Melting a frozen Rydberg gas with an attractive potential

We report clear evidence of the role of the attractive dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of $np$ Rydberg cesium atoms does not occur for states with $n<42$, which have repulsive potentials, but it does occur for states of $n$ larger than 42, which have attractive potentials. Blackbody radiation is mostly responsible for the background and initial ionization, although ion–Rydberg-atom collisions and population transfer due to limited superradiance may have to be considered.

Figure 1

(Color online) (a) The energy difference $\left(W\right)$ between the molecular $pp$ and $s{s}^{’}$ states at $R=\infty$ for different $n$. (b) and (c) Schematic potential curves of pair of atoms as function of internuclear distance.

Figure 2

(Color online) Rydberg and ion signal with two different laser powers (42 and $278\mathrm{mW}$) for (a) $40p$ state (repulsive) and (b) $43p$ (attractive). Data are taken after a time delay of $10\mu \mathrm{s}$, between the end of the laser pulse and the field ionization pulse.

Figure 3

(Color online) Ion density as function of initial Rydberg density for different $n$ states and three delay times: (a) $0.45\mu \mathrm{s}$, (b) $5\mu \mathrm{s}$, and (c) $10\mu \mathrm{s}$. At $10\mu \mathrm{s}$ the “$40p^{\star }$” is without $6p$ cold and hot atoms. Each point corresponds to the maxima of spectral scans, such as those shown in Fig. 2.