Excitation of the Cs‐5p core level in cesium halides at 30 K

The excitation of the Cs‐5p core level in Cs halides which starts a t about 13 eV was studied by measuring the reflection spectra of evaporated films between 12.5 and 26 eV at room temperature and a t 30 K. Synchrotron radiation was used as a light source. In comparison to the spectra at room temperature new remarkable fine structure was found at 30 K. The very complex spectra are discussed with regard to band calculations including excitons associated with critical points. I n a complementary view the spectra are compared with transitions of the free Cs+ ion. A sideband at the high energy part of the most prominent exciton was observed for every Cs halide. It is, presumably, caused by excitonphonon interaction.