When ultracold plasma is generated using the photoionization of laser-cooled atoms, some atoms reach only up to Rydberg states. These, in turn, interact with the free electrons of the plasma and get ionized further, thus enhancing the plasma. Theoretical models until date have used a semiclassical approach that does not explain all the properties of Rydberg ionization. We present an analytical solution for the Rydberg–electron ionization cross section using the Quantum-mechanical scattering process, which shows a good agreement with experimental data. In particular, the dependency of ionization on the Rydberg state is clearly shown in the form of relation with orbital size and the scattering length. This calculation is valid for all alkali atoms and, hence, is useful for estimating the electron density of ultracold plasma.
While generating ultracold plasma (UCP) by photoinization of laser-cooled atoms, only a small fraction of atoms are ionized, and the remaining neutrals interact with the electrons present therein. These interactions, in addition to the Coulomb interactions between ions and electrons, cause phenomena such as ionization of Rydberg atoms and three body recombination, all of which affect the overall behaviour of the ultracold plasma. We had earlier developed a quantum treatment to analyze these interactions and investigated the ionization of Rydberg atoms in Cesium, which showed good agreement with measured results. We now extend it to other atomic species to investigate Rydberg ionization, and other effects such as three-body recombination and a resulting additional `quantum pressure' which causes a faster expansion of the UCP. Our results successfully explain experimental observations which were hitherto deemed as `anomalies'.
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.
We have studied the behavior of cold Rydberg atoms embedded in an ultracold plasma. We demonstrate that even deeply bound Rydberg atoms are completely ionized in such an environment, due to electron collisions. Using a fast pulse extraction of the electrons from the plasma we found that the number of excess positive charges, which is directly related to the electron temperature
Te, is not strongly affected by the ionization of the Rydberg atoms. Assuming a Michie-King equilibrium distribution, in analogy with globular star cluster dynamics, we estimate
Te. Without concluding on heating or cooling of the plasma by the Rydberg atoms, we discuss the range for changing the plasma temperature by adding Rydberg atoms.
Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magneto-optical trap. Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy. The dependences of recombination on initial electron temperature (1–70 K) and initial ion density (∼10 10 cm −3 ) are investigated. The measured dependence on initial ion density is N 1.547+0.004 at a delay time of 5 μs. The recombination rate rapidly declines as initial electron temperature increases when delay time is increased. The distributions of Rydberg atoms on different values of principal quantum number n, i.e. n = 30–60, at an initial electron temperature of 3.3 K are also investigated. The main experimental results are approximately explained by the three-body recombination theory.
We report a systematic experimental and numerical study of the expansion of ultracold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities,
v0, of ultracold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature,
Te,0=mionv02/kB (where
mion is the
Rb+ ion mass), as a function of the original Rydberg atom density and binding energy,
Eb,i. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range ${10}^{7}--{10}^{9} {\mathrm{cm}}^{\ensuremath{-}3}$, for states with principal quantum number
n>40,Te,0 is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity
kBTe,0 is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|\ensuremath{\lesssim}{k}_{B}{T}_{e,0}\ensuremath{\lesssim}2.5\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n\ensuremath{\lesssim}40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where
Te,0 is determined by their Coulomb coupling parameter, ${\mathrm{\ensuremath{\Gamma}}}_{e}\ensuremath{\sim}0.01$.
In the last decade, ultracold plasmas have been created in the laboratory by photo-ionizing laser-cooled atoms. To understand the overall dynamics of ultracold plasmas, one needs to understand Rydberg collisional processes at ultracold temperatures. The two kinds of problems addressed in this thesis are: study of Rydberg atomic processes at ultracold temperatures, and a study of the overall dynamics of \nthe ultracold plasmas. \nTheoretical methods based on quantal-classical correspondence is used to understand Rydberg atomic processes such as radiative cascade, and radiative recombination. A simulation method suitable for ultracold collisions is developed and tested. \nThis method is then applied to study collisional-Stark mixing in Rydberg atoms. \nTo study the dynamics of the ultracold plasmas, a King model for the electrons in plasmas is proposed. The King model is a stationary, finite sized electron distribution for the electrons in a cloud of fixed ions with a Gaussian distribution. A Monte-Carlo method is developed to simulate the overall dynamics of the King distribution.