Through a combination of experimental measurements, theoretical computations, and phenomenological modeling, our group studies the structure of highly excited and highly ionized atoms. These ions are studied experimentally using a particle accelerator to create a beam of singly ionized atoms, all moving with the same speed and direction. A very thin foil interposed in the beam path causes some atomic electrons to be stripped away and others to be excited to higher states. Because the speed of the beam is a few percent of the speed of light, the time evolution of the birth, life, and death of these ions is clearly revealed on a scale of billionths of a second by the emission of light along the flight path.
With this method, the high density excitation conditions produce copious single and multiple electron excitation, and the low density decay conditions provide a field-free and collision-free environment for their time-resolved study. Position sensitive detection methods are used to obtain simultaneous multiplexed measurements as functions of both the wavelength and the time after excitation. Because the atom undergoes successive decay in which each electron cascades from one orbit to another, the mathematical content of this decay process can be very complicated. To eliminate this problem, our group has devised methods that relate the decay curves for the final and the next-to-final steps, which remove these cascade effects and permit highly precise determination of the lifetimes.
The program involves extensive national and international collaboration utilizing not only the 330 kV University of Toledo Heavy Ion Accelerator (THIA), but also accelerators at the University of Lund in Sweden, the University of Bochum in Germany, and Argonne National Laboratory. Many of the measurements have been directed toward applications in the interpretation of astrophysical data from the Hubble Space Telescope, for the diagnosis of plasma data from the Joint European Torus (JET) controlled nuclear fusion tokamak reactor in Culham, England, and in applications relevant to the development of X-ray lasers.