State-Mixing of nS Rydberg Atoms in an External Electric Field

State-mixing effect of ultracold nS cesium Rydberg atoms in an external electric field is investigated in a magneto-optical trap. Populated high-l Rydberg atoms due to the state-mixing through avoided crossings are measured with a state-selective field ionization technique. The measured transfer rates of high-l states increase with the electric field and get to the maximum at the field of about 3.0 V/cm for 49S1/2 Rydberg state, and show decrease behavior when the electric field increases further. The decrease behavior of the transfer rate is explained with the slower m-mixing effect caused by decreasing dipole–dipole interactions between high-l Rydberg atoms. During the m-mixing process the ultracold plasma is formed by the Penning ionization.