Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as 100 cm −1 . Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after 1 s caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high n states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.

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Abstract. We have measured the radiative lifetimes of ns, np and nd Rydberg states of rubidium in the range 28 ≤ n ≤ 45. To enable long-lived states to be measured, our experiment uses slow-moving (∼100 µK) 85 Rb atoms in a magnetooptical trap (MOT). Two experimental techniques have been adopted to reduce random and systematic errors. First, a narrow-bandwidth pulsed laser is used to excite the target nℓ Rydberg state, resulting in minimal shot-to-shot variation in the initial state population. Second, we monitor the target state population as a function of time delay from the laser pulse using a short-duration, millimetre-wave pulse that is resonant with a one-or two-photon transition to a higher energy "monitor state", n ′ ℓ ′ . We then selectively field ionize the monitor state, and detect the resulting electrons with a micro-channel plate. This signal is an accurate mirror of the nℓ target state population, and is uncontaminated by contributions from other states which are populated by black body radiation. Our results are generally consistent with other recent experimental results obtained using a method which is more prone to systematic error, and are also in excellent agreement with theory.

We have studied the behavior of cold Rydberg atoms embedded in an ultracold plasma. We demonstrate that even deeply bound Rydberg atoms are completely ionized in such an environment, due to electron collisions. Using a fast pulse extraction of the electrons from the plasma we found that the number of excess positive charges, which is directly related to the electron temperature Te, is not strongly affected by the ionization of the Rydberg atoms. Assuming a Michie-King equilibrium distribution, in analogy with globular star cluster dynamics, we estimate Te. Without concluding on heating or cooling of the plasma by the Rydberg atoms, we discuss the range for changing the plasma temperature by adding Rydberg atoms.PACS numbers: 32.80. Pj, 52.25.Dg, 98.10.+z One challenge in ultra-cold plasma physics is to reach the correlated regime where the Coulomb energy dominates the kinetic energy. One suggested way, unfortunately limited to non alkali ions, is to cool the plasma ions by lasers [1]. An alternative way might be to use the binding energy of Rydberg states as "ice cubes" to cool the plasma. The physics of ultracold Rydberg gases [2,3] and ultracold plasmas [4] formed by laser excitation of a cold atomic sample have strong similarities. Indeed, Rydberg atom formation in an ultracold plasma [5] and spontaneous evolution of an ultracold Rydberg gas to plasma [6] have been demonstrated. The Rydberg ionization process starts with blackbody photoionization and initial electrons leave the cloud region. A second phase occurs when the positive ion potential is deep enough to trap subsequent electrons, which then collide with Rydberg atoms creating more electrons in an avalanche ionization process [7,8,9]. However, other relevant processes have been proposed whose effects need to be investigated, such as continuum lowering [10], and manybody effects or long-range interactions [11] that can lead to autoionization of Rydberg atom pairs [12].In this letter, we report the behavior of a mixture of an almost neutral ultracold plasma and a cold Rydberg atom sample. A related experiment has been reported, but in the case of rubidium Rydberg atoms created in a purely ionic plasma [13]. In this letter, we analyze the fast avalanche ionization of deeply bound Rydberg states embedded in a quasi-neutral plasma. We also study, with a theory based on analogy with globular star cluster dynamics, the evolution of the temperature of the plasma when Rydberg atoms are added.The cesium magneto-optical trap (MOT) apparatus has been described in a previous paper [6]. Two dye lasers pulses (Coumarin 500) that are focused to the cold atom cloud diameter excite atoms initially in the 6p 3/2 state. The time origin of the experiment is the first laser (L 1 ) pulse, with typical energy P 1 = 10 µJ, which creates a quasi-neutral plasma of N i ≈ 4 × 10 5 ions with peak density 10 10 cm −3 . The second laser (L 2 ) pulse (ASE < 1%), has a 18 ns delay and excites typically 4 × 10 5 Rydberg atoms. The Rydberg number fluctuates from pulse to pulse due to...