92 TheLS-coupling scheme
Fig. 5.12The normal Zeeman effect
on the 1s2p^1 P 1 –1s3d^1 D 2 line in he-
lium. These levels split into three and
fiveMJstates, respectively. Both lev-
els haveS=0andgJ= 1 so that the
allowed transitions between the states
give the same pattern of three compo-
nents as the classical model (in Sec-
tion 1.8)—this is the historical reason
why it is called the normal Zeeman ef-
fect. Spectroscopists called any other
pattern an anomalous Zeeman effect,
although such patterns have a straight-
forward explanation in quantum me-
chanics and arise whenever S =0,
e.g. all atoms with one valence elec-
tron haveS =1/2. Theπ-andσ-
components arise from ∆MJ=0and
∆MJ =±1 transitions, respectively.
(In this example of the normal Zeeman
effect each component corresponds to
three allowed electric dipole transitions
with thesamefrequency but they are
drawn with a slight horizontal separa-
tion for clarity.)
2
1
0
− 1
− 21
0
− 1FrequencyFig. 5.13The anomalous Zeeman ef-
fect for the 6s6p^3 P 2 –6s7s^3 S 1 transi-
tion in Hg. The lower and upper levels
both have the same number of Zeeman
sub-levels (orMJstates) as the levels in
Fig. 5.12, but give rise to nine separate
components because the levels have dif-
ferent values ofgJ. (The 6s7s config-
uration happens to have higher energy
than 6s6p, as shown in Fig. 5.9, but the
Zeeman pattern does not depend on the
relative energy of the levels.)
1120
− 1
− 20− 1Frequency