Physical Chemistry , 1st ed.

forbidden transitions. For molecules, there is a convention based on the char-

acter table for the symmetry point group, but wide-ranging generalities are dif-

ficult (except perhaps for conjugated electron systems, as described by the

Hückel theory). Most in-depth discussion of electronic transitions of mole-

cules is best confined to a specific discussion of that molecule instead of a gen-

eral discussion of electronic spectroscopy (unlike rotational and vibrational

spectroscopy, which can be well developed without defining the molecular sys-

tem). Transitions among electronic energy levels produce laser beams (as we

know thanks to Einstein’s analysis of electronic transitions), but laser action is

not limited to electronic energy level transitions. In the visible region of the

spectrum, however, a laser represents a superb modern example of how we

have been able to understand and utilize transitions between the electronic

energy levels of atoms and molecules.

15.13 Summary 557

Energy Pumping

Carbon dioxide Nitrogen

Decay

4

0 0

3

1

2

Decay

9.6 m

10.6 m

Figure 15.35 Energy levels of the CO 2 laser.
Lasing at 10.6 m, which is in the IR region of
the spectrum, means that a CO 2 laser can heat
objects very quickly. CO 2 lasers are actually used
for cutting and welding.Source:From M. J.
Beesley,Lasers and Their Applications.© Taylor
& Francis. Reprinted with permission from the
publisher.