Fast Penning Ionization of Cold Rydberg atoms in an Electric Field

Abstract

We observe a fast Penning ionization in a dilute gas of cold rubidium Rydberg atoms, in the presence of a static electric field of 50 V/cm, with the ionization rate coefficients for two specific states being measured, which are orders of magnitude higher than the theoretical predictions in field-free space. Our analysis based on a polarized two-atom model reveals that the ionization threshold of Rydberg atoms is lowered by the static electric field, reducing the energy exchange required for Penning ionization and increasing the ionization rate. Beyond this, the dipole-dipole interaction strengthened by the electric field between two Rydberg atoms at a micrometer-scale distance leads to double ionization of the atoms pair, opening a new autoionization channel. Such enhancement of the Penning ionization by a static electric field poses both a threat to the stability and a potential control strategy for quantum systems composed of cold Rydberg atoms with micrometer-scale interatomic separations.

Figures (4)

• Figure 1: (a) Diagrammatic sketch of the experimental setup, the compositions of MOT and ion spectrometer. (b) The extraction pulsed electric field (dash line) and the ionic ToF spectrum (solid line) for the initial 49d Rydberg atoms with density of $1\times10^8\ {\rm cm}^{-3}$. The atoms are excited by the pulsed laser at t=0, and the pulsed electric field is applied 1 $\mu$s later, shown by the labels on the top and right. The time interval between the laser and the electric field is minimized as much as possible, preventing the loss and redistribution of initial Rydberg atoms (less than $5\%$). The sharp peak and the broad peak in TOF spectrum represent the free ions and the ions from field-ionization of high-n Rydberg atoms, respectively. (c) The image recorded by VMI, arising from the ions in the interval [19.6-20.6 $\mu$s] in TOF. The image was accumulated for 1000 repeat rimes.The two-dimensional velocity distribution of ions can be derived through the calibrated scale from space to velocity.
• Figure 2: (a) For Rydberg atom i, the shortest interval is denoted as $R_{i,min}$. (b) The shortest interval distribution within the range of 0 to 150 $\mu$m in case of N=4900, with a bin width of 1 $\mu$m. The inset shows the shorter range of 0 to 10 $\mu$m, with a bin width of 0.25 $\mu$m. (c) The rate $\gamma_{penning}(N)$ normalized to $\gamma_{1\mu m}$. Circles represent the numerical simulation results, while the dashed line corresponds to the fitted line following a 0.91 power law relationship. (d) The plot of experimental average rates $\gamma_{exp}$ vs simulated relative Penning ionization rates. Experimental measurement results are represented by circles, while the dashed line depicts the fitted line.
• Figure 3: (a) The electrons ($e_1$, $e_2$) and nuclei ($n_1$, $n_2$) of two Rydberg atoms 1 and 2 in a static electric field. (b) The electronic potential energy $V_e$ as a function of electron-nucleus distance $r$, for $E=$ 50V/cm, $\theta = \pi/2$ and various values of $R$. (c) The maximum of potential energy $V_e^{\rm max}$ as a function of $R$ and $\theta$, for various electric field strength $E$. The red plane indicates $2E_{ \rm Ryd}$ for two $^{87}$Rb atoms in the 51$s$ Rydberg state. (d) A plot in polar coordinates $(R,\theta)$. The condition (\ref{['con']}) is satisfied for two $^{87}$Rb atoms in the 51$s$ Rydberg state in the areas enclosed by the points.
• Figure 4: The threshold interatomic distance as the functions of Rydberg state (principal quantum number n) and electric field strength (E in V/cm). The interatomic distance is represented by different colors in this figure. A necessary condition for double ionization of a Rydberg atom pair is that their separation should be less than this threshold distance. The blank region in the upper-right part of the figure corresponds to the conditions where Rydberg atoms undergo direct field ionization. The lower-left section represents a parameter range where our model is not applicable since the interatomic distance is too small for a classical treatment