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Atomic Spectra 261

When only a single electron is involved, L0 is prohibited and Ll    1 is

the only possibility. Furthermore, Jmust change when the initial state has J0, so

that J 0 →J0 is prohibited.

The helium energy-level diagram is shown in Fig. 7.24. The various levels repre-

sent configurations in which one electron is in its ground state and the other is in an

excited state. Because the angular momenta of the two electrons are coupled, the levels

are characteristic of the entire atom. Three differences between this diagram and the

corresponding ones for hydrogen and sodium are conspicuous:

1 There is a division into singlet and triplet states. These are, respectively, states in

which the spins of the two electrons are antiparallel (to give S0) and parallel (to

give S1). Because of the selection rule S0, no allowed transitions can occur

between singlet states and triplet states, and the helium spectrum arises from transi-

tions in one set or the other.

Figure 7.23Energy-level diagram for sodium. The energy levels of hydrogen are included for

comparison.

Excitation

energy, eV

n = 2

4 s

3 p

3 s

7 p

6 p

5 p

4 p

3 d

6 d

5 d

4 d

n = 3

n = 6

n = 5

n = 4

6 f

5 f

4 f

1

0

2

3

4

5.13

7 s

6 s

5 s

n = ∞

SP D FHydrogen

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