Cold dense samples of Rydberg atoms spontaneously evolve into ultracold plasmas. Initially some cold ions are formed by black body photo ionization and collisions, and they trap electrons freed later. It appears that these trapped electrons initiate an avalanche of ionization of the remaining Rydberg atoms. How quickly the avalanche occurs is determined in part by how rapidly energy is supplied to the system.
Cold dense samples of Rydberg atoms spontaneously evolve into ultracold plasmas. Initially some cold ions are formed by black body photo ionization and collisions, and they trap electrons freed later. It appears that these trapped electrons initiate an avalanche of ionization of the remaining Rydberg atoms. How quickly the avalanche occurs is determined in part by how rapidly energy is supplied to the system.
We have observed the spontaneous evolution of a dense sample of Rydberg atoms into an ultracold plasma, in spite of the fact that each of the atoms may initially be bound by up to 100 cm(-1). When the atoms are initially bound by 70 cm(-1), this evolution occurs when most of the atoms are translationally cold, <1 mK, but a small fraction, approximately 1%, is at room temperature. Ionizing collisions between hot and cold Rydberg atoms and blackbody photoionization produce an essentially stationary cloud of cold ions, which traps electrons produced later. The trapped electrons rapidly collisionally ionize the remaining cold Rydberg atoms to form a cold plasma.
By photoionizing cold, trapped atoms it is possible to produce ultracold plasmas with temperatures in the vicinity of 1 K, roughly 4 orders of magnitude colder than conventional cold plasmas. After the first photoelectrons leave, the resulting positive charge traps the remaining electrons in the plasma. Monitoring the dynamics of the expansion of these plasmas shows explicitly the flow of energy from electrons to the ionic motion, which is manifested as the expansion of the plasma. The electron energy can either be their initial energy from photoionization or can come from the energy redistribution inherent in recombination and superelastic scattering from recombined Rydberg atoms. If the cold atoms are excited to Rydberg states instead of being photoionized, the resulting cold Rydberg gas quickly evolves into an ultracold plasma. After a few percent of the atoms are ionized by collisions or blackbody radiation, electrons are trapped by the resulting positive charge, and they quickly lead to ionization of the Rydberg atoms, forming a plasma. While the source of this energy is not clear, a likely candidate is superelastic scattering, also thought to be important for the expansion of deliberately made plasmas.
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.
The spontaneous evolution from Rydberg atoms in different fine states to plasmas is investigated. Two-photon excitation is used to excite ultracold cesium atoms from 6S1/2 to 47D3/2 and 47D5/2 Rydberg states, respectively. The ramp electric field is used to ionize the Rydberg atoms and drive the ions to the micro-channel plate detector. In this experiment, The atom number of 47D5/2 state, which has a high oscillator strength compared with 47D3/2, is about 7 times larger than that of 47D3/2 state. Based on the different initial atom numbers of two fine states, the evolutions of plasma and Rydberg atoms are observed. The initial ionization time of Rydberg atoms and avalanche ionization rate of different states are presented. The initial ionization of Rydberg atoms is ascribed to the combining effect of state transfer from repulsive interaction to attractive interaction induced by the blackbody radiation and superradiation and Rydberg atoms collision. Moreover, the avalanche ionization and production of plasma are explained as being due to the rapid collision between Rydberg atoms and electrons in a local potential trap produced by positive ions.
We have observed the evolution of cold Rydberg gases and ultracold plasmas in an environment that allows for the control of interactions of the blackbody radiation temperature from 4K to 300K. The work in this thesis describes the influence of initial densities, populations, excitation state, evolution time and thermal background temperature on the evolution of cold Rydberg gases as well as directly excited ultracold plasmas. With some initial ionization, cold-Rydberg-atom gases easily develop a positive space charge that traps electrons for times on the order of tens of mus. This acts as a catalyst for electron-Rydberg atom collisions, increasing the collision probabilities by orders of magnitude and profoundly altering the final state of the Rydberg gas. Using state-selective field ionization, we have provided direct proof that these collisions cause changes in l-states, and we have seen the first experimental evidence that they also lead to inelastic n-changing collisions, as predicted by theory. We have also found evidence for Rydberg-Rydberg collisions, enabled by the attractive electric-dipole forces between atoms in high-l states and possibly by van-der-Waal's interactions at very high-n . Thermal interactions are seen to influence the formation of a plasma from a cold Rydberg gas. Plasmas form more easily in the presence of a 300K radiation field than at 4K, due to the contribution of thermal ionization to the plasma formation. The background temperature also appears to affect the electron temperature in both the directly excited plasma and the plasma spontaneously formed from a cold Rydberg gas. In Rydberg atom gases that have undergone partial ionization, electron temperatures were estimated using the dependence of n-changing collisions on the electron temperature.
By photoionizing samples of laser-cooled atoms with laser light tuned just above the ionization limit, plasmas can be created with electron and ion temperatures below 10 K. These ultracold neutral plasmas have extended the temperature bounds of plasma physics by two orders of magnitude. Table-top experiments, using many of the tools from atomic physics, allow for the study of plasma phenomena in this new regime with independent control over the density and temperature of the plasma through the excitation process. Characteristic of these systems is an inhomogeneous density profile, inherited from the density distribution of the laser-cooled neutral atom sample. Most work has dealt with unconfined plasmas in vacuum, which expand outward at velocities of order 100 m/s, governed by electron pressure, and with lifetimes of order 100 μs, limited by stray electric fields. Using detection of charged particles and optical detection techniques, a wide variety of properties and phenomena have been observed, including expansion dynamics, collective excitations in both the electrons and ions, and collisional properties. Through three-body recombination collisions, the plasmas rapidly form Rydberg atoms, and clouds of cold Rydberg atoms have been observed to spontaneously avalanche ionize to form plasmas. Of particular interest is the possibility of the formation of strongly coupled plasmas, where Coulomb forces dominate thermal motion and correlations become important. The strongest impediment to strong coupling is disorder-induced heating, a process in which Coulomb energy from an initially disordered sample is converted into thermal energy. This restricts electrons to a weakly coupled regime and leaves the ions barely within the strongly coupled regime. This review will give an overview of the field of ultracold neutral plasmas, from its inception in 1999 to current work, including efforts to increase strong coupling and effects on plasma properties due to strong coupling.