The inelastic light scattering of the localized vibration of the interstitial hydrogen atom in the alkali halides

Abstract

The Raman spectrum of the interstitial hydrogen atom (H i0) centre, also called U2 centre, is measured in a series of eight alkali halides. The vibration of the hydrogen atom gives rise to a single narrow line in the spectrum, with a frequency shift, ωH, ranging from 237 cm−1 in RbI to 523 cm−1 in NaCl. This vibrational mode possesses the T2 symmetry of the tetrahedral group as confirmed by measurements of the scattered light in different polarization geometries. In OD‐ doped crystals of KBr and KI the localized mode frequency, ωD, of the analogous deuterium (D i0) centre is measured, and it is found to obey the relation \documentclass{article}\pagestyle{empty}\begin{document}$ \omega _{\rm H} /\omega _{\rm D} = \sqrt 2 $\end{document}. This proves the involvement of the hydrogen atom in the vibrational mode, and shows that this vibration is not strongly coupled to the phonons. A “soft sphere” potential calculation of the vibrational frequency of the H i0 centre is presented, which uses parameters taken from the literature. The results of this first calculation are in bad agreement with the experiments. Possible improvements of the model are discussed: The distorsion of the crystal lattice near the H i0 centre is estimated from ENDOR data and taken into account in the further calculation. Finally, the parameters of the “soft sphere” potential between the hydrogen atom and the nearest neighbour halogen ions are readjusted in order to fit the measured vibrational frequencies of the H i0 centres.