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The Hanle effect offers something seldom seen in an undergraduate laboratory, a direct measurement of the lifetime of an excited atomic state. The principle is simple. Excite an atom using polarized light, and when the atom decays, it will emit radiation in a dipole pattern, with the z-axis parallel to the direction of the electric field (the plane of polarization) of the incoming photon. While in its excited state the atom has a magnetic dipole moment oriented along the same z-axis. A photodetector placed along that axis will receive no light from the decay of the atom, unless we rotate the atom while it is still excited, which we can do with a magnetic field. By applying such a field and varying its magnitude, we can essentially directly measure the lifetime of the excited state.
Participants in this immersion will measure the lifetime of Mercury atoms in the 3P1 state, and they will cool the mercury vapor and observe how this affects the outcome of the experiment. This lab provides an excellent introduction to some very useful techniques and ideas of modern atomic physics.
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