Structured photoionization bands of alkali diatomic molecules

We present a review on the photoionization bands that can be found in the far ultraviolet part of the spectrum using all sapphire cells in absorption experiments with hot alkali vapor. We describe cesium and rubidium dimers which have very pronounced photoionization bands together with bialkali mixtures like KCs and RbCs. We explain the origin of these peculiar bands as special molecular transitions between the ground state of the neutral molecule and exited states of the ionized molecule as a direct ionization process. We also described the diffuse bands as transition from the same ground state molecule to doubly excited molecular state, as an indirect ionization process. Finally, we believe that these two pathways may interfere resulting in a complex structure revealing the observed diffuse bands.