Ionization of Rydberg atoms embedded in an ultracold plasma

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 $T_e$, 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 $T_e$. 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.

Figure 1

(Color online) Electron signal GI2 monitored at $t_1=4.7\mu \mathrm{s}$ after plasma creation when scanning the Rydberg laser. Upper curve: plasma present. Lower curve: plasma absent. In the inset: Lower trace: a schematic view of the voltage between the two grids. Middle trace: MCP signal (the first peak is noise from the laser shot). Upper trace: integrator gates.

Figure 2

(Color online) Rydberg ionization efficiency (in percent) versus plasma laser power ($P_1=10\mu \mathrm{J}$ corresponds roughly to $4\times {10}^5\text{ions}$) at $t_1=1\mu \mathrm{s}$. The GI3 remaining Rydberg atom signal is monitored.

Figure 3

(Color online) Number of electrons ejected (GI1) when varying the voltage $V_1$ for the plasma alone and for $45d$ Rydberg atoms embedded in the plasma after $t_1=1\mu \mathrm{s}$ of evolution time. The dashed lines are a guide for the eye to interpret Eq. (1). Each datum has been calibrated to the multiple shots average electron number ${\overline{N}}_e$; i.e., the signal shown is ${\overline{N}}_e\times \mathrm{GI}1∕(\mathrm{GI}1+\mathrm{GI}2)$.