| The availability of francium has made possible a
number of measurements of its atomic structure and
properties. The big advantage of the trap is that the
atoms are basically at rest and can be interrogated many
times with lasers or other forms of radiation before they
leave the trap or decay into some other element. The energy level structure of francium is partially known. Several of the low lying levels remain undetected.The group at Stony Brook has found two energy levels. The search is guided by theory and semi-empirical calculations. Groups at Notre Dame University and New South Wales University in Australia have predicted the energy of atomic levels by solving ab-initio the electronic structure. The accuracy of those calculations should be better than one part in a thousand. On the other hand by using the energy levels already known quantum defect calculations permit predictions to within one part in ten thousand. |
 Energy Levels of the Francium Atom |
| Because in the trap, some of the trapped atoms spend time in the excited state, a second laser can be used to excite them to higher levels. When the wavelength of the second laser is at about 851 nm the 9S state can be excited. The 9S level has many paths to decay down and two of them produce characteristic blue photons(see figure above). Interference filters block background light allowing the detection of blue light as the resonant signature of the transition. Simultaneous with the transition to the 9S state there is a decrease in the amount of fluorescence from the captured atoms at the trapping transition that serves to confirm the resonance. We measured the wavenumber of the transition between the 7s and 9s states as 25671.021 cm-1 with an accuracy of 0.006 cm-1. |  |
 Click here to see an mpeg movie (909kB) of the trap as the laser scans. |
In February of 1997, we started to study the first excited state in the S family of francium. This state is particularly interesting for our long term goal of parity non-conservation. For the 8S state we needed the uncommon laser with wavelength of 1.7µm. We used an InGaAs-InP diode laser from EOSI in Boulder Colorado. Again we looked for a distinctive photon produced in the decay down at 817 nm. We recorded the change in the trap fluorescence with a CCD camera and you can look at images of the trap that we acquired. The film presents the trap as the 1.7 µm laser scans 300 MHz as marked in the resonance figure. |
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The lifetime of an excited state decaying into a ground state is directly related to the quantum mechanical matrix element between the two states. It is a very sensitive probe of our understanding of the wavefunctions of the two states. We have measured the lifetime of the 7p 2P3/2 state in francium, the D2 line. Since we keep the atoms in a trap we use a time-correlated photon counting technique. We turn off the trap laser very fast compared to the time scale involved and observe the decaying exponential of the fluorescence. We repeat the cycle many times until we build up enough statistics to fit a curve to the data. |
| Above: Decay curve of the P3/2 state of Francium. Right: Comparison of measured P3/2 lifetime with theories. |
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