High resolution reflection spectra of alkali halides in the far ultraviolet

The reflectivity spectra of KCl and KBr have been measured at room temperature in the photon energy range 13.5-20.5 eV using a helium discharge source. Care has been taken to ensure high instrumental resolution so that several new structures have been revealed which previously were recorded only at low temperatures. The current interpretation which ascribes them to interband transitions from the valence-band to high-energy conduction-band states has been found to be not completely satisfactory. The possibility of double excitations-a conduction electron plus a surface plasmon-has been analyzed