EN-35 Electron Spin Resonance
In 1925 Uhlenbeck and Goudsmit showed that small discrepancies between Bohr's quantum description of the hydrogen atom and experimental values could be explained if the electron behaved as if it had angular momentum and a magnetic moment. They were proved to be correct, and an intrinsic angular momentum is now one of the fundamental properties of a particle.
This elegant apparatus, designed by the Physics Department of Dartmouth College, lets the student measure the value of the magnetic moment of a single electron. The paramagnetic organic compound DPPH (diphenyl-picri-hydrazyl) has an unbound electron that can be oriented in an external magnetic field. The electron spins like a top in the polarizing field with a frequency proportional to the magnitude of the field. If the polarizing magnetic field is swept through a range, the electron's precession varies in frequency with the field change. If a weak high frequency field is set up at right angles to the primary field, it will interact with the electron when their frequencies are the same.
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The polarizing magnetic field is set up by a pair of Helmholtz coils powered by an ac current. The probe containing the DPPH is inserted into the field. A tiny coil, which provides the inductance for the oscillator, surrounds the crystals and provides the transverse high frequency signal. The coil is mounted in a rugged metal probe with an acrylic end cap so that the coil can be easily seen but is well protected. As the magnetic field varies from positive to negative and back, the electron's frequency matches the oscillator's frequency at particular field values. The oscillator detects the energy transfer and displays it on the oscilloscope screen. |
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