Off-shell effect in Rydberg-atom--alkali-metal-atom scattering

We study the Rydberg-atom--ground-state-atom collisions within the framework of the quasi-free-electron model. We show that the general formulation of the problem based on the three-body Faddeev equations requires the off-shell extension of the two-body amplitude for electron-atom scattering. The two-body off-shell amplitudes for electron scattering by Rb and Cs atoms are calculated using the pseudopotential approach. The cross section for collisional quenching and broadening of Rydberg states due to perturbations by Rb and Cs atoms are calculated, and the results are compared with experimental data. The off-shell extension of the scattering amplitude usually improves agreement with the experiment. However, both on-shell and off-shell results fail in the region of low principal quantum numbers, where the multiscattering effects become important. Our calculations and their comparison with the experiments confirm that the Cs-( 3P) state is a resonance rather than a bound state