Since 1994 at Florida State University we have pursued a program of precision laser spectroscopy of hydrogen-like and helium-like ions, i.e. highly stripped atoms with only one or two electrons remaining. The purpose is to provide precise tests of relativistic atomic structure theory which has made important progress in the last 10 years. One and two-electron systems provide an ideal testing ground for ab initio, quantum-electrodynamics based atomic structure theory. Relativistic and QED effects scale rapidly with atomic number Z, and so such effects can be sensitively investigated by performing measurements at different Z. Many theory groups are actively engaged in this area including those at the University of Notre Dame, Windsor (Canada), Lawrence Livermore, NIST Gaithersburg, Lund (Sweden), St. Petersburg (Russia), Warsaw (Poland), Dresden (Germany) and elsewhere. Measurements completed so far have been of intercombination (between singlets and triplets) and fine-structure transitions in the n=2 manifold of helium-like nitrogen, fluorine, magnesium and silicon (see publication list). In all cases the precision of these measurements matches or exceeds that of the theory. Currently we are working to improve the precision of our measurement on helium-like silicon and we are also developing a precise measurement on the 2s1/2 - 2p3/2 (fine-structure - Lamb shift) transition in N6+. This will constitute a direct test of QED theory of a bound electron. Apart from the intrinsic importance of testing and stimulating fundamental atomic theory, this work has application to the interpretation of precision spectroscopy of hydrogen and helium, which is aimed at obtaining improved values for the Rydberg constant and fine-structure constant, respectively.
These measurements have been carried out using the fast-beam laser technique and the ions have been produced by foil-stripping less highly-charged ions from the FSU Van de Graaff accelerator in a thin carbon foil. The ions typically move at 5% of the speed of light resulting in very short ion-laser interaction times and large doppler shifts. To partially overcome these problems we have developed techniques of co-linear laser spectroscopy, and the use of two lasers, at different frequencies, to interact with the ions in co- and counter-propagating geometry. We have also used an ultra-high-finesse optical build-up cavity to achieve high (over 2 kW cw) laser power at the interaction region to enhance signal to noise and enable weak, magnetic dipole, or first-order forbidden electric dipole transitions to be observed.
An alternative method for producing few-electron ions is to ionize, using an electron beam, a cloud of ions trapped in a Penning trap. Over the last few years we have collaborated with the Oxford EBIT (Electron Beam Ion Trap) group in laser spectroscopy of ions in an EBIT. Using a CO2 laser we have recently succeeded in observing the 2s1/2 - 2p3/2 transition in N6+. To our knowledge this is the first laser spectroscopy of trapped few-electron ions in an EBIT. Nevertheless the conventional EBIT produces relatively hot (typically several 100 eV) highly-charged ions. As such it is not suited for precision laser spectroscopy. However very high precision spectroscopy can, and has been, performed on cooled single ions in precision Penning traps (and also Paul traps.) The goal of applying single-ion Penning trap techniques to spectroscopy of few-electron ions has led us to precision Penning trap techniques.
Selected Publications on Laser Spectroscopy of Few-Electron Ions:
"Measurement of the 1s2s 1S0 - 1s2p 3P1 intercombination transition in helium-like silicon", M. Redshaw and E.G. Myers, Phys. Rev. Lett. 88, 023002 (2002).
"Laser spectroscopy of hydrogenlike and heliumlike ions", E.G. Myers, in: "The Hydrogen Atom: Precision Physics of Simple Atomic Systems", ed. S.G. Karshenboim et al, Springer, 2001, p179.
"Measurement of the 1s2p 3P0 - 3P1 fine structure interval in heliumlike magnesium'', E.G. Myers and M.R. Tarbutt, Phys. Rev. A61, 010501(Rapid Communication), (1999).
"Precision measurement of the 1s2p 3P2 - 3P1 fine structure interval in heliumlike fluorine'', E.G. Myers, H.S. Margolis, J.K. Thompson, M.A. Farmer, J.D. Silver, and M.R. Tarbutt, Phys. Rev. Lett. 82, 4200 (1999).
"Measurements of the 1s2s 1S0 - 1s2p 3P1,0 transitions in heliumlike nitrogen'', J.K. Thompson, D.J.H. Howie, and E.G. Myers, Phys.Rev. A 57, 180 (1998).
"Hyperfine-Induced 1s2s 1S0 - 1s2p 3P0 transition and fine-structure measurement in helium-like nitrogen'', E.G. Myers, D.J.H.Howie, J.K. Thompson, and J.D. Silver, Phys. Rev. Lett. 76, 4899 (1996).
(Last update March 2006)