Many-Body Ionization in a Frozen Rydberg Gas

In a dense gas of $300\mu \mathrm{K}$ ${}^{85}\mathrm{Rb}$ atoms of $n\sim 50$ ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at $n=88$ at a density of $5\times {10}^{10}{\mathrm{cm}}^{-3}$. The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.

Figure 1

Oscilloscope trace of signals obtained with a field ionization pulse applied 80 ns after dye laser excitation of the Rb $40d$ state. With low density we observe the $40d$ signal at $t=1.1\mu \mathrm{s}$. At high density we observe signals from a range of final states: predominantly free ions at $t=0.46\mu \mathrm{s}$ and the $42p$ state at $t=0.8\mu \mathrm{s}$.

Figure 2

Excitation spectra of the Rb $32d$, $45d$, and $88d$ states. The frequency is relative to the center of gravity of the atomic $5p_{3/2}-nd$ transition frequency. (a) The $32d$ state at density $5.0\times {10}^{10}{\mathrm{cm}}^{-3}$. Neither the $32d$ signal nor the free ion signal exhibits broadening beyond the laser linewidth. (b) The $45d$ state at a density of $5\times {10}^{10}{\mathrm{cm}}^{-3}$. The central free ion signal feature is 800 MHz broad, and is several hundred megahertz broader than the $45d$ signal. Also a new feature appears at $\nu -{\nu }_0=1$. The hole at relative frequency zero is an artifact due to trap depletion. Here the peak atom and free ion signals are equal. (c) The $88d$ and $90s$ states total (atom plus free ion) signals. At $2.3\times {10}^9{\mathrm{cm}}^{-3}$ (light blue or light gray) the $88d$ and $90s$ are well resolved features. At $1.6\times {10}^{10}{\mathrm{cm}}^{-3}$ (blue or dark gray) the two states are still well resolved, but at $5.0\times {10}^{10}{\mathrm{cm}}^{-3}$ (green or gray) the states are completely overlapped; the spectrum has become a continuum.

Figure 3

Excitation spectra of the $51d$ state at different relative laser intensities, 0.01 (cyan or lightest gray), 0.03 (blue or dark gray), 0.2 (green or light gray), 0.65 (red or gray), and 1.0 (black). The frequency is relative to the atomic $5p_{3/2}-51d$ transition frequency. At low intensity the free ion signal is only visible at the atomic transition frequency, but as the intensity is raised the free ion signal broadens and develops new features, for example, at $\nu -{\nu }_0=-1.0\mathrm{GHz}$ and 0.6 GHz. Inset, signals from the field ionization of the $51d$ state at the same laser intensities. The signals are essentially confined to the atomic line except at the highest density.

Figure 4

Schematic of ionization in a simple binary model in terms of (a) molecular two-atom energy levels and (b) the single-atom levels. (a) Pairs of atoms initially excited to the $ndnd$ state are coupled to other nearly degenerate pairs of states composed of atomic states of higher and lower energy. (b) The atomic population rapidly diffuses over a band of levels, which includes the ionization continuum.