Physical Chemistry , 1st ed.

energy level diagram for this laser is shown in Figure 15.33, with Figure 15.34

showing an example of a He-Ne laser in operation. The He-Ne laser is an ex-

ample of an ion laser,since He^ and Ne^ ions are the important electronic sys-

tems. Figure 15.33 shows that two of the excited electronic states of He^ are

almost coincident with excited electronic states of Ne^ , thereby increasing the

ability to transfer electrons to Ne^ excited states and produce a population in-

version. The He-Ne system is an example of an electronic system that has sev-

eral possible lower-energy states that can participate in the lasing process, pro-

ducing different wavelengths of laser light. How does one select which state

produces the laser beam? For the He-Ne system, one can modify the reflectiv-

ity of the mirrors so that only one wavelength of light is effectively reflected

back and forth through the laser medium, stimulating more photons of the

exact same wavelength. In this way, the transition occurring at 632.8 nm, in the

red part of the spectrum, is preferred and is the dominant color of He-Ne

lasers. (Green He-Ne lasers are also available, which lase at 543 nm.) The other

transitions, at 3.39 m and 1.15 m, occur in the infrared portion of the spec-

trum but are largely unused. The 1.15-m laser beam is historically important

because it was the wavelength of the first laser made from a gaseous laser

medium.

Because four energy levels are involved in the laser action of He-Ne lasers

(consult Figure 15.33), they are referred to as four-level laser systems.He-Ne

lasers are perhaps the most common lasers in use, among other things for sur-

veying and for scanning prices in a grocery store. Although not considered

high-energy lasers, they are a very bright light and can cause eye damage if one

looks into the laser beam.

An electrical discharge using the Ar^ ion also produces laser action at mul-

tiple wavelengths. These lasers are among the more common higher-energy

lasers, producing laser light in the green (514.6 nm) and blue (488.8 nm)

region of the spectrum.Tuningof the laser light frequency is done by chang-

ing the resonance characteristics of the lasing chamber. For example, a prism

can be used to direct one wavelength of light toward a mirror for stimulated

emission, and (because the index of refraction is different for different wave-

lengths of light) to direct other wavelengths in a slightly different direction to

avoid reinforcing them through stimulated emission.

The carbon dioxide laser uses a population inversion based on the rovibra-

tional energy levels of the CO 2 molecule (and so technically is not welcome in

a discussion of electronic energy levels). The energy level diagram is shown in

Figure 15.35. The CO 2 laser is infamous because it does what lasers are ex-

pected to do in science fiction: blast holes in objects. The wavelength of CO 2

lasers is 10.6 m, which is in the infrared portion of the spectrum. Many solid

objects absorb infrared light very well, and since CO 2 laser light is a very po-

tent source of infrared photons, it has the capability of heating up things very

quickly. This is coupled with the25% efficiency of conversion of the excita-

tion energy into light energy, an efficiency almost unmatched by any other

popular laser medium.

15.13 Summary

Transitions among electronic energy levels are among the most crucial processes

that occur in all of chemistry. For atomic systems, they are fairly well under-

stood. At the very least, there is an established convention for labeling the elec-

tronic energy levels and a rather rigorous understanding of the allowed and

556 CHAPTER 15 Introduction to Electronic Spectroscopy and Structure

Electron energy

Pumping

Ground state

Helium Neon

3391 nm

543.5 nm

632.8 nm

1152 nm

1118 nm

Collisions

Figure 15.33 Energy levels of the He-Ne laser,

one of the most prevalent lasers in society (wit-

ness grocery store scanners). Helium atoms are

initially excited, but they transfer energy very ef-

fectively to neon atoms, which participate directly

in the lasing process. Note that several possible

transitions are possible for laser action, each one

having a different color. Most common is the

632.8-Å laser, which is bright red.Source:From

M. J. Beesley,Lasers and Their Applications.

© Taylor & Francis, 1971. Reprinted with permis-

sion from the publisher.

Figure 15.34 A He-Ne laser in operation.

© Richard Magna/Fundamental Photographs