Surface optical phonons in ionic crystals

The review is devoted to the present status of theory and experiment in the region of surface optical oscillation (SO) in ionic crystals. Results are presented of a phenomenological analysis of the SO, and a microscopic theory of SO is developed. Results of recent experiments on the observation and investigation of SO by the methods of electron and infrared spectroscopy are reported and analyzed. The conclusion deals briefly with other types of surface excitations (surface plasmons, magnons, and excitons.)