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Unformatted text preview: The Hanle Effect 1 Introduction The object of this experiment is to measure the intensity (related to the polarization state) of the fluorescent light from a sample cell containing mercury vapor, as a function of the applied magnetic field. From this data, you will determine the lifetime of the 3 P 1 excited state of mercury. [ Please read pages 18 of the Zeeman Effect writeup for additional review of magnetic moments and angular momentum addition. ] Unlike many of the experiments conducted in this modern physics lab, Hanles explanation of his 1924 experiment (that demonstrated the variation of polarization of the resonance fluorescent light in a weak magnetic field) was not well accepted at first. For that matter, Hanle was not even the first to observe this phenomenon. It was actually reported first by Wood in 1912; and in 1922, R.W. Wood and A. Ellett published a paper describing the effect of a magnetic field on the polarization of resonance fluorescence radiation. It turned out that for the 253.7 nm light from Hg absorption cell, magnetic fields of a few gauss were sufficient to depolarize the resonance fluorescence light. Wood and Ellett soon realized that this behavior could not be interpreted as a Zeeman effect since the Zeeman separation in such fields is very small compared to typical Doppler-broadened linewidths of such radiation. Hanles real credit comes for his classical explanation of the effect. However, typical of the criticism of his interpretation is the statement made by the distinguished physicist Max Planck who said to Hanle, your interpretation cannot be correct, it contradicts quantum theory. Of course, modern quantum mechanics was not yet born and most theorists were convinced that his effect was a kind of Faraday effect (see below). 1.1 The Faraday Effect It is known that the Faraday effect yields a rotation of the plane of polarization in a magnetic field when light travels through a dispersive medium. The Faraday rotation has the same sense as the Larmor precession associated with the Zeeman effect in both the right and left wings of the finite-width absorption line, but the rotation sense is reversed for frequencies close to the center of the absorption line. For the Hanle effect, only the central part of the line is active and the observed rotation of the polarization is always in the same sense as the precessional motion. Furthermore, the Faraday effect causes the rotation to be proportional to the length of the path in the medium within the magnetic field. For the Hanle effect, the rotation effect reaches its maximum as soon as the sample experiences the longitudinal magnetic field and there is absolutely no dependence on the length of the path traveled by the exciting light in the magnetic field, nor on the path traveled by the fluorescence light in the magnetic field before detection....
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