B.S., Trinity College (CT) (1969)
Ph.D. Harvard University (1975)
Fellow of the American Physical Society
Resonant Excitation Stark Ionization Spectroscopy
Our research is focused on a special class of excited states of atoms, ions, and molecules, Rydberg states with L > 5. In these “high-L” Rydberg states, the excited Rydberg electron is excluded virtually completely from the vicinity of the ion core by the repulsive contrifugal potential. As a result, the forces between the Rydberg electron and the ion core, aside from the dominant Coulomb force from the ion core’s net charge, are so feeble that the appearance and behavior of these High-L Rydberg states can be quite different from other excited states of the same system. In simple systems like helium and H2, the high-L Rydberg structure can be predicted precisely from first principles, and measurements can pose tests of fundamental theory. In more complex systems, the high-L Rydberg electron can be regarded as a sensitive probe of many properties of the positive ion core that are otherwise difficult to measure.
To study these unusual states, we use a special experimental technique which we call Resonant Excitation Stark Ionization Spectroscopy (RESIS). It is based on the use of a Doppler-tuned CW CO2 laser to selectively excite and ionize particular Rydberg levels in a fast Rydberg beam formed by charge capture in a Rydberg target from an accelerated ion beam. The main advantage of this technique for studying high-L levels is that upwards excitation is allowed for every value of L. Very high resolution microwave spectroscopy can be carried out based on the selective RESIS detection of different fine-structure levels. Another attractive feature of this method is that it is immediately applicable to many different Rydberg systems simply by changing the identity of the initial ion beam.
One of the main goals of our current work is to use the RESIS method to learn more about the properties of chemically important actinide ions, like U6+. This ion occurs as a unit in uranium chemistry, but predicting its properties from first principles is very difficult because of its large nuclear charge and numerous electrons. By forming high-L Rydberg ions with U6+ as the core ion, we hope to measure its dipole polarizibility and other properties. These measurements could provide a check on existing calculations, and also be used directly in some chemical models of actinide chemistry. Our first experimental result of this type is the recently reported measurement of the dipole polarizability of Rn-like Th4+, described in reference 7) listed below.
- “Polarizabilities of Rn-like Th4+ from spectroscopy of high-L Rydberg levels of Th3+“, M.E. Hanni, Julie A. Keele, S.R. Lundeen, and C.W. Fehrenbach, Phys. Rev. A 82, 022512 (2010)
- “Properties of Fr-like Th3+ from spectroscopy of high-L Rydberg levels of Th2+“, Julie. A. Keele, M.E. Hanni, Shannon L. Woods, S.R. Lundeen , and C.W. Fehrenbach, Phys. Rev. A 83, 062501 (2011)
- “Polarizabilities of Rn-like Th4+ from rf spectroscopy of Th3+ Rydberg levels”, Julie A. Keele, S.R. Lundeen, and C.W. Fehrenbach, Phys. Rev. A 83, 062509 (2011).
- “Effective Potential Model for high-L Rydberg atoms and ions” Shannon L. Woods and S.R. Lundeen, Phys. Rev. A 85, 042505 (2012)
- “Measurements of dipole and quadrupole polarizabilities of Rn-like Th4+”, Julie. A. Keele, Chris S. Smith, S.R. Lundeen, and C.W. Fehrenbach, Phys. Rev. A 85, 064502 ( 2012)
- “Microwave Spectroscopy of high-L, n=9 Rydberg levels of Nickel: Polarizabilities and Moments of the Ni+ ion”, Shannon Woods, Chris Smith, Julie Keele, and S.R. Lundeen, Phys. Rev. A 87, 022511 (2013)
- “Properties of Fr-like Th3+ from rf spectroscopy of high-L Th2+ Rydberg ions”, Julie A. Keele, Chris Smith, S.R. Lundeen, and C.W. Fehrenbach, Phys Rev A 88, 022502 (2013)