Helium atom scattering alkali halides

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. 

Table 16.1 Alkali halide (001) surfaces and fluorite (111) surfaces that have been investigated with helium atom scattering (HAS), low-energy electron diffraction (LEED), and atomic force microscopy (AFM).

It has long been known that thermal displacements of the surface ions lead to temperature-dependent attenuation of the intensity of elastically scattered helium atoms. This Debye-Waller effect has been extensively used to determine the surface Debye temperature of alkali halide single-crystal surfaces, especially... 

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