Emission of fast atoms from a cold Rydberg gas

Rydberg-Rydberg collisions in a cold, dense Rydberg gas can lead to the conversion of internal energy into center-of-mass energy of the colliding atoms, resulting in Rydberg-atom velocities much larger than the initial velocities in the gas. We prepare cold Rydberg-atom gases by laser excitation of laser-cooled atom clouds and use time-of-flight measurements to demonstrate the production of fast Rydberg atoms. The velocity distributions of Rydberg atoms emerging from the Rydberg-atom gases are obtained. State-selective field ionization spectra indicate a correlation between the production of fast Rydberg atoms and the presence of Penning ionization signatures. The dependence of the yield of fast Rydberg atoms on Rydberg atom density is measured.

Figure 1

Experimental setup for time-of-flight measurement. A fraction of the fast Rydberg atoms emitted from the cloud travels into the remote field ionization region between the semicircular electrode and the first extraction electrode. Rydberg atoms in this region are field-ionized, and the resulting electrons are counted by the microchannel plate detector.

Figure 2

Time-of-flight (top) and velocity distribution (bottom) of Rydberg atoms emitted from a cold Rydberg-atom gas with initial principal quantum number $n_0=90$, central Rydberg atom density $1.4\times {10}^8{\mathrm{cm}}^{-3}$, and Rydberg atom number $3\times {10}^5$. The time-of-flight $t$ is the elapsed time between the Rydberg excitation pulse and the detection of the Rydberg electron by the MCP detector.

Figure 3

Total production of fast Rydberg atoms (∎) vs initial Rydberg atom density $\rho$ at $n_0=90$, where ${\rho }_{\mathrm{scale}}=9.1\times {10}^8{\mathrm{cm}}^{-3}$ is a convenient density unit. The production of fast Rydberg atoms is compared with integrated Penning ionization $(▴)$ and plasma $(▾)$ signals obtained from state-selective FI spectra. For the numbered data points, the FI spectra are shown in Fig. 4.

Figure 4

Field ionization (FI) spectra for $n_0=90$ and Rydberg atom densities identified by the labels on the left and matching labels in Fig. 4. The Rydberg gas is excited at $t=0$, and the FI pulse (thick line) is applied between $40\mathrm{\mu }\mathrm{s}<t\lesssim 80\mathrm{\mu }\mathrm{s}$. Characteristic FI signatures: (A) plasma electrons, (B) initial state, (C) $\ell$-mixing, (D) Penning ionization.

Figure 5

Total production of fast Rydberg atoms vs initial Rydberg atom density $\rho$ in the low-density limit at $n_0=75$, where ${\rho }_{\mathrm{scale}}=7.7\times {10}^8{\mathrm{cm}}^{-3}$. A constant background of 17 counts was subtracted from the data set to compensate for dark counts on the MCP. The uncertainty for these counting measurements arises mainly from statistical noise, which is $\approx \sqrt{N}$. The data are fitted with the indicated power law.