16
W
E HAVE MADE PASSING MENTION of the effects of magnetic fields
on atomic or molecular energy levels. For example, we have stated
several times that magnetic fields remove the degeneracies of the various m
levels within a single value. However, we have not really considered the idea
of magnetic spectroscopy. We have alluded to the fact that magnetic fields can
change the energies of electrons having different mquantum numbers. You
may also be aware of the medical imaging technique called magnetic resonance
imaging (MRI), which is an important application of magnetic spectroscopy.
The very term “magnetic spectroscopy” is a bit of a misnomer. Although
electricity and magnetism are elements of the same phenomenon (hence the
word electromagnetism), we typically experience electricity as an ability to do
electrical work and magnetism as a sort of static field. Perhaps in an experien-
tial sense, this is accurate. But magnetic fields do affect quantum states at the
atomic and molecular level, and so have an effect in spectroscopy. In magnetic
spectroscopy,a magnetic field is imposed on a sample while it is probed with
electromagnetic radiation. Although the presence of a magnetic field might not
seem to make a lot of difference, we can get a lot more information about the
quantum states of an atom or molecule when we use a magnetic field along
with light. In this chapter, we will consider a few examples of magnetic spec-
troscopy.16.1 Synopsis
Initially, we will look at magnetic fields and magnetic dipoles and see how they
interact. Even though most of the magnetic spectroscopies we will discuss are
based on quantum mechanics, an understanding of magnetic fields and dipoles
from classical mechanics will be useful. When we get to magnetic spectro-
scopies, we will first consider the Zeeman effect: a splitting of the electronic
energy levels of an atomic system by a magnetic field. Observation of the
Zeeman effect went almost hand in hand with the development of quantum
mechanics and provided some useful and necessary experimental evidence for
the existence of the various quantum numbers. The Zeeman effect is a rela-
tively straightforward and useful type of magnetic spectroscopy. More involved
and more intricate are the various types of magnetic resonance (or MR) spec-560
16.1 Synopsis
16.2 Magnetic Fields, Magnetic
Dipoles, and Electric Charges
16.3 Zeeman Spectroscopy
16.4 Electron Spin Resonance
16.5 Nuclear Magnetic Resonance
16.6 Summary