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Photochemistry I I'>

where h i s the Planck constant (6.6262 x 1 0 - 3 4 J s). From Eqs . ( 7 . 1 )
and (7. 2 )
he
W = - = h c il (7. 3 )
A
Thus, the amount of energy in a photon of em radiation is inversely
proportional to the wavelength and directly proportional to the wave
number of the radiation.
I n photochemistry we are often interested in an Avogadro' s number
(NA 6 .022 x 1 0 2 3) of photons , which could be considered as "l mole
=
of photons"; this is called an einstein. One einstein of photons has
energy

(7.4)

7 .2 Some photochemical terminology and principles
An important principle of photochemistry is that each photon ab­
sorbed by a molecule activates that molecule in the initial (or absorp­
tion) step of a photochemical proces . For example, the absorption of
s
a photon by a general molecule, indicated by XY, can be represented
by
XY + h v � XY* (7. 5 )

where h v i s used to indicate one photon [hv s actually the energy of
i
*
one photon - see Eq. (7.2)] and XY indicates that the molecule XY
has been raised to an electronically excited state. If one mole of XY is
considered in Reaction (7 .5), that is NA molecules of XY , then I
einstein of photons (i.e. , NA photons) would be involved.
Exercise 7.2 . In sunlight, N02 in the air undergoes dissociation

NOz(g) + h v � N o;(g) � N O(g) + O(g) (7. 6 )
, what
If the energy of dissociation for this reaction is 304 kJ mole - 1
is the minimum wave number of the EM radiation that can cause
this dissociation?
Solution. If we consider the dissociation of 1 mole of N02 , we will
need to have 1 einstein of photon , which will need to have an energy
s
of at least 304 kJ to produce the dissociation. Therefore, from Eq. (7.4)

N Ah v ?:. 304 x 1 0 3