Page 95 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 89

Figure 5.11. Prior to exposure, the photosensitive grain is composed of a silver bromide
single crystal with silver sulfide impurities or sensitive spots introduced during manu-
facture. The bromine atoms are so large that they stay fixed in place, but the silver
atoms are smaller and much more mobile. Professor Shelly Errington of the University
of California at Santa Cruz drew this original illustration.

the large figure on the left-hand side of Figure 5.11. The bromine an-
ion is physically much larger than the silver cation. This size differ-
ence plays an important role in the events to follow. Over on the
right, we see the silver sulfide molecule. This molecule gives the film
its sensitivity to light. This molecule is so sensitive, it is speaking
French! Two new characters are introduced in Figure 5.12, and the
action begins. A photon is absorbed by the silver bromide molecule,
breaking a bond and freeing both a bonding electron and the silver
atom. The molecule is split apart into its atomic components. The
newly liberated electron moves very quickly through the crystal and
is attracted to the silver sulfide site on the right-hand side of the
cartoon. This action gives the silver sulfide site the charge it needs
to attract the silver atom.
The silver atom is attracted to the silver sulfide site, and diffuses
through the silver bromide crystal. The silver atom can move
through the silver bromide crystal because of its smaller size relative
to bromine. Eventually, it reaches the silver sulfide site that has
been activated by the presence of an extra electron. The dénouement
is shown in Figure 5.13. The silver atom, the silver sulfide, and the
electron form a new group on the left-hand side of the cartoon. The
bromine atom becomes a stand-alone figure as shown on the right.
Photons having an energy in the visible wavelength range (~ 2 eV
to 4 eV) can be absorbed by the silver bromide, breaking the sil-

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.