Evolution of the pairs of ultracold Rydberg atoms in the repulsive potential.

We present the explicit dynamics process of Rydberg cesium atoms initially experiencing a repulsive van der Waals (vdW) interaction by measuring the line width and intensity of the Rydberg ionization spectra. The signals of Rydberg atoms and free ions are recorded simultaneously within an initial 3.5 micros delay time between the excitation laser and the ramp electric field. For high-density gases, a rapid decrease of Rydberg atoms and an increase of free ions are observed, which is not found to be the case for low-density gases. The experimental results indicate that superradiance is the main cause of the redistribution of Rydberg atoms from the repulsive potential to the attractive potential for high density, which provides the initial ionization. The corresponding theoretical calculation is also given.