Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59

Revised Pages

764 • Chapter 19 / Optical Properties

Electron

excitation,

ΔE = E 4 – E 2

= h 42

Incident photon

of frequency

 42

E 1

E 2

E 3

E 4

E 5

Energy

Figure 19.3 For an isolated atom, a schematic

illustration of photon absorption by the

excitation of an electron from one energy state

to another. The energy of the photon (hv 42 )

must be exactly equal to the difference in

energy between the two states (E 4 – E 2 ).

A second important concept is that a stimulated electron cannot remain in an

excited state excited stateindefinitely; after a short time, it falls or decays back into itsground

state,or unexcited level, with a reemission of electromagnetic radiation. Several

ground state decay paths are possible, and these are discussed later. In any case, there must be a

conservation of energy for absorption and emission electron transitions.

As the ensuing discussions show, the optical characteristics of solid materials

that relate to absorption and emission of electromagnetic radiation are explained

in terms of the electron band structure of the material (possible band structures

were discussed in Section 12.5) and the principles relating to electron transitions, as

outlined in the preceding two paragraphs.

Optical Properties of Metals

Consider the electron energy band schemes for metals as illustrated in Figures 12.4a

and 12.4b; in both cases a high-energy band is only partially filled with electrons.

Metals are opaque because the incident radiation having frequencies within the

visible range excites electrons into unoccupied energy states above the Fermi energy,

as demonstrated in Figure 19.4a; as a consequence, the incident radiation is absorbed,

in accordance with Equation 19.6. Total absorption is within a very thin outer layer,

usually less than 0.1μm; thus only metallic films thinner than 0.1μm are capable of

transmitting visible light.

All frequencies of visible light are absorbed by metals because of the continu-

ously available empty electron states, which permit electron transitions as in Figure

19.4a. In fact, metals are opaque to all electromagnetic radiation on the low end of the

frequency spectrum, from radio waves, through infrared, the visible, and into about

the middle of the ultraviolet radiation. Metals are transparent to high-frequency

(x- andγ-ray) radiation.

Most of the absorbed radiation is reemitted from the surface in the form of visible

light of the same wavelength, which appears as reflected light; an electron transition

accompanying reradiation is shown in Figure 19.4b. The reflectivity for most metals

is between 0.90 and 0.95; some small fraction of the energy from electron decay

processes is dissipated as heat.

Since metals are opaque and highly reflective, the perceived color is determined

by the wavelength distribution of the radiation that is reflected and not absorbed.

A bright silvery appearance when exposed to white light indicates that the metal is