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Lifetime measurement of the 8s level in francium



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PHYSICAL REVIEW A 71 062504 2005 Lifetime measurement of the 8s level in francium E Gomez 1 L A Orozco 2 A Perez Galvan 2 and G D Sprouse1 1 Department of Physics and Astronomy SUNY Stony Brook Stony Brook New York 11794 3800 USA 2 Department of Physics University of Maryland College Park Maryland 20742 4111 USA Received 22 February 2005 published 22 June 2005 We measure the lifetime of the 8s level of 210Fr atoms on a magneto optically trapped sample with timecorrelated single photon counting The 7P1 2 state serves as the resonant intermediate level for two step excitation of the 8s level completed with a 1 3 m laser Analysis of the fluorescence decay through the 7P3 2 level gives 53 30 0 44 ns for the 8s level lifetime DOI 10 1103 PhysRevA 71 062504 PACS number s 32 70 Cs 32 10 Dk 32 80 Pj We present in this paper a measurement of the 8s level lifetime of francium the heaviest of alkali metal atoms Fr is yet to be used in parity nonconservation PNC measurements 1 but work toward that goal requires understanding of the excited state properties of the atom The 8s state is the preferred candidate for an optical PNC measurement the dipole forbidden excitation between the 7S1 2 ground state and the first excited 8S1 2 state becomes allowed through the weak interaction The equivalent state in Cs 7s has been used in PNC experiments by the Boulder 2 3 and Paris 4 groups and a quantitative understanding of this state its lifetime and its branching ratio is critical to the successful extraction of weak interaction physics in these experiments Our measurement is a test of the modern techniques of ab initio calculations using many body perturbation theory MBPT 5 6 Quantitative measurements on Fr and comparison with theoretical calculations validate the same MBPT techniques used for Cs and other atoms with a more relativistic atom where correlations from the 87 electrons are large The lifetime of an excited state is determined by its individual decay rates 1 i through the matrix element associated with the i partial decay rate The connections between lifetime partial decay rates and matrix elements are 1 1 i i 1 4 3 J r J 2 i 3 c2 2J 1 1 2 where is the transition energy divided by c is the speed of light is the fine structure constant J and J are respectively the initial and final state angular momenta and J r J is the reduced matrix element 7 Equation 2 links the lifetime of an excited state to the electronic wave functions of the atom Comparisons of measurements with theoretical predictions test the quality of the computed wave functions especially at large distances from the nucleus due to the presence of the radial operator The lifetimes of the low lying states in Fr are reaching a level of precision comparable to that of the other alkali metals 7 9 The atomic theory calculations for these transitions 10 12 predict lifetimes measured with impressive agree1050 2947 2005 71 6 062504 4 23 00 ment strengthening the possibility of a PNC experiment in a chain of francium isotopes We use the method of time correlated single photon counting to obtain the lifetime of the 8s level in Fr in a magneto optical trap MOT We populate the 8s level with a two step excitation and then we turn off the excitation suddenly and observe the exponential decay through the fluorescence photons 13 The production cooling and trapping of Fr online with the superconducting linear accelerator at Stony Brook has been described previously 14 Briefly a 100 MeV beam of 18O ions from the accelerator impinges on a gold target to make 210Fr radioactive half life 3 min We extract 1 106 francium ions s out of the gold and transport them 15 m to a cold yttrium neutralizer where we accumulate the Fr atoms We then close the trap with the neutralizer and heat it for one second 1000 K to release the atoms into the dry film coated glass cell where they are cooled and trapped in a MOT The cycle of accumulating and trapping repeats every 20 s Figure 1 shows the states of 210Fr relevant for trapping and lifetime measurements The nuclear spin of this isotope FIG 1 Energy levels of 210Fr The figure shows the trapping and repumping transitions thin solid lines the two step excitation thick solid lines the fluorescence detection used in the lifetime measurement dashed line and the undetected fluorescence dotted line 062504 1 2005 The American Physical Society PHYSICAL REVIEW A 71 062504 2005 GOMEZ et al FIG 2 Timing diagram for the 8s level excitation and decay cycle 100 kHz is I 6 with a ground state hyperfine splitting of 46 768 GHz A Coherent 899 21 titanium sapphire Ti sapphire laser operating at 718 nm excites the trapping and cooling transition 7S1 2 F 13 2 7P3 2 F 15 2 trap in Fig 1 A Coherent 899 21 Ti sapphire laser operating at 817 nm repumps any atoms that leak out of the cooling cycle via the 7S1 2 F 11 2 7P1 2 F 13 2 transition repumper in Fig 1 The first step for the 7S1 2 8S1 2 excitation comes from a Coherent 899 01 Ti Sapphire at 817 nm it resonantly populates the 7P1 2 F 13 2 state first step in Fig 1 The second step at 1 3 m originates from an EOSI 2010 diode laser to excite also resonantly the 7P1 2 8S1 2 transition second step in Fig 1 A Burleigh WA 1500 wavemeter monitors the wavelength of all lasers to about 0 001 cm 1 We lock the trap first step and repumper lasers with a transfer lock 15 while we lock the second step laser with the aid of a Michelson interferometer that is itself locked to the frequency stabilized HeNe laser used in the transfer lock The MOT consists of three pairs of retroreflected beams each with 15 mW cm2 intensity 3 cm diameter 1 e intensity and red detuned 31 MHz from the atomic resonance A pair of coils generates a magnetic field gradient of 9 G cm We work with traps of 104 atoms a temperature lower than 300 K with a diameter of 0 5 mm and a typical lifetime between 5 and 10 s Figure 2 displays the timing sequence for the excitation and decay cycle for the measurement Both lasers of the two step excitation are on for 50 ns before they are switched off while the counting electronics are sensitive for 500 ns to record the excitation and decay signal The trap laser turns off 500 ns before the two photon excitation We repeat the cycle at 100 kHz We turn the trap light on and off with an electro optic modulator EOM Gs nger LM0202 and an acousto optic modulator AOM Crystal Technology 3200 144 The combination of the two gives an extinction ratio of better than 1600 1 after 500 ns AOM s Crystal


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