Optical Properties of the Rubidium and Cesium Halides in the Extreme Ultraviolet

The absorption spectra of evaporated thin films of all rubidium and cesium halides in the 50- to 250-eV region are reported. In this range, transitions from the $3d$ shell of ${\mathrm{Rb}}^{+}$ and from the $4d$ and $4p$ shells of ${\mathrm{Cs}}^{+}$ can be seen, as well as some transitions from inner shells of the halogen ions. Besides the absorption fine structure near the threshold for inner-shell transitions, broad absorption structure is observed and explained as due to $d\to f$ continuum transitions. The number of effective electrons whose oscillator strength has been exhausted in our spectral region has been computed from the absorption data; it is particularly strong for materials containing either Cs or I. Differences in the spectra of materials with NaCl and with CsCl structure are discussed. The measurements were performed using the DESY electron synchrotron as a light source.