Two-Electron Excitation of an Interacting Cold Rydberg Gas

We report the creation of an interacting cold Rydberg gas of strontium atoms. We show that the excitation spectrum of the inner valence electron is sensitive to the interactions in the Rydberg gas, even though they are mediated by the outer Rydberg electron. By studying the evolution of this spectrum we observe density-dependent population transfer to a state of higher angular momentum $l$. We determine the fraction of Rydberg atoms transferred, and identify the dominant transfer mechanism to be $l$-changing electron-Rydberg collisions associated with the formation of a cold plasma.

Figure 1

(a) Schematic of the experiment. The MOT is formed between two segmented ring electrodes, which direct the ions towards the MCP. (b) Energy level diagram. The first electron is excited to the $5s56d{}^{1}D_2$ state by a two-step excitation (${\lambda }_1+{\lambda }_2$). After a variable delay $\Delta t$, the second electron is excited at ${\lambda }_3$ and the atom autoionizes. (c) An example of the time-resolved ion signal. The autoionization signal is much larger than the signal from spontaneously created background ions.

Figure 2

The autoionization signal at $\Delta t=0.5\mu \mathrm{s}$ as a function of the detuning ${\Delta }_3$, for $P_2=1\mathrm{mW}$. The solid line is a fit using a six-channel MQDT model for the $5s56d$ state.

Figure 3

(a–c) Autoionization spectrum for three different values of $\Delta t$. The Rydberg laser power $P_2=10\mathrm{mW}$. The background ionization signal has been subtracted from the data. The solid lines are fits using the combined $5s56d$ and $5s54f$ MQDT model. (d) Autoionization spectra for the $5s54f{}^{1}F_3$ (black dots) and $5s56p{}^{1}P_1$ (blue triangles) Rydberg states. The solid line is a two-channel MQDT fit to the $5s54f$ data.

Figure 4

(a) Variation in the lifetime, derived from a single exponential fit to the decay of the autoionization signal, with ${\Delta }_3$. (b) Decay of the autoionization signal at the two positions A (red triangles) and B (black dots) indicated in Fig. 3b, measured at $P_2=10\mathrm{mW}$. The solid lines are fits using a single (red or gray) and double (black) exponential decay. At $150\mu \mathrm{s}$ the signal at A has reached the noise floor.

Figure 5

Variation in the autoionization signal due to atoms in the $5s54f$ state ($S_F$, black dots), measured after $\Delta t=100\mu \mathrm{s}$ at point B [Fig. 3b], with the autoionization signal due to atoms in the $5s56d$ state $S_D$, measured at $\Delta t=0.5\mu \mathrm{s}$. The variation in the background ionization signal ($S_{\mathrm{bg}}$, red crosses) is also shown.