The nature of the surface and pseudosurface modes in ionic-crystal slabs is clarified, and some apparent discrepancies among the previous studies are resolved. The first shell-model calculations of surface-mode spectra are reported; the results for NaCl are significantly different from those obtained with the rigid-ion model. A new type of long-wavelength optical surface mode has been found in the shell-model results for RbF. 1971 The American Physical Society.
