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Electron Spin Resonance Experiment
Setup
Aim
This is a model experiment for electron spin resonance, for clear demonstration of
interaction between the magnetic moment of the electron spin with a superimposed
direct or alternating magnetic field.
Introduction
Electrons have intrinsic spin angular momentum that is independent its orbital
angular momentum. In a magnetic field, the magnetic moment associated with the
spin precess with a known frequency, the Larmor frequency.
Full setup of experiment
If a photon or electromagnetic radiation of the same frequency is applied to the
electrons in the magnetic field, the spin of the electron will either flip. The flip is
either from a lower energy spin to a higher energy spin or vice versa (stimulated
transition).
Electrical connections of experiment
Procedures
Observations and parallel to electron spin
When the direct magnetic field Bo is applied, the magnetic axis of the balll
precess. Since the precession frequency is proportional to the applied field, one
may want to change the voltage from the power supply to observe this effect.
Ball with central
rod magnet
(red is north pole)
1.
2.
3.
4.
Main switch to control
direct magnetic field
Commutator switch
control alternating
magnetic field
Once the equipment is setup properly, start the air blower which will cause the
ball to spin.
When the spin is steady, a constant magnetic field is switched on and the ball
will precess. The period of precession is determined by the strength of the
field.
Choose two reference points on the precession cycle that are on opposite
sides, flip the commutator switch from one side to another when the north
pole of the magnet reaches these reference points.
If your timing is good, the pole of the magnet will flip
A photon of frequency matching the energy difference of an electron in a uniform
field causes its spin to flip. This experiment provides learners the opportunity to
form a mental picture of quantum effect.
Teacher guide
It may be useful to help students to recall on the concept of resonance before the
experiment/demonstration.
To equip students about gyroscopic precession (if it is not covered before) teachers
may want to use the bicycle wheel on turntable demonstration.
Electron spin has no classical equivalent. To provide a classical analog to spin
resonance mental model, a spinning ball with a central magnet is required. It is
good to remind students that although there are similarities with electron spin in
this demonstration, electron spin is actually an intrinsic property.