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Molecular Beams$
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Norman Ramsey

Print publication date: 1986

Print ISBN-13: 9780198520214

Published to Oxford Scholarship Online: September 2007

DOI: 10.1093/acprof:oso/9780198520214.001.0001

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Molecular Beams

Norman F. Ramsey

Oxford University Press

A brief history of early magnetic resonance methods is given. In 1938, Rabi published the description of the first successful magnetic resonance experiment which he had used with collaborators to measure the nuclear magnetic moments in LiCl. With other collaborators, it became clear that the method provided precision radio-frequency and microwave spectroscopy. The theory of the method and the calculations of transition probabilities are given in detail. Although Rabi's method gives more accurate results than any previous method, the accuracy is limited by the fact that the accuracy increases with the length of the apparatus, but the length can not exceed ½ a wavelength of the oscillatory radiation without getting cancellations from the opposite phases. In 1949, Ramsey pointed out that this problem could be overcome by using two separated oscillatory field regions with each one less than ½ a wavelength long, but with the two regions being many wave lengths apart and with the oscillations in the two regions being coherently driven. This method (often called the Ramsey method) works well and provides resonances that can be hundreds of times narrower than that by Rabi's method. Sometimes the intensity of the resonances are increased by added space focusing or by the use of very slow molecules. Possible distortions of molecular beam resonances and means for preventing them are discussed.

Keywords:   transition probabilities, nuclear magnetic moments, Rabi method, Ramsey method, separated oscillatory fields, space focusing, distortions

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