Radio Frequency Spectroscopy with a Fast Atomic Beam
Measurements of the fine structure of the excited states of simple atoms give important tests of basic theory and important sources for determination of the fundamental constants.1,2 The most familiar such system is the n=2 state of atomic hydrogen — the 2S1/2 → 2P1/2 Lamb shift interval provides a fundamental test of quantum electrodynamics, the 2P3/2 → 2P1/2 fine structure interval gives a theoretically unambiguous source for the determination of the fine structure constant. The most precise measurements of these intervals have come from atomic beam experiments which depend critically on the metastability of the 22S1/2 state. In the atomic beam technique developed by Lamb and coworkers in their pioneering measurements of the hydrogen fine structure, hydrogen atoms produced by thermal dissociation in a hot oven are excited by a crossed electron beam into the metastable 2S1/2 state. They then pass through a microwave cavity located in a magnetic field and are detected through the ejection of electrons from a wire. When the magnetic field is such that the radiofrequency field produces transitions to the 2P1/2 or 2P3/2 states, number of metastable atoms striking the wire decreases.
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