First resonance doublet absorption in very dense cesium vapor

Alkali metal vapours are well known to be easily ionizable and many applications have been developed. Despite its high chemical reactivity which cause great troubles, the caesium vapour is the ideal medium to be investigated. The ionization of Cs is enhanced by its low potential E=2.89 eV, and a high atomic polarizability a-65 Angstrom. The coordinates of the critical point enable the authors to operate at relatively high density. Moreover the caesium vapour exhibits anomalously high electric conductivity. Tentative explanations have been given, including the contribution of positive cluster ions having a lower ionization potential. The authors have developed an experimental procedure sensitive to the species present in the vapour. The resonant optical absorption measurements show the relative variation of the neutral Cs atoms in the ground state, the electrons, and the ions in a wide density range up to the critical region. The absorption results around the 8521 Angstrom line are given: the relation between them and the vapour composition, plus the possible influence of anomalous values of the dielectric constant is discussed