Atomic physics is concerned with high precision measurements that test our very basic understanding of many-electron systems. Apart from its fundamental interest, atomic physics has implications for plasma physics, astrophysics, and chemistry. Atomic physics at the University of Kentucky is focussed on the structure and evolution in time of excited states of atoms, the interaction of excited atoms with strong external fields, and collisions between excited atoms.
Professor Keith MacAdam
Through the process of photoexcitation, the outermost electron of an atom can be raised into a highly excited orbital state; these atoms are called Rydberg atoms. They are characterized by extremely large diameters (frequently 1000 times their normal size), enhanced collision probabilities, and an extreme sensitivity to the effects of externally applied electromagnetic fields. A sufficiently strong static electric field can actually tear away the highly excited electron, a process called field ionization.
Rydberg atoms may be studied in great detail both experimentally and theoretically. Among the features amenable to these comparisons are energy level shifts, positions, and widths; the appearance of continua; autoionization rates; and the transfer of population among states. The effects of time-varying electric fields on atoms are significantly more complicated than the effects of static fields since population may be transferred among levels in a manner that depends on the rate at which the field changes. Strong oscillating fields---e.g., intense laser light or strong microwaves---may cause absorption and emission of several photons simultaneously in a single atom and give the participating atoms unusual absorption and ionization properties that have no counterpart in the classical theory of light.
Topics of current theoretical research at the University of Kentucky include studies of atomic Rydberg spectra, electron correlations in multiply-excited atomic states, multichannel quantum-defect theory, strong-field Stark and Zeeman effects, selective field ionization, collision- and field-induced dynamical state mixing in networks of interacting energy levels, multiphoton effects, and muonic atoms.
Experimental work in Rydberg atoms at UK is centered on the study of collisions between ions and laser-excited Rydberg atoms. In these collisions, the highly excited Rydberg electron is sufficiently far removed from the remainder of the atom so that the interaction among the ion, electron and atomic core can be formulated as the familiar ``three-body problem'' of celestial mechanics---although transposed into a quantum-mechanical context. The study of Rydberg atom collisions with ions becomes a model for the processes that occur in nuclear fusion plasmas, interstellar clouds, and gas lasers. Furthermore, it offers empirical information about three-body dynamics in a regime that presents a difficult challenge to atomic-collision theories, due principally to the comparable speeds of the projectile ion and the orbiting electron. In the well-equipped Rydberg laboratory at UK, investigations are under way to determine the population distributions following charge-transfer and other inelastic collisions, the response of highly excited atoms to pulsed or ramped electric fields, and microwave resonance.
An 
spectrometer at UK is being used to investigate autoionization in
metal vapor atoms. Autoionization is a spontaneous process of ionization that
occurs whenever quantized energy levels lie above the ionization potential of
an atom. The existence of such states has a significant effect on the
ionization balance in plasmas. In 
spectroscopy, a beam of electrons is
scattered by an atomic vapor target, and both the scattered electron and an
electron ejected from the atom by autoionization are detected in coincidence.
Sophisticated 
coincidence experiments have shown that complex
quantum-mechanical interference effects play a significant role in the
electron-impact ionization of atoms.
Faculty and Research Staff