(Click to check out schematic drawings of the beam apparatus.)

Molecular Beam Spectroscopy was brought to the United States by I. I. Rabi. He became interested in molecular beam work while he was in Hamburg visiting with Stern, of the famous Stern-Gerlach Experiment. Rabi was charmed by the simplicity of the experiments, coupled with the power they give one to influence an atom or molecule. Rabi went on to win the Nobel Prize in 1944 for his work on developing the resonance method to record the magnetic properties of nuclei. One of Rabi's star graduate students at Columbia Univerisity, Norman Ramsey, also became a leader in the field of molecular beam spectroscopy. After earning his Ph.D., Ramsey (pictured left) went on to be a professor at Harvard. There, he built the molecular beam apparatus that we use here at St. Olaf. In order to get more precise measurements of nuclear magnetic moments, Ramsey wanted to improve upon Rabi's molecular beam design. This led to his development of the method of successive oscillatory fields in 1949. In fact, he won the 1989 Nobel Prize for inventing this technique and applying it to the development of atomic clocks.

Our project at St. Olaf College makes use of an electric resonance spectrometer built by Ramsey and his students at Harvard around 1970. It was moved to St. Olaf following Ramsey's retirement from teaching in 1981. The project was initially supported by the Research Corporation, and has been funded by the National Science Foundation since 1984.

In the method we use, a beam of molecules emerges from the source, which is heated by an oven. In the first third of the apparatus, a quadrupole lens focuses the beam. The beam then passes through the middle region of the spectrometer, where transitions occur. These transitions in the hyperfine structure are induced by an oscillating electric field which extends over the 2-meter length of the region. The final third of the apparatus contains a second quadrupole lens to focus the beam further. The resonance transitions taking place in the middle region cause the beam to become less focused, thus weakening the detector signal and allowing us to observe the transition.

Molecular Beam Spectroscopy came to St. Olaf in 1981 when Dr. James Cederberg and Dr. David Nitz went to Harvard to pick up the apparatus. Dr. Cederberg had been a graduate student under Ramsey at Harvard, so when Ramsey retired he donated the apparatus to St. Olaf. Currently, Dr. Cederberg and Dr. Duane Olson are overseeing the project at St. Olaf. As of now, the molecular beam group has studied the hyperfine spectrum of KCl, NaBr, KF, LiF, KOH, LiI, CsF, RbBr, RbCl, RbF, KBr, and KI.

During the summer of 1998, Dr. Cederberg and the summer researchers attended the 53rd annual International Symposium on Molecular Spectroscopy at Ohio State University in Columbus, Ohio. The group sat in on talks given by Ph.D.'s and graduate students from around the world and gave one talk as well. Senior Matt Feig reported on the progress which had recently been made in determining the hyperfine constants of rubidium chloride. In the spring of 1999 Matt received word that he had been awarded an NSF Graduate Fellowship. Congratulations Matt!!!
The following year, two other beamers from the class of 1999 also received NSF graduate fellowships: Ann Gabrys in electrical engineering, and Thomas Höft in applied mathematics. Congratulations Ann and Thomas!!!!!!

During the summer of 1999, the four students (Jess Ward, Geoff McAlister, Garrett Hilk, and Erik Beall) decided to tackle a new molecule, CsF. A New Zealand chemist, Peter Schwerdtfeger, and his coworkers had made theoretical calculations of the field gradient at the Cs site, and needed a better measurement of the nuclear quadrupole coupling constant to allow an improved determination of the Cs nuclear electric quadrupole moment. They started the study in late June, and by the end of July had completed it. A manuscript of a paper was submitted to J. Chem. Phys., and was accepted for publication, without revision, on August 13!

On March 28, 2001, Norman Ramsey visited St. Olaf to give a seminar and meet the beamers. Here is a picture of him with Garrett Hilk, Jason Buysman, Trisha Khanna, and Geoff McAlister.

During 2001-2003 the research has concentrated on the rubidium salts RbF, RbCl and RbBr. The spectra are quite dense because of the relatively large rubidium spins (5/2 for the mass 85 isotope, 3/2 for the mass 87), the large number of rotational and vibrational states populated at the source temperature, and the multiple isotopes (2 each for Rb, Cl, and Br). The observable transitions range in frequency from zero up to about 23 MHz. Nevertheless we are able to fit the data well, as shows in the example here, of a set of lines. It shows a theoretical fit of the data in which the Stark components are resolved. The fitting process has determined the three separate root frequencies, the effective amplitude of the oscillating electric field, the background, and made small adjustments in the separate Stark amplitudes and the time spent traversing the oscillating field region. The Stark splittings and the detailed lineshapes are calculated, not fitted.

The Nobel site contains other information on Dr. Ramsey and his Nobel Prize winning work. Check here for the web site on the 1989 Nobel Prize winners.

In 2002 the American Physical Society awarded its annual Prize for Research in an Undergraduate Institution to our project! Click here for a link to the citation.

For a list of publications from the molecular beam project click here.
For a list of students who have participated in the molecular beam project click here.

---

Information on I. I. Rabi and Norman Ramsey from:
Rigden, J.S. Rabi. New York: Basic Books, Inc., 1987.
Information on Method of Successive Fields from:
Ramsey, N.F. "The Method of Successive Oscillatory Fields." Physics Today, 33, 1980, pp. 25 - 30.

Disclaimer