Physical Chemistry , 1st ed.

of photons, all having the same wavelength (and phase, as it turns out), occurs.

Because all of these photons have the same wavelength, the collection of pho-

tons is called monochromatic(“same color”). (The individual photons usually

have the same phase and specific polarization properties, too, but we will omit

discussion of these properties. They are, however, important for various appli-

cations of lasers.) This process is shown in Figure 15.29. It is called “light am-

plification by stimulated emission of radiation,” which produced the acronym

LASER and later entered the language as laser.The very first laser was built by

the American physicist Theodore Maiman in 1960 using a ruby rod, but the

concept of stimulated emission was first demonstrated by the Columbia

University physicist Charles Townes in 1953. Using ammonia, he and his stu-

dents constructed a device that amplified microwave radiation using a similar

stimulated-emission principle (“microwave amplification ...,”leading to the

term “maser”). Realizing a similar process for visible light, Townes and Arthur

Schawlow published such ideas in 1958, and in 1964 Townes shared a Nobel

Prize with the Soviet scientists Alexander Prokhorov and Nikolai Basov, who

developed the theory of lasers independently.

A simplified diagram of a laser is shown in Figure 15.30. Although the pop-

ulation inversion is the key to producing laser action (“lasing”), the engineer-

ing of the laser is also crucial. In most cases, the active material is tubular with

each of the transverse ends of the material meeting a mirror. These two mir-

rors are important because they make the photons travel back and forth

through the laser medium, thereby increasing the chances that they will induce

stimulated emission. Even if one of the mirrors reflects 100% of the photons

and the other reflects only 95–99% of the light (the leftover 1–5% is transmit-

ted), the transmitted light makes a monochromatic beam of very high inten-

sity. This emitted laser beamis a rich source of photons of a particular energy.

A more complete discussion of lasers is beyond our scope. On the other

hand, it is worth discussing a few points using specific lasers as examples. The

very first laser was built around a ruby rod (Figure 15.31). Ruby is crystalline

sapphire, aluminum oxide, Al 2 O 3 , that has been doped with a few hundredths

of a percent of Cr^3 ions. A partial electronic energy diagram of Cr^3 is given

in Figure 15.32. The ground state of Cr^3 is^4 A 2. Pulses of visible light are used

to excite electrons into either an excited^4 T 2 or^4 T 1 state. (These term symbols

are actually irreducible representations combined with multiplicities.) Within

10  7 s, there is a nonradiative transitioninto an E electronic excited state. Since

this transition occurs very quickly, a population inversion is established in

554 CHAPTER 15 Introduction to Electronic Spectroscopy and Structure

Laser medium

Many monochromatic,

in-phase photons:

a laser beam

Single

photon

Figure 15.29 When a population inversion is
established, stimulated emissions can build until
the number of photons is extremely high, yield-
ing a very bright light. This is called light ampli-
fication by stimulated emission of radiation, or
laser.

Laser beam

Excitation source

(electric discharge,

flash lamp, other)

Glass tube filled with

gaseous laser medium, or

solid rod of laser medium

100%

reflecting

mirror

95– 99%

reflecting

mirror

Power

supply

Figure 15.30 Simple diagram of a laser. Photons bounce back and forth inside the lasing

medium, reflected off mirrors on each end, stimulating the production of photons of the same

wavelength and phase. One of the mirrors lets some of the photons out.