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We have observed resonant energy transfer between cold Rydberg atoms in spatially separated cylinders. Resonant dipole-dipole coupling excites the 49s atoms in one cylinder to the 49p state while the 41d atoms in the second cylinder are transferred down to the 42p state. We have measured the production of the 49p state as a function of separation of the cylinders (0-80 microm) and the interaction time (0-25 micros). In addition, we measured the width of the electric field resonances. A full many-body quantum calculation reproduces the main features of the experiments.

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We study the stabilization behavior of the circular 5g state in neon in a two-pulse experiment. A first pulse prepares the state. Comparison of the single-photon ionization yield, due to a second laser pulse for both short (0.1 ps) intense and long (1 ps) less intense pulses, shows an intensity-dependent suppression of ionization. The smaller yield due to the short pulses, at intensities of several times 10 13 W/cm 2 , is in accordance with recent predictions of stabilization.PACS numbers: 32.80.Rm, 42.50.HzThe photoionization rate of an atom will usually increase when the atom is exposed to increasing intensities. Recently, however, different mechanisms have been proposed that could prevent photoionization of an atom, even at the highest intensities. Collectively these are called stabilization. One such mechanism is dynamic or transient stabilization, in which part of the population is temporarily inaccessible for ionization [l].Adiabatic stabilization [2-5] is a different mechanism that suppresses photoionization. More precisely, it corresponds to an ionization rate which decreases with increasing intensity. This type of stabilization is predicted to occur in the high-frequency regime, where the photon energy is much larger than the binding energy. Classically, this means that the electron motion due to the light field is much faster than its motion in the Coulomb potential. Quantum mechanically, it means that the electron wave function cannot adiabatically respond to the oscillating light field. As a result, in the high-frequency regime, the wave function of the electron will be driven in an oscillatory motion by the light field.Adiabatic stabilization may occur once the amplitude of the oscillatory motion becomes comparable to the size of the unperturbed wave function, which usually implies a high intensity [6]. In the rest frame of the oscillating electron cloud (the Kramers-Henneberger frame), the atomic core will appear to oscillate. The electron wave function is determined by the potential of the atomic core, which, in this rest frame, is time dependent. However, in the high-frequency regime, it is valid to time average the motion of the atomic core. The potential which results is no longer Coulombic, but is deepest at the outer turning points of the oscillation, where the atomic core spends most of its time. In response the probability of finding the electron is largest at these points. One result of the electron spending more of its time further away from the nucleus is that ionization is suppressed and stabilization occurs. Ionization depends on transfer of momentum from the core to the emitted electron and once the wave function is far away from the core this becomes impossible [7], Originally, stabilization was predicted for the ground state of atomic hydrogen [2], and was later confirmed by other calculations [3], However, with present-day laser technology, the stabilization regime can only be reached when the initial state is a Rydberg state. For that case, different theoretical groups [4,5] give s...