Abstract
The authors report the results of extensive absorption measurements in dense alkali vapours with the main emphasis on the diffuse bands, located close to the short wavelength end of the B1 Pi u-X1 Sigma g+ alkali-band system. At extremely high pressures they succeeded in observing Li2 and Na2 diffuse bands at about 422.5 and 436.5 nm, respectively. Detailed measurements of the absorption coefficient of potassium diffuse band at 575 nm show square dependence on the atom density and independence on the temperature. The relevant data for dense rubidium and caesium vapours have been remeasured. The authors believe that diffuse bands originate in a transition from the ground triplet state towards the 23 Pi g state, which suffer peculiar configuration interaction with the first excited ionic state of the same symmetry.