Page 151 - Basic physical chemistry for the atmospheric sciences

7

Photochemistry

A molecule may absorb electromagnetic (em) radiation and, in the
s
process, break down into its atomic or molecular component . Unsta­
ble atoms and molecular fragments may also combine to form more
s
stable molecule , disposing of their excess energy in the form of em
radiatio . These chemical reactions are called photochemical, and the
n
process by which a photochemical reaction occurs is called photolysis.
Photochemical reactions play very important roles in many aspects of
environmental chemistry. Therefore, this book concludes with a brief
account of some of the basic principles of photochemistry, which we
will then apply to ozone in the Earth' s stratosphere and the problem
of the stratospheric ozone hole.

7 1 Some properties of electromagnetic waves
.
Electromagnetic radiation has both wave and particle characteristics.
Considered as a wave, em radiation may be viewed as an ensemble of
waves that travels through a vacuum with the speed of light,
2
c = . 99 8 x 1 0 8 m s - 1 . The distance between two successive crests in
the intensity of the radiation is called the wavelength (A) of the radia­
tion. The f r equency ( v ) of the radiation is the number of crests in

intensity that pass a given point in one second. The units of v are s - 1
or hertz (Hz). If v crests in intensity pass a given point in one second,
one crest passes a given point in llv seconds. Therefore, a crest in
intensity of an em wave travels a distance A in 1/v seconds. Hence,
since speed = distance + time,

c = v A (7. 1 )
.
The em spectrum s shown in Figure 7 1 . A s can be seen, a portion
i
of the spectrum constitutes visible light. Within this portion, different
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