Lifetime measurement of the 8s level in franciumE. Gomez,1L. A. Orozco,2A. Perez Galvan,2and G. D. Sprouse11Department of Physics and Astronomy, SUNY Stony Brook, Stony Brook, New York 11794-3800, USA2Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA共Received 22 February 2005; published 22 June 2005兲We measure the lifetime of the 8s level of210Fr atoms on a magneto-optically trapped sample with time-correlated single-photon counting. The 7P1/2state serves as the resonant intermediate level for two-stepexcitation of the 8s level completed with a 1.3-␮m laser. Analysis of the fluorescence decay through the 7P3/2level gives 53.30±0.44 ns for the 8s level lifetime.DOI: 10.1103/PhysRevA.71.062504 PACS number共s兲: 32.70.Cs, 32.10.Dk, 32.80.PjWe present in this paper a measurement of the 8s levellifetime of francium, the heaviest of alkali-metal atoms. Fr isyet to be used in parity nonconservation 共PNC兲 measure-ments 关1兴, but work toward that goal requires understandingof the excited-state properties of the atom. The 8s state is thepreferred candidate for an optical PNC measurement; thedipole-forbidden excitation between the 7S1/2ground stateand the first excited 8S1/2state becomes allowed through theweak interaction. The equivalent state in Cs 共7s兲 has beenused in PNC experiments by the Boulder 关2,3兴 and Paris 关4兴groups and a quantitative understanding of this state—itslifetime and its branching ratio—is critical to the successfulextraction of weak-interaction physics in these experiments.Our measurement is a test of the modern techniques of abinitio calculations using many-body perturbation theory共MBPT兲关5,6兴. Quantitative measurements on Fr and com-parison with theoretical calculations validate the sameMBPT techniques used for Cs and other atoms with a morerelativistic atom where correlations from the 87 electrons arelarge.The lifetime␶of an excited state is determined by itsindividual decay rates 1/␶ithrough the matrix element asso-ciated with the i partial decay rate. The connections betweenlifetime, partial decay rates, and matrix elements are1␶=兺i1␶i, 共1兲1␶i=43␻3c2␣兩具J储r储J⬘典兩22J⬘+1, 共2兲where␻is the transition energy divided by ប, c is the speedof light,␣is the fine-structure constant, J⬘and J are, respec-tively, the initial- and final-state angular momenta, and兩具J储r储J⬘典兩 is the reduced matrix element 关7兴. Equation 共2兲links the lifetime of an excited state to the electronic wavefunctions of the atom. Comparisons of measurements withtheoretical predictions test the quality of the computed wavefunctions especially at large distances from the nucleus dueto the presence of the radial operator.The lifetimes of the low-lying states in Fr are reaching alevel of precision comparable to that of the other alkali met-als 关7–9兴. The atomic theory calculations for these transitions关10–12兴 predict lifetimes measured with impressive agree-ment, strengthening the possibility of a PNC experiment in achain of francium isotopes.We use the method of time-correlated single-photoncounting to obtain the lifetime of the 8s level in Fr in amagneto-optical trap 共MOT兲. We populate the 8s level with atwo-step excitation, and then we turn off the excitation sud-denly and observe the exponential decay through the fluores-cence photons 关13兴.The production, cooling, and trapping of Fr online withthe superconducting linear accelerator at Stony Brook hasbeen described previously 关14兴. Briefly, a 100-MeV beamof18O ions from the accelerator impinges on a gold targetto make210Fr 共radioactive half-life 3 min兲. We extract⬃1⫻ 106francium ions/s out of the gold and transport them15 m to a cold yttrium neutralizer where we accumulate theFr atoms. We then close the trap with the neutralizer and heatit for one second 共⬃1000 K兲 to release the atoms into thedry-film-coated glass cell where they are cooled and trappedin a MOT. The cycle of accumulating and trapping repeatsevery 20 s.Figure 1 shows the states of210Fr relevant for trappingand lifetime measurements. The nuclear spin of this isotopeFIG. 1. Energy levels of210Fr. The figure shows the trappingand repumping transitions 共thin solid lines兲, the two-step excitation共thick solid lines兲, the fluorescence detection used in the lifetimemeasurement 共dashed line兲, and the undetected fluorescence 共dottedline兲.PHYSICAL REVIEW A 71, 062504 共2005兲1050-2947/2005/71共6兲/062504共4兲/$23.00 ©2005 The American Physical Society062504-1is I=6 with a ground-state hyperfine splitting of46.768 GHz. A Coherent 899-21 titanium-sapphire 共Ti:sap-phire兲 laser operating at 718 nm excites the trapping andcooling transition 共7S1/2, F=13/2→ 7P3/2, F=15/2兲共trap inFig. 1兲. A Coherent 899-21 Ti:sapphire laser operating at817 nm repumps any atoms that leak out of the cooling cyclevia the 7S1/2, F=11/2→ 7P1/2, F=13/2 transition 共repumperin Fig. 1兲. The first step for the 7S1/2→ 8S1/2excitationcomes from a Coherent 899-01 Ti:Sapphire at 817 nm, itresonantly populates the 7P1/2, F=13/2 state 共first step inFig. 1兲. The second step at 1.3␮m originates from an EOSI2010 diode laser to excite also resonantly the 7P1/2→ 8S1/2transition 共second step in Fig. 1兲.A Burleigh WA-1500 wavemeter monitors the wavelengthof all lasers to about ±0.001 cm−1. We lock the trap, firststep, and repumper lasers with a transfer lock 关15兴, while welock the second step laser with the aid of a Michelson inter-ferometer that is itself locked to the frequency-stabilizedHeNe laser used in the transfer lock.The MOT consists of three pairs of retroreflected beams,each with 15 mW/cm2intensity, 3 cm diameter 共1/e inten-sity兲, and red detuned 31 MHz from the atomic resonance. Apair of coils generates a magnetic field gradient of 9 G/cm.We work with traps of ⬇104atoms, a temperature lower than300␮K, with a diameter of 0.5 mm and a typical lifetimebetween 5 and 10 s.Figure 2 displays the timing sequence for the excitationand decay cycle for the measurement. Both lasers of the twostep excitation are on for 50 ns before they are switched off,while the counting electronics are sensitive for 500 ns torecord the excitation and decay signal. The trap laser turnsoff 500 ns before the two-photon excitation. We repeat thecycle at 100 kHz.We turn the trap light on and off with an