Electron Spin Resonance Experiment

1
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. 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).
Full setup of experiment
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. 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.
Commutator switch control alternating magnetic
field
Main switch to control direct magnetic field
Ball with central rod magnet (red is north pole)
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.

• 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