  High resolution laser Stark excitation of np (60 < n < 85) Rydberg states of
ultra-cold cesium atoms shows an efficient blockade of the excitation
attributed to long-range dipole-dipole interaction. The dipole blockade effect
is observed as a quenching of the Rydberg excitation depending on the value of
the dipole moment induced by the external electric field. Effects of eventual
ions which could match the dipole blockade effect are discussed in detail but
are ruled out for our experimental conditions. Analytic and Monte-Carlo
simulations of the excitation of an ensemble of interacting Rydberg atoms agree
with the experiments indicates a major role of the nearest neighboring Rydberg
atom.
