The Use of Molecular Beam Spectroscopy to Measure Hyperfine Interactions Sara Fortman, John Nichol, Jimmy Randolph and James Cederberg St. Olaf College June 22, 2006

What we do We use an electric resonance molecular beam spectrometer to study pure hyperfine transitions in diatomic molecules. These transitions are found in the tens of MHz range.

Energy Levels

The Molbeam The molecules travel from the source on the right to the detector on the left.

Source Our source of molecules is a hollow cylinder filled with an alkali-halide salt. We use an effusive oven to vaporize our molecules and send a beam through our spectrometer.

Quadrupole Lens Our quadrupole lenses are made up of four 43 cm electrically charged rods arranged in a diamond. By setting the voltage across the lens,we can focus certain m j states. Molecules in a non-selected state will not be focused by the lens.

Transition Region The transition region contains two long parallel plates. We apply an oscillating RF electric field and an DC field to the plates. We look for second order transitions induced by these fields.

Quadrupole Lens The second quadrupole lens will refocus molecules so that they hit the detector. Molecules that have undergone a transition with a change in the m j state, will not be refocused and will miss the detector.

Detecting Transitions Source Transition Region Detector

For our detector we use a thin strip of tantalum foil to ionize the molecules. A nearby metal blade attracts the positive ions. We can measure the current created by the positive ions.

Raw Data We subtract the RF on value from the RF off value to create a plot like shown above. We can fit these peaks to a Rabi lineshape.

Fitting Data To fit these lines, we use a sum of Rabi lineshapes that take into account the following:  Stark splitting  DC field  RF field  Quadrupole lens voltage

Effect of DC field

Effect of RF field

Effect of Quadrupole Voltage

Fitted Data

Final Calculations After we have observed many transitions and calculated their corresponding frequencies, we imput the data into a program we call spectfit. We then use a singular value decomposition routine to fit for hyperfine coupling constants.

Acknowledgements I would like to thank the St. Olaf College Physics Department, the Whittier endowment, and the American Physical Society for their resources and funding. My colleagues John and Jimmy and my advisor Dr. James Cederberg.