Physical Chemistry , 1st ed.

15.7 show two Grotrian diagrams, illustrating the allowed transitions. As might

be expected, atoms that have more unpaired valence shell electrons have a

more complicated Grotrian diagram.

The spectra of atoms can be measured by generating the atoms, for exam-

ple by vaporizing them into the gas phase. This can be difficult sometimes, es-

pecially for a substance like tungsten, which has a normal boiling point of

5660°C. However, many atoms and ions exist in the solid state as either metal

complexes or as ionic crystals. Although the imposition of other chemical

species around the atom or ion affects the spectrum (this is known as crystal

field theory), the previous discussion provides the basis for understanding the

electronic transitions of atoms and ions in compounds.

15.6 Electronic Spectra of Diatomic Molecules

The electronic spectra of molecules, even the smallest diatomic molecules, are

more complicated than those of atoms because more than one nucleus is

present. However, now we can take advantage of molecular symmetry. Just as

with vibrational spectroscopy, electronic spectroscopy of molecules uses group-

theoretical ideas for simplification. Since all diatomic molecules have either

534 CHAPTER 15 Introduction to Electronic Spectroscopy and Structure

5,000

(^2) S1/2 (^2) F7/ 2, 5/2
45,000
cm^1 n
7
8
6
5
4
n
6
5
4
n
6
7 7
5
4
8 7 6 5 4 3
8
7
6
5
nn
4
3
n
5
6
4
(^2) P3/2 (^2) P1/2 (^2) D5/2 (^2) D3/2
10,000
15,000
20,000
25,000
30,000
35,000
40,000
Figure 15.7 A partial Grotrian diagram for aluminum atoms in the gas phase. The thickness
of the lines indicates the relative intensity of the electronic transition.Source:G. Herzberg,Atomic
Spectra and Atomic Structures,trans. J. W. T. Spinks, Dover, New York, 1944. Reprinted with per-
mission from the publisher.