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Atomic PhysicsClick a menu option to jump or scroll down for more:
Laser Spectroscopy of Highly-Charged IonsSince 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. Precision Penning Trap Mass Spectrometry and Single Ion Spectroscopy (New Program!)Some of the most precise measurements of atomic masses, at <0.1 ppb relative precision, have been obtained with the single-ion, “Ion Cyclotron Resonance” Penning trap mass spectrometer developed by David E. Pritchard and co-workers at MIT [1,2]. In their final work the MIT ICR group succeeded in pushing atomic mass comparisons below the 10 ppt level by simultaneously measuring the cyclotron frequencies of two ions in the same Penning trap [3], leading to a precision direct test of Einstein’s “E=mc2” [4]. In the process they discovered a new perturbation of the cyclotron frequency of molecular ions due to polarizability, which can be used to accurately measure dipole moments [5]. In May 2003, following a pre-arranged agreement, the MIT lab was closed and the apparatus was moved by the FSU atomic physics group to Tallahassee. Here it was set up again to form a new precision ICR laboratory. Following apparatus set-up and shake-down our initial measurements were of the mass of 32S and of the most abundant isotopes of the heavier rare gases 84,86Kr and 129,132Xe, all at sub 10-10 relative precision [6]. We then developed a technique for mass comparison with two ions simultaneously trapped in a Penning trap, but with the ions alternately cooled to the center of the trap - where the cyclotron frequency is measured - or else parked in a large radius cyclotron orbit. This technique was used to measure the mass of 31P [7], to re-investigate polarizability induced cyclotron frequency shifts in CO+, to investigate such shifts in 31PH+, and for preliminary work at using polarizability shifts to detect laser-induced transitions. These precision mass measurements have immediate value in providing improved reference masses for many other atomic masses and hence strengthening the global Atomic Mass Evaluation [8]. We have also recently measured the mass of 136Xe. This is needed for determining the Q-value for searches for the neutrino-less double-beta decay of 136Xe to 136Ba. In parallel we are re-developing the ultra-high precision simultaneous cyclotron frequency measurement technique. We intend to apply this to more high-precision mass comparisons relevant to fundamental constants in general, and in particular to a high precision measurement of the mass difference between tritium and helium-3. This will determine the beta-decay Q-value and hence the end-point of the tritium beta-decay electron spectrum, an important parameter in the determination of limits to the mass of the electron neutrino. Collaborations, People and FundingCollaborations: Laser Spectroscopy of Highly Charged Ions:Edmund Myers has collaborated with the Electron Beam Ion Trap group at Oxford for several years. Exchange visits have involved fast-beam laser spectroscopy and Penning trap measurements (at FSU), and EBIT laser spectroscopy (at Oxford). Penning Trap Mass Spectrometry:Myers was a visiting scientist in David Pritchard’s mass spectrometry group at MIT for 20 weeks in 2002 and 2003 working with Simon Rainville and James Thompson. In May 2003 the FSU group moved the MIT apparatus to Tallahassee and set up the new ICR lab. John M. Brown of Oxford University has provided detailed calculations of the polarizabilities of molecular ions needed for correcting cyclotron frequencies for stark-induced polarization shifts. People:The following have contributed to laser spectroscopy of few-electron ions at FSU: Evangelos P. Gavathas (FSU post-doc) The following have contributed to the Penning trap program at FSU: Matthew Redshaw (FSU grad. student) The following have worked in the Penning trap lab as FSU Young Scholar Program (High School, Math/Science Program) participants: Matthew Wierman, Nicholas Horton, Jonathan Malmaud, Andrea McElveen, Anna Alexandrova. Funding:Support has been received from the NATO Collaborative Research Grants program, from the National Science Foundation, and the National Institute of Standards and Technology Precision Measurement Grants program.
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