We present time-resolved spectroscopic measurements of Rydberg-Rydberg interactions in an ultracold gas, revealing the pair dynamics induced by long-range van der Waals interactions between the atoms. By detuning the excitation laser, a specific pair distribution is prepared. Penning ionization on a microsecond timescale serves as a probe for the pair dynamics under the influence of the attractive long-range forces. Comparison with a Monte Carlo model not only explains all spectroscopic features but also gives quantitative information about the interaction potentials. The results imply that the interaction-induced ionization rate can be influenced by the excitation laser. Surprisingly, interaction-induced ionization is also observed for Rydberg states with purely repulsive interactions.PACS numbers: 32.80. Rm, 34.20.Cf, 34.10.+x, 34.60.+z Long-range dipolar interactions ubiquitously appear in nature as the cause for binding forces ranging from atomic and molecular gases [1] all the way to large biological systems [2]. Rydberg atoms have attracted much interest in this context, as they represent an ideal system to study quantum dynamics under the influence of dipolar interactions. As a prominent example from cavity quantum electrodynamics, attractive forces between Rydberg atoms and conducting surfaces have been observed as level shifts in atomic beam experiments [3]. RydbergRydberg interactions leading to resonant energy transfer have been studied in thermal beam experiments [4] and later in ultracold Rydberg gases [5,6,7]. The longrange dipolar interactions can be used to block multiple Rydberg excitation [8,9] and to create many-particle entangled states, which may be employed for quantum information processing [10,11]. So far, investigations of Rydberg-Rydberg interactions have mainly focused on the electronic degrees of freedom neglecting the center-ofmass motion ("frozen Rydberg gas" [7]). In this Letter, we present real-time measurements of the motion of interacting pairs of Rydberg atoms revealing the character and strength of the long-range interparticle interactions.In most molecular and atomic systems the relevant timescales of interparticle dynamics are in the sub-ns regime calling for very fast probes. Ultracold atoms bridge to the ns regime as the thermal energy (T <1 mK) is negligible which allows one to study systems with much weaker interactions or large interatomic distances. Due to the negligible kinetic energy, the dynamics of the gas is fully determined by the interatomic interactions. Interactions in a cold atomic sample have been studied time-resolved in the case of ground state atoms using a pump-probe scheme [12] allowing for coherent control of the collision process [13]. Evidence for interactioninduced motion in cold Rydberg gases was recently found spectroscopically as the cause for Penning ionization [14]. By combining time-resolved and spectroscopic measure- ments, we quantitatively examine the van der Waals (vdW) interactions between two Rydberg atoms. Our measurements can be dir...

Abstract. We investigate a possible mechanism for the autoionization of ultracold Rydberg gases, based on the resonant coupling of Rydberg pair states to the ionization continuum. Unlike an atomic collision where the wave functions begin to overlap, the mechanism considered here involves only the long-range dipole interaction and is in principle possible in a static system. It is related to the process of intermolecular Coulombic decay (ICD). In addition, we include the interaction-induced motion of the atoms and the effect of multi-particle systems in this work. We find that the probability for this ionization mechanism can be increased in many-particle systems featuring attractive or repulsive van der Waals interactions. However, the rates for ionization through resonant dipole coupling are very low. It is thus unlikely that this process contributes to the autoionization of Rydberg gases in the form presented here, but it may still act as a trigger for secondary ionization processes. As our picture involves only binary interactions, it remains to be investigated if collective effects of an ensemble of atoms can significantly influence the ionization probability. Nevertheless our calculations may serve as a starting point for the investigation of more complex systems, such as the coupling of many pair states proposed in [1].

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