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Zeeman Effect Experiment
Introduction
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012-14266B
Introduction
In 1896, the Dutch physicist Pieter Zeeman discovered that, in a magnetic field, the spectral lines of
atoms split into a number of closely spaced lines. This is called the (anomalous) Zeeman Effect. The
Zeeman Effect was very important in the development of quantum mechanics, and especially the devel-
opment of quantum chemistry. It provides direct evidence that the orbital angular momentum of the
atoms (the magnetic moment of atoms) is quantized. The number of lines that a single spectral line
splits into allows the determination of the total angular momentum of the energy levels involved in the
transition that produced the spectral line.
In this experiment, the student observes the interference pattern from a Fabry-Perot interferometer that
results from the 546.1 nanometer (nm) spectral line of a mercury lamp that is immersed in a uniform
magnetic field. The magnetic field is varied from zero to almost 1 tesla (T). Initially, the light is viewed along an axis perpen-
dicular to the magnetic field axis. A polarizer is used to show the three lines due to light that is polarized parallel to the mag-
netic field axis and to show the six lines that are polarized perpendicular to the magnetic field axis. The interference pattern
may also be observed along the magnetic field axis in the case where the light is circularly polarized. The included CMOS
(complementary metal-oxide semiconductor) camera sends the interference pattern to a computer where the PASCO Capstone
software (included) records and analyzes the pattern. Data from the pattern that is polarized perpendicular to the magnetic field
axis is then used to calculate the Bohr magneton, the intrinsic magnetic dipole moment of an electron in the ground state. (All
atomic magnetic moments are integral or half-integral multiples of the Bohr magneton.)
Background Information
Pieter Zeeman observed the “anomalous Zeeman Effect” in 1896 and shared the Nobel Prize in Physics in 1902 with his
teacher, Hendrik Lorentz, who observed the “normal Zeeman Effect”.
In the Zeeman-Lorentz explanation, the electron moving in a magnetic field experiences a Lorentz force that slightly changes
the orbit of the electron and hence its energy. The change in energy depends on the orientation of the orbit. If the plane of the
orbit is perpendicular to the magnetic field, then the change in energy,
E, is either positive or negative depending upon
whether the motion of the electron is clockwise or counter-clockwise. If the field lies in the plane of the orbit, the net Lorentz
force is zero (averaged over one orbit) and the change in energy is zero. This reasoning thus predicts that the spectral emission
line will split into three lines when a field is applied. More detailed arguments predict the same result for an arbitrary orienta-
tion of the plane of the orbit. A more modern argument, not using forces, can be made by noting that the orbital motion of the
electron (angular momentum L) produces a magnetic moment.
The Zeeman Effect is very important in applications such as nuclear magnetic resonance spectroscopy, electron spin resonance
spectroscopy, magnetic resonance imaging (MRI), and Mössbauer spectroscopy.
Theory
The 546.1 nm green spectral line in mercury is due to a transition of an electron from a
3
S
1
(6s7s) energy level to a
3
P
2
(6s6p)
level as shown (see Figure 1). The (6s7s) notation means that mercury has two valence electrons; one in a 6s orbital and one in
a 7s orbital. The orbital angular momentum quantum number for an S state is L = 0 and for a P state is L = 1. Mercury has two
valance electrons which couple here to give a spin quantum number S = 1 (called triplet states and annotated with the super-
script 3). This results in total angular momentum quantum numbers for the states of J = 1 and J = 2 respectively. The J values
are indicated by the subscripts. In the presence of a magnetic field, each level splits into 2J + 1 closely spaced levels. NOTE: