Ionisation of excited caesium atoms (9.4⩽n*⩽ or ≈29) in collisions with Cs(6S)

The ionisation yields of the optically excited caesium atoms (9.4<or=n*<or approximately=29) are measured in a vapour cell at a caesium vapour density of about 10 14 cm -3 . Direct ionisation rate constants are given for Rydberg atoms with 9.4<or=n*<or=21.4. Collisions with caesium ground-state atoms and black-body radiation strongly modify the population of Rydberg levels with n*>or approximately=25 so the concept of direct collisional ionisation cannot be applied to those levels. The diffusional ionisation mechanism of Rydberg atoms in vapour cells, atomic beams or buffer gases is discussed.