Abstract Penning ionisation (PI) processes involving pairs of Rydberg alkali-metal atoms, excited to different quantum states and experiencing dipole–dipole interactions, have a wide range of important properties in atomic physics. Within the framework of the semiclassical approximation, we have used both numerical and analytical approaches to examine the Penning autoionisation width dependence on the state quantum numbers in a quasi-molecule formed by the interacting partner atoms. We described the characteristics of optimal quantum numbers that lead to enhanced PI widths for the interacting Rydberg atom pairs of all alkali-metal atoms. The excited states of atoms in these pairs are asymmetric, resulting in a large atomic shell size difference: inspired by Efimov et al (2016 J. Phys. B: At. Mol. Opt. Phys. 49 125302), we call such a pair ‘Tom’ and ‘Jerry’ (for ‘big’ and ‘small’). Compared to symmetric pairs, the optimal asymmetric pairs display a significant (by several orders of magnitude) increase in the PI rate. This property makes PI a relevant source for producing charged particles in cold Rydberg systems that spontaneously evolve into cold plasma. Contrary to hydrogen atoms examined in (Efimov et al 2016 J. Phys. B: At. Mol. Opt. Phys. 49 125302), the difference in quantum defects in alkali-metal atoms results in a strong Penning width dependence on the orbital quantum numbers l of the quasi-molecule. In particular, alkali-metal atoms exhibit two PI channels associated with bound–bound optical transitions showing Δ l = ±1—individual and closely spaced (doublet-like) configurations of optimal pairs. Furthermore, we demonstrate that the presence of Förster resonances can lead to a notable (up to five times) increase in the PI efficiency.
Abstract Penning ionisation (PI) processes involving pairs of Rydberg alkali-metal atoms, excited to different quantum states and experiencing dipole–dipole interactions, have a wide range of important properties in atomic physics. Within the framework of the semiclassical approximation, we have used both numerical and analytical approaches to examine the Penning autoionisation width dependence on the state quantum numbers in a quasi-molecule formed by the interacting partner atoms. We described the characteristics of optimal quantum numbers that lead to enhanced PI widths for the interacting Rydberg atom pairs of all alkali-metal atoms. The excited states of atoms in these pairs are asymmetric, resulting in a large atomic shell size difference: inspired by Efimov et al (2016 J. Phys. B: At. Mol. Opt. Phys. 49 125302), we call such a pair ‘Tom’ and ‘Jerry’ (for ‘big’ and ‘small’). Compared to symmetric pairs, the optimal asymmetric pairs display a significant (by several orders of magnitude) increase in the PI rate. This property makes PI a relevant source for producing charged particles in cold Rydberg systems that spontaneously evolve into cold plasma. Contrary to hydrogen atoms examined in (Efimov et al 2016 J. Phys. B: At. Mol. Opt. Phys. 49 125302), the difference in quantum defects in alkali-metal atoms results in a strong Penning width dependence on the orbital quantum numbers l of the quasi-molecule. In particular, alkali-metal atoms exhibit two PI channels associated with bound–bound optical transitions showing Δ l = ±1—individual and closely spaced (doublet-like) configurations of optimal pairs. Furthermore, we demonstrate that the presence of Förster resonances can lead to a notable (up to five times) increase in the PI efficiency.
Abstract We consider Penning ionization of Rydberg atom pairs as an Auger-type process induced by the dipole–dipole interaction and employ semiclassical formulae for dipole transitions to calculate the autoionization width as a function of the principal quantum numbers, n d , n i , of both atoms. While for symmetric atom pairs with n d = n i = n 0 the well-known increase of the autoionization width with increasing n 0 is obtained, the result for asymmetric pairs is counterintuitive—for a fixed n i of the ionizing atom of the pair, the autoionization width strongly increases with decreasing n d of the de-excited atom. For H Rydberg atoms this increase reaches two orders of magnitude at the maximum of the n d dependence, and the same type of counterintuitive behavior is exhibited also by Na, Rb and Cs atoms. This is a purely quantum-mechanical effect, which points towards existence of optimal (we call them ‘Tom’ and ‘Jerry’ for ‘big’ and ‘small’) pairs of Rydberg atoms with respect to autoionization efficiency. Building on the model of population redistribution in cold Rydberg gases proposed in [1], we demonstrate that population evolution following the initial laser excitation of Rydberg atoms in state n 0 would eventually lead to the formation of such Tom–Jerry pairs with n i > n 0 > n d which feature autoionization widths that are enhanced by several orders of magnitude compared to that of two atoms in the initial laser-excited state n 0 . We also show that in the high-density regime of cold Rydberg gas experiments the ionization rate of Tom–Jerry pairs can be substantially larger than the blackbody radiation-induced photoionization rate.
Penning ionization (PI) processes for cold Rydberg alkali metal atoms are investigated. Contrary to the reference case of a hydrogen atom, the corresponding autoionization widths demonstrate a sharp dependence (by orders of magnitude) on the orbital quantum numbers of the atoms exposed to long-range dipole-dipole interaction. An important feature of PI is the nontrivial dependence of its efficiency on the size of Rydberg particles. For all types of alkali atoms, the existence of optimal Rydberg pairs has been demonstrated (highly asymmetric configurations of Rydberg pairs), which lead to an explosive intensification (by several orders of magnitude) of the formation of free electrons due to PI processes. This property makes PI an important source of the formation of primary charged particles during the formation of cold Rydberg plasma. The presented numerical data for potassium atomic pairs demonstrate a significant effect of the Förster resonance on the values of PI rate constants.
