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Journal of the Physical Society of Japan

Many-Body Ionization of a Dense Rydberg Gas in Repulsive Potential

Linjie Zhang, Zhigang Feng, Jianming Zhao, Changyong Li, Suotang Jia

Published Apr 26, 2010• 3 citations
Publisher

Abstract

We investigate the initial ionization process of Rydberg cesium atoms prepared in repulsive van der Waals interaction potential. The time-of-flight spectrum is used to detect the free ions and ionized Rydberg atoms. Free ions are observed immediately after the production of Rydberg atoms and increase significantly in number on the blue detuned side of the resonance of 6P 3/2 →40D 5/2 with increasing delay time between the excitation laser pulse and the ramp ionization field. The initial ionization is explained as the excitation of pair states in attractive potential due to the ac Stark shifts induced by the excitation laser. The attractive dipole–dipole interaction results in fast collision process and penning ionization. We also measure free-ion fraction as a function of delay time. A rapid increase in the number of free ions is observed after a delay time of 1.8 µs. The experimental observations are in accordance with qualitative explanation of the initial ionization mechanism.

DOI
10.1143/jpsj.79.054301

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We investigate the initial ionization process of Rydberg cesium atoms prepared in repulsive van der Waals interaction potential. The time-of-flight spectrum is used to detect the free ions and ionized Rydberg atoms. Free ions are observed immediately after the production of Rydberg atoms and increase significantly in number on the blue detuned side of the resonance of 6P 3/2 →40D 5/2 with increasing delay time between the excitation laser pulse and the ramp ionization field. The initial ionization is explained as the excitation of pair states in attractive potential due to the ac Stark shifts induced by the excitation laser. The attractive dipole–dipole interaction results in fast collision process and penning ionization. We also measure free-ion fraction as a function of delay time. A rapid increase in the number of free ions is observed after a delay time of 1.8 µs. The experimental observations are in accordance with qualitative explanation of the initial ionization mechanism.
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.
We report clear evidence of the role of dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of np Rydberg Cesium atoms is prevented for states with in n < 42, which correspond to a repulsive potential, but it does not occur for n larger than 42, corresponding to an attractive potential. Blackbody radiation is mostly responsible for the background and initial ionization, although ion-Rydberg collisions and population transfer due to limited superradiance may have to be considered.
The spontaneous evolution from Rydberg atoms in different fine states to plasmas is investigated. Two-photon excitation is used to excite ultracold cesium atoms from 6S1/2 to 47D3/2 and 47D5/2 Rydberg states, respectively. The ramp electric field is used to ionize the Rydberg atoms and drive the ions to the micro-channel plate detector. In this experiment, The atom number of 47D5/2 state, which has a high oscillator strength compared with 47D3/2, is about 7 times larger than that of 47D3/2 state. Based on the different initial atom numbers of two fine states, the evolutions of plasma and Rydberg atoms are observed. The initial ionization time of Rydberg atoms and avalanche ionization rate of different states are presented. The initial ionization of Rydberg atoms is ascribed to the combining effect of state transfer from repulsive interaction to attractive interaction induced by the blackbody radiation and superradiation and Rydberg atoms collision. Moreover, the avalanche ionization and production of plasma are explained as being due to the rapid collision between Rydberg atoms and electrons in a local potential trap produced by positive ions.
We present time-resolved spectroscopic measurements of Rydberg-Rydberg interactions between two Rydberg atoms 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 time scale 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.
We have observed the ionization of a cold gas of Rb Rydberg atoms which occurs when nsnsnsnsnsns van der Waals pairs of nsnsns atoms of $n\ensuremath{\approx}$ 40 on a weakly repulsive potential are transferred to an attractive dipole-dipole nsnpnsnpnsnp potential by a microwave transition. Comparing the measurements to a simple model shows that the initial 300-$\ensuremath{\mu}$K thermal velocity of the atoms plays an important role. Excitation to a repulsive dipole-dipole potential does not lead to more ionization on a 15-$\ensuremath{\mu}$s time scale than leaving the atoms in the weakly repulsive nsnsnsnsnsns state. This observation is slightly surprising since a radiative transition must occur to allow ionization in the latter case. Finally, by power broadening of the microwave transition, to allow transitions from the initial nsnsnsnsnsns state to the nsnpnsnpnsnp state over a broad range of internuclear spacings, it is possible to accelerate markedly the evolution to a plasma.
In a recent work [Phys. Rev. Lett. 98, 023004 (2007)], we investigated the influence of the attractive van der Waals interaction on the pair distribution and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this description to atoms initially prepared in Rydberg states exhibiting repulsive interaction. We present calculations based on a Monte Carlo algorithm to simulate the dynamics of many atoms under the influence of both repulsive and attractive long-range interatomic forces. Redistribution to nearby states induced by blackbody radiation is taken into account, changing the effective interaction potentials. The model agrees with experimental observations, where the ionization rate is found to increase when the excitation laser is blue detuned from the atomic resonance.