Physical Chemistry , 1st ed.

short order and a pulse of lasing action occurs at a wavelength of 694.3 nm,
which is in the red region of the spectrum. Typically, ruby crystals are a few
millimeters wide and several centimeters long, and are still used today in some
lasers. Because three individual electronic states contribute to the laser action,
the ruby laser is an example of a three-level laser system.If the electronic exci-
tation is caused by a pulse of light, the resultant laser action is also a pulse of
laser light. This is an example of a pulsed laser.It can also be operated contin-
uously using constant illumination by an excitation source. In such a configu-
ration, the ruby laser is an example of a continuous-wave(or cw) laser.
Direct electrical excitation using an electrical discharge is used to excite
atoms in the helium-neon or He-Ne (actually pronounced “hee-nee”) laser. An

15.12 Lasers 555

Figure 15.31 A ruby laser in operation.

25,000

0

3600 Å

(violet)

6943 Å

5100 Å

(green)

Energy (cm

^1

)

0.4 cm^1

14,000

(^4) T 1
(^4) T 2
(^4) A 2
(^2) E 2 A
E
6934 Å
29 cm^1
Figure 15.32 Energy levels of the Cr^3 ion involved in the ruby laser, the first laser to be de-
veloped. The ions are excited by green or violet light, a population inversion is established, and red
laser light is produced by stimulated emission.Source:Reprinted with the permission of Simon &
Schuster from The Laser Bookby Clifford Laurence. Copyright © 1986 by Clifford L. Laurence.
© Bettmann/CORBIS