Collisional broadening of Rydberg states by alkali-metal perturbers

Collisional broadening of nS Rydberg states by K, Rb, and Cs atoms is studied using the adiabatic approach for the ³Σ quasimolecular state and impulse approximation for all other contributions to the collisional width. The calculations are based on the use of the low-energy phase shifts for electron scattering by alkali metals obtained from ab initio close-coupling calculations and their extrapolation according to the modified effective range theory. We obtain a good agreement for the magnitude of the width with experimental data and satisfactory agreement for the oscillatory part of the width as a function of the principal quantum number. We find that the theory overestimates the amplitude of the oscillations. In the case of Cs as a perturber, the agreement with experimental data is good if we assume the existence of the low-energy ³P resonance of Cs^-. However, recent ab initio bound-state and scattering calculations indicate the existence of the Cs (³P) bound state rather than the resonance.