Lattice dynamics, second-order vibrational spectra and fluorescence spectra in magnesium oxide

Abstract

The spectra of single crystals of magnesium oxide have been studied using He/Ne laser excitation and infra-red radiation. The laser-excited spectrum is extremely intense and appears characteristic of fluorescence rather than second-order Raman scattering. The one- and two-phonon density of states distributions have been calculated using a shell model and have shown a complete lack of correlation with laser-excited experimental spectra. An interpretation of the infra-red spectrum has been made on the basis of the calculated two-phonon density of states distribution. Analysis of the crystals using e.s.r. and emission spectral techniques revealed the presence of some transition metal ions as trace impurities. The observed “Raman” spectrum has been analyzed on the basis of fluorescence due to Cr(III), with associated vibronic sidebands. An interpretation of the vibronic structure on the basis of a perturbed complex ion species CrO^9–₆ is given.