Abstract Features of Penning ionization in cold gaseous media of Rydberg alkali-metal atoms have been investigated. In contrast to the hydrogen atom, the corresponding autoionization widths exhibit a strong (by orders of magnitude) dependence on the orbital quantum numbers of the atoms involved in the long-range dipole–dipole interaction. An important feature of Penning ionization is the nontrivial dependence of its efficiency on the Rydberg particle size. For all types of alkali-metal atoms, the optimal highly nonsymmetric configurations of Rydberg pairs have been found that lead to the explosive (by several orders of magnitude) intensification of the free electron formation by means of the Penning ionization processes. This property makes Penning ionization an important means of forming primary charged particles during the cold Rydberg plasma formation. The numerical data for pairs of potassium atoms are reported that demonstrate a significant effect of the Förster resonance on the values of the Penning ionization rate constants.
We have considered two types of interactions between particles resulting in autoionization of the intermediate Rydberg collisional complex.The first type is caused by the charge exchange processes inside the ionic core of the complex and is responsible for the formation of the final associative ionization channel due to the departure of a Rydberg electron (RE) of the complex to the energy continuum.We have investigated the evolution of the orbital momentum L of RE and discovered a non-trivial time dependence of L, demonstrating a contrasting oscillatory structure in both direction orientation and its absolute value.The second type of interaction, dipole-dipole interaction, dominates in cold highly excited gaseous media and leads to Penning ionization of various pairs of Rydberg atoms.We have found the optimal quantum configurations of atomic pairs, which intensify the PI rates by several orders of magnitude, and become important suppliers of charged particles upon the evolution of Rydberg gas into cold plasma.
The development of atom trapping and cooling techniques has been a boon for the field of low temperature plasma physics. By forming plasmas through photoionization (PI) of cold atoms, researchers are able to create cold, dense plasmas that often lie in the strong-coupling regime. This is especially remarkable considering that many naturally-occurring plasmas in the strong-coupling regime are in inaccessible environments (e.g. the insides of stars and gas planets), whereas these laboratory-based plasmas prepared in cold-atom systems are relatively easy to produce and analyze with standard AMO techniques. In this thesis, I present my work from several projects on photoionization, plasma formation and dynamics, and plasma trap development. I first discuss an experiment that I spearheaded on expansion dynamics in cold, non-neutral plasmas. In this experiment, cold plasmas are formed by photoionizing rubidium atoms in a MOT, and the resulting plasma ions are allowed to expand for a variable wait time before being imaged with a single-ion imaging system. This ion imaging system, along with the rest of the experimental setup, was originally used to study Rydberg atoms. I have adapted the setup for plasmas by installing a new laser and changing the experimental timings. I report on plasma features seen in this experimental setup, such as shock shells and time-evolved ion pair correlations. I also present findings from a computational study of wavelength-dependent PI of Rydberg atoms, namely Rydberg-state rubidium and cesium. The main goal of the study is to find PI minima, wavelengths where the atom’s PI cross section decreases dramatically. These minima can be caused by Cooper minima or shape resonances, which each have their own telltale signs. This computational study paves the way for an informed experimental measurement of the locations of some of the identified PI minima, and comparing their experimental locations with the computational results. This is, in part, interesting because the latter depend on whether length or velocity gauge is used. An experimental measurement of PI cross sections could be a good way to explore the significance of gauge dependence in l-dependent potentials. I conclude with a discussion of a project centered on creating a small MOT with diverging cooling beams inside of a metal enclosure. This project was a collaboration with researchers at Eastern Michigan University. The idea to make a MOT with these design features was born out of plans for cryostatic plasma experiments. Now, it has potential applications in plasma experiments, precision measurements, and quantum information. Design-wise, the diverging beams are formed by sending narrow laser beams into 1.5 mm ball lenses, which cause the beams to diverge rapidly. Six ball lenses, one for each cooling beam, are mounted on a metal structure called the ball lens optical box (BLOB), which encases the trapped atom cloud. This enclosure provides immediate benefits for plasma experiments, as it offers Faraday shielding from stray electric fields. The first build successfully produced and maintained a MOT, making it the first known case of a MOT made with diverging beams from ball lenses. No further experiments are planned for this first build, but the concept of a ball lens MOT will certainly be implemented into future experimental efforts. Subsequent designs may include more design features, such as interior electrodes for custom field configurations.
Penning ionization (PI) occurring during the collision of a Rydberg atom Rb(nl) with a resonant state K(4P) or Rb(5P) is analyzed theoretically. By using a semiclassical approach, we calculate the rate coefficients corresponding to both the direct and exchange decay channels. We implement the method by deriving various approximations for the exchange and direct decay widths. The radiative mechanism is found to dominate the thermal-energy quenching of alkali atoms by PI, the exchange being negligible. The calculated values are in reasonable agreement with the available experimental results.