Do a Google search on "electron paramegnetic resosonance", and you'll
get numerous hits that have basic to advanced explanations of this
technique. EPR is also known as electron spin resonance.
Briefly, electrons have a quantum property known as "spin", which
gives rise to a magnetic moment for the electron. (You can crudely
think of each electron behaving like a bar magnet.) Like other
properties of quantum systems, an electron's spin can only take on
certain discrete values, in this case, +1/2 or -1/2. Using the
analogy to a bar magnet, these correspond to having the north pole of
the magnet pointed in one direction, or in exactly the opposite
direction.
In the absence of another magnetic field, there is no difference in
energy between these two different orientations, but if one imposes an
external magnetic field, then one spin orientation will have lower
energy (corresponding to the case in which the N pole of a bar magnet
is pointed at the S pole of the magnet producing the external field --
in this case the poles *attract* one another), and the other
orientation will have higher energy (corresponding to the case in
which the N pole of a bar magnet is pointed at the N pole of the
external magnet -- in this case the poles *repel* one another). The
stronger the external magnetic field, the larger the difference in
energy between the two possible "states", or orientation. In science
jargon, the differentiation between the two spin states of an electron
cause by the external magnetic field is called "splitting the energy
degeneracy" of the states.
Left to itself, an electron in a magnetic field will orient itself so
that it is in the lower energy state. If, however, we "pump" energy
into the system, then it is possible to "excite" the electron to the
higher energy state. Specifically, an electron can interact with
("absorb") a photon that has the same energy as the energy difference
between the two spin states, and thus be "flipped" to the higher
energy state. It turns out that for reasonable external magnetic
field strengths, the difference in energy between the two electron
spin states corresponds to the energy of photons in the microwave
regions of the electromagnetic spectrum. EPR measures the absorption
of such microwave radiation, and thus "observes" the transition from
one spin state of an electron to another.
EPR only works on atoms or molecules that have what are called
"unpaired" electrons. If all the electronic orbitals in an atom or
molecule contain two electrons (the most that can be put into any
given orbital), then the electrons in those pairs *must* have spins
that are oriented opposite to one another (due to a law known as the
Pauli Exclusion Principle). If a such a system of paried spins is put
into a magnetic field, the energies of the two spin orientations are
"split", just as before, but now there are already electrons in both
the low-energy and high-energy state of each orbital. The electron in
the low-energy state can't absorb a photon and be excited to the
higher energy state because that state is already occupied, and there
is "no room" for another electron. If, however, an atom or molecule
has an electron orbital that contains a single, unpaired electron,
then the higher energy state of the orbital will be unoccupied, and
the low-energy electron can absorb a photon and be excited to that
state. |
