Ionization of Rydberg atoms embedded in an ultracold plasma

Abstract

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...

Evolution dynamics of a dense frozen Rydberg gas to plasma

“…Soon after this work, a sample of purely cold Rydberg atoms was observed to ionize spontaneously, and electron emission from such a sample was observed for more than 20 ms after the excitation of the initial Rydberg state [15]. A similar phenomenon had been observed previously in a thermal sample of cesium Rydberg atoms by Vitrant et al [16] at a density a factor of 10 4 higher than those used in Refs. [14] and [15].…”

“…Such a plasma can be created by photoionization of atoms in a magneto-optical trap (MOT) using a pulsed laser. The initial observation of this phenomenon was made using 50 K Xe atoms, but since then similar results have been found using cold Rb, Cs, and Sr atoms [2][3][4][5]. To a very good approximation, the initial electron energy is determined by the excess photon energy above the ionization limit of the photoionizing laser, while the initial positive ion temperature is that of the initially trapped atoms before ionization.…”

External Control of Electron Temperature in Ultra-Cold Plasmas

“…In later experiments, it was found that electron-Rydberg-atom collisions that occur once the trap depth is sufficient do in fact cause redistribution of the population from the initial Rydberg state to nearby high-nl states. As a result of these collisions, approximately 2 3 of the Rydberg atoms ionize while 1 3 are de-excited to lower energy states. Li.…”

“…This is an experimentally attractive setup because of its simplicity of construction and optimal optical access. The quadrupole trap is commonly used because of these attributes [2,3,4,5].…”

“…Our ultra-cold plasma is created by photoionization of a sample of trapped 85 Rb atoms in a magneto-optical trap (MOT) using a pulsed dye-amplified diode laser. The initial observation of this used 50 µK Xe atoms, but the results have been reproduced using cold Rb, Cs, and Sr atoms [1,2,3]. To a good approximation, the initial electron temperature is obtained from the remaining photon energies in excess of the ionization threshold of 85 Rb; and the initial positive ions maintain the same temperature as the initially trapped atoms [1,2,3].…”