Collisional broadening of Rydberg-atom transitions by ground-state alkali atoms

A theory is developed that allows a qualitative explanation and quantitative calculation of the widths and shifts of the two-photon Doppler-free transitions from the ground states of alkali atoms to Rydberg n S-2 and n D-2 states. The widths and shifts result from collisions between Rydberg atoms and ground-state alkali atoms, either of the same or of other species. The collisions are represented by means of a pseudopotential that is a function of theoretically calculated free-electron elastic scattering resonant and nonresonant phase shifts, which depend upon the Rydberg electron momentum. The momentum, in turn, is correlated with the electronic position within the Rydberg atom. This theory assumes classical straight-line trajectories and takes into consideration the multicollisional nature of the Rydberg-electron-perturbing-atom interaction within the Rydberg atom. Impact theory calculations of the linewidths and shifts for K** (n S)-K, K** (n S)-Rb, Rb** (n S)-Rb, Rb** (n S)-K, K** (n D)-K, K** (n D)-Rb, Rb* * (n D)-Rb and Rb** (n D)-K show substantial agreement with experiment for 10 less than or equal to n less than or equal to 40.