Electron Spin Resonance

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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 proven correct, and 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 students measure the value of a single electron's magnetic moment. The parametric organic compound DPPH (diphrenyl-picri-hydrazyl) has an unbound electron that orients 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 students set up a weak high frequency field at right angles to the primary field, the weak field will interact with the electron when their frequencies are the same. A pair of Helmholtz coils, powered by an ac current, set up the polarizing magnetic field. A probe containing the DPPH is inserted into the field. A tiny coil, which provides the inductance for the oscillator, surrounds the crystals and supplies the transverse, high-frequency signal. The coil mounts in a sturdy metal probe with an acrylic end cap. Students can see the coil, but it is well-protected. As the magnetic field varies from positive to negative and back, the electron's frequency matches the oscillator's frequency at each particular field value. The oscillator detects the energy transfer and displays it on the oscilloscope screen.

Measures the electron's precession frequency in a magnetic field.
Intrinsic angular momentum of the electron.
Variation of frequency with field strength.

Electron Spin Resonance Apparatus includes:

Power supply, Two Helmholtz coils, Probe and electronics, complete instruction manual

Required Accessory:

An oscilloscope, which is not included.