Cvor Dhsymmetry, for the present those two point groups will be impor-
tant. There are also some similarities in electronic spectra and rotational spec-
tra for diatomic molecules, so a review of rotational motions might be useful.
As for atoms, term symbols can be defined for diatomic molecules. The
term symbols for diatomic molecules are determined similarly to the Kquan-
tum number for diatomic rotations: the term symbol is determined by the
total orbital angular momentum of electrons about the internuclear axis.
Figure 15.8 shows how this is determined.
The term symbols look very similar to those for atoms, but the quantum
numbers involved are given different labels. Instead of^2 S^1 LJ, the term symbol
for diatomic molecules is2 S (^1)
(15.16)
where ,, and Sare as illustrated in Figure 15.9. The quantized total angu-
lar momentum vector Jis the combination of the orbital angular momentum
of the electrons L(note that electrons paired in molecular orbitals contribute
no overallangular momentum to the total), the spin orbital momentum of the
electrons S(again, paired electrons contribute no overall spin angular mo-
mentum), and the rotational angular momentum of the molecule itself,R.
With the exception ofR, which is perpendicular to the axis of the molecule
anyway, all of these angular momenta have components that lie along the mol-
ecular axis. The axial component ofLis , the axial component ofSis , and
the axial component ofJis (see Figure 15.9). These components are quan-
tized, having integer or half-integer (for some cases ofS) values of, like all
angular momenta. It is the values ofand , along with S, that make up the
term symbol. But just as with Lfor multielectronic atoms, instead of using the
numerical value ofin the term symbol, a letter is used. Unlike atomic term
symbols, diatomic term symbols use capital Greek letters:
Letter designation
0
1
2
3
..
..
..
Care should be taken to not confuse , the quantum number for the axial
spin component, with as a term symbol. As with the quantum number J, the
15.6 Electronic Spectra of Diatomic Molecules 535
Molecular
axis
Total angularmomentum
Figure 15.8 Electronic spectra of diatomic
molecules are described on the basis of the com-
ponent of electron angular momentum about the
molecular axis, as shown.
Molecular
axis
Total angular
momentum
J R
S
L
Figure 15.9 Electronic spectra of diatomic
molecules are more specifically defined in terms
of,, and S.is defined in terms of the or-
bital angular momentum of the electrons,L.is
defined in terms of the total angular momentum,
J. The vector difference between Land Jis, of
course,S.