Fig. 1 these bands appeared only as a weak modulation of the oscillations against the background of the continuous spectrum at those places which, in wavelength, approximately correspond to the positions of the above-mentioned bands of the MgO molecule. In Table 1, the first column gives the positions of the intense bands of the green system of molecular

*) V. M. Kudryavtseva and P. S. Litvinova, for CaO and ZnO, also used the ordinary formulas of electron-vibrational transitions for the molecule, with an appropriate selection of the vibrational quanta of the upper and lower states. The rather satisfactory agreement with experiment obtained in this way indicates that the character of the vibrations of molecules or lattice ions, and their interaction with electronic transitions, are to some extent similar to the vibrations of atoms in a molecule and to their influence on its optical radiation.

spectra of MgO according to literature data [4]; in the second column are also indicated the positions of the most intense bands in the spectrum shown in Fig. 2; the third column gives those maxima that appeared in the MgO spectra shown in Fig. 1 and that, by their position, approximately correspond to the positions of the intense MgO bands indicated in the table.

Fig. 2

correspond to the position of the intense MgO bands indicated in the table. The weak maxima in the spectrum of Fig. 2, located between the two groups of intense bands, as well as all the other maxima of the spectra of Fig. 1, generally speaking, can be calculated, like the intense bands, from the same electron-vibrational formula for MgO, with a corresponding selection of quantum numbers.

Table 1

In both cases corresponding to Fig. 1, we are dealing mainly with temperature spectra of solid bodies (incandescent complexes of solid oxide particles), when the gas phase is absent. The vibrations of particles under the conditions of a solid-body lattice are, to a considerable degree, coupled in comparison with a molecular gas and therefore, naturally, they appear in the spectrum only as a weak modulation against the background of continuous radiation, which is not in all cases perfectly distinct and easily revealed. Under the conditions of an energetic and short-lived flash of magnesium, however, owing to the higher temperature, conditions are created for the existence of a gas phase of the MgO being formed, along with the formation of complexes of solid oxide particles that give a continuous radiation background. The existence—

a large ensemble of individual thermally excited molecules and leads to the appearance in the flash spectrum of a clearly expressed vibrational structure, which always proves to be readily detectable. A. N. Gorban’ [5], having repeated our experiments on the spectroscopic study of the magnesium flash, obtained results that are also entirely analogous to ours.

On the basis of the foregoing, it seems possible to us to draw the following conclusions.

In the spectra of the so-called candoluminescence of magnesium oxide excited by a flame, which are essentially spectra of the temperature radiation of this oxide [6], under certain conditions a weakly modulated structure is observed, caused by the interaction of electronic transitions with the vibrations of the particles constituting the crystal lattice of the given substance. A similar, weakly expressed structure also appears in the spectra of the flame of slowly burning magnesium, when the glow of the flame is due to the temperature radiation of the solid particles of the oxide being formed. In the case of a magnesium flash, the molecular character of the spectrum is manifested quite distinctly, owing to the presence in the combustion zone of the gas phase of MgO.

As was mentioned above, some authors have also found vibrational structure in the spectra of the temperature glow (“candoluminescence”) of other oxides, in particular CaO and ZnO. Moreover, a similar structure has also been noted in the photoluminescence spectra of these same substances. In this connection it is of interest to note that a report has recently appeared on the presence of vibrational structure also in the photoluminescence spectra of chromium-activated magnesium oxide [7], which was interpreted as a combination of an electronic transition in the activator with vibrations of the lattice particles.

Thus, the presence of close interaction between lattice vibrations and the electronic transitions responsible for the glow is apparently a common property of solid crystalline bodies; and the possibility of detecting and studying, in a number of cases, the fine structure of the spectra of temperature glow or luminescence of these substances marks the beginning of a new, distinctive branch of solid-state spectroscopy.

REFERENCES

1. Nichols E. L., Howes H. L., Wilber D. T. Cathodoluminescence and Luminescence of Incandescent Bodies, Washington, 1928.

2. Kudryavtseva V. M. Temperature luminescence of ZnO and CaO. DAN AN SSSR, 52, 585, 1946.

3. Litvinova P. S. On the vibrational structure in the spectra of ZnO. ZhETF, 27, 636, 1954.

4. Pearse R., Gaydon A. The Identification of Molecular Spectra, IL, 1949.

5. Gorban’ A. N. Diploma thesis. Tomsk, Polytechnic Institute, 1956.

6. Sokolov V. A. On certain features of thermal radiation. Izv. Tomsk Polytechnic Institute, vol. 86 (in press).

7. Saksena B., Viswanthain S. Fundamental frequency of the MgO lattice. Proc. Phys. Soc., B, 69 (2), 129, 1956.