Electron drift in alkali-metal vapours

Electron drift velocities in sodium, potassium, rubidium and caesium vapours are calculated using theoretical momentum-transfer cross sections for electron-atom scattering. The results strongly disagree with experimental data for saturated vapours at low values of E/N (ratio of electric field to vapour number density). We show, using the case of sodium as an example, that the disagreement could be explained by the presence of alkali dimers in saturated vapours which lead to the processes of electron-impact vibrational excitation and de-excitation and dissociative attachment. These processes strongly affect the electron energy distribution function and reduce drift velocities. Results of model calculations show that inclusion of inelastic processes involving the dimers leads to a better agreement with the experiment. (author)