Radiation-Induced Expansion and Increase in Refractive Index of Magnesium Oxide; Evidence for theFCenter

Magnesium oxide single-crystal fragments bombarded with ${\mathrm{H}}^{+}$, ${\mathrm{D}}^{+}$, and ${\mathrm{He}}^{+}$ at several energies between 0.14 and 4 MeV were examined polarimetrically. The dilatation was determined from photoelastic measurements (order of magnitude 0.01), and an increase in refractive index (despite a concomitant decrease in density) was inferred from an interference effect noted in the polarimeter. The depth of the radiation-induced dilatation for the 0.14-MeV ions was determined by an etching procedure. The increase in refractive index was confirmed interferometrically for specimens bombarded with 40-MeV ${\mathrm{He}}^{+}$, and also the dispersion of the change in refractive index was measured for the visible region. Anomalous dispersion caused by a small absorption band in the visible region was found, but most of the refractive index change was associated with a very large band in the ultraviolet. On thermal annealing the refractive-index increase vanished at ∼450°C, but the dilatation remained until much higher temperatures were reached. From the refractive-index change and its dispersion, the wavelength of the ultraviolet absorption peak (∼0.26 μ) and the associated number of color centers (multiplied by their oscillator strength) were calculated. Thus the absorption was determined despite impurity absorption which prevented its direct observation. From a comparison with the annealing behavior reported by others for the magnetic resonance signal identified as the $F$ center, it is concluded that the optical absorption band is caused by the $F$ center. It is shown that the expansion can be accounted for largely by vacancies which were generated during ion bombardment and which have formed $F$ centers. Thus the magnesium oxide behaves very much like the alkali halides, except that in the case of the alkali halides vacancies may be introduced by ionization and on annealing may vanish with the centers, whereas in the case of magnesium oxide corpuscular radiation is required to produce the vacancies and on annealing may remain even when the color centers have vanished.