Surface vibration alkali halides

Bryksin, V. V., Yu. M. Gerbshtein, and D, N. Mirlin, 1971. Optical effects in alkali-halide crystals due to surface vibrations, Sov. Phys.-Solid State, 13, 1342-1347. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

The effects of relaxation on the calculated surface phonon dispersion in Rbl have apparently been verified, particularly by the observation of a surface optical mode which lies above the bulk phonon optical bands. Except for the mysterious acoustic band mode in Rbl, the Shell model calculations have generally been quite accurate in predicting surface vibrational mode energies in both high-symmetry directions of the alkali halide (001) surfaces. 

The next phase for the theorists in connection with this work lies in predictions of helium atom scattering intensities associated with surface phonon creation and annihilation for each variety of vibrational motion. In trying to understand why certain vibrational modes in these similar materials appear so much more prominently in some salts than others, one is always led back to the guiding principle that the vibrational motion has to perturb the surface electronic structure so that the static atom-surface potential is modulated by the vibration. Although the polarizabilities of the ions may contribute far less to the overall binding energies of alkali halide crystals than the Coulombic forces do, they seem to play a critical role in the vibrational dynamics of these materials. 

Even in the absence of the special effects of hydrogen bonding, bandwidths of 10 cm or more are common from adsorbed species on polycrystalline substrates owing to interactions with sites that differ in their detailed environments. Absorptions obtained from adsorbed species on single-crystal planes with uniform and well defined sites can, in contrast, be very sharp with bandwidths of less than 1 cm i. In the case of methane itself, and of a number of other molecules such as CO and CO2, the resolution of the spectra on alkali metal halide single-crystal surfaces are such that even the fine-structure splitting caused by the vibrational couplings of more than one molecule in the surface unit cell can readily be resolved. 

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