Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as $100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high
n states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.
We have observed the ionization of a cold gas of Rb Rydberg atoms which occurs when
nsns van der Waals pairs of
ns atoms of $n\ensuremath{\approx}$ 40 on a weakly repulsive potential are transferred to an attractive dipole-dipole
nsnp potential by a microwave transition. Comparing the measurements to a simple model shows that the initial 300-$\ensuremath{\mu}$K thermal velocity of the atoms plays an important role. Excitation to a repulsive dipole-dipole potential does not lead to more ionization on a 15-$\ensuremath{\mu}$s time scale than leaving the atoms in the weakly repulsive
nsns state. This observation is slightly surprising since a radiative transition must occur to allow ionization in the latter case. Finally, by power broadening of the microwave transition, to allow transitions from the initial
nsns state to the
nsnp state over a broad range of internuclear spacings, it is possible to accelerate markedly the evolution to a plasma.
We have excited helium atoms from their metastable 2
3S state to Rydberg states in the range
13<n<50 in a two-step process via the 3
3P state using light at $\ensuremath{\lambda}=389$ nm and 785--815 nm. Atoms in a thermal beam (100 K) cross partially overlapping laser beams of the appropriate frequencies in the counterintuitive order to exploit the high efficiency of stimulated rapid adiabatic passage. The interaction region is between two plates that can be used for Stark tuning in a few V/cm field or for field ionization. At fields much too low for field ionization, we observe signals attributed to ionization by blackbody radiation. Multiple tests confirm this attribution as the cause of ionization. For example, by heating the plates we observe the expected signal increases. Our experiments reinforce previous work where the interaction between Rydberg atoms and room temperature blackbody radiation is important for experiments.
We report clear evidence of the role of dipole-dipole interaction in Penning ionization of Rydberg atoms, leading to the formation of an ultracold plasma. Penning ionization of np Rydberg Cesium atoms is prevented for states with in n < 42, which correspond to a repulsive potential, but it does not occur for n larger than 42, corresponding to an attractive potential. Blackbody radiation is mostly responsible for the background and initial ionization, although ion-Rydberg collisions and population transfer due to limited superradiance may have to be considered.
Ion formation in sodium vapor (\ensuremath{\sim}
1011 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) containing Rydberg atoms (18\ensuremath{\le}n\ensuremath{\le}35) was investigated. Both atomic ions and diatomic molecular ions resulting from collisions between highly excited sodium,
Na∗∗, and Na(3s) were observed. However, the predominant production mechanism was found to be photoionization of
Na∗∗ by blackbody radiation, the rate of which was measured at 500 K and found to be \ensuremath{\simeq}5.7\ifmmode\times\else\texttimes\fi{}
103 ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=18, in good agreement with theory. The
Na+ signal from blackbody photoionization is observed to be independent of n, which we interpret as implying rapid and nearly complete self-l-mixing, mixing by collisions with Na(3s) atoms, for both s and d states. It is probable, however, that the s states are mixed to the adjacent, n-1, manifold of l states. It was also found that the heavy-body collision cross sections for
Na+ and
Na2+ formation, while roughly three orders of magnitude lower than our estimate of the self-l-mixing cross section, are appreciable, of order 100 A
˚2.
We have studied ionization of alkali-metal Rydberg atoms by blackbody radiation (BBR). The results of theoretical calculations of ionization rates of Li, Na, K, Rb and Cs Rydberg atoms are presented. The calculations have been performed for nS, nP and nD states for principal quantum numbers n=8-65 at ambient temperatures of 77, 300 and 600 K. The calculations take into account the contributions of BBR-induced redistribution of population between Rydberg states prior to photoionization and field ionization by extraction electric field pulses. The obtained results show that these phenomena affect both the magnitude of the measured ionization rates and their n-dependence. A Cooper minimum for BBR-induced transitions between bound Rydberg states of Li has been found. The calculated ionization rates are compared with our earlier measurements of BBR-induced ionization rates of Na nS and nD Rydberg states with n=8-20 at 300 K. A good agreement for all states except nS with n> 15 is observed. Useful analytical formulas for quick estimates of BBR ionization rates of Rydberg atoms are presented. Application of BBR-induced ionization signal to