Page 154 - Basic physical chemistry for the atmospheric sciences

1 4 0 Basic physical chemistry

or,
304 1 0 3
x
1 0 1 0 -
� 2 3 34
v (6.022 x )(6.6262 x )
� 7 . 6 2 x 1 0 14 Hz
v
l
Therefore, from Eq. (7. )
c
-� 7 . 6 2 x 1 0 1 4
,\
or,
7 . 6 c 1 0 2 . 9 98 x 1 0 8
1
,\ :::; 4 0 . 3 9 x 0 - 6 m or 0 . 3 9 µ,m
2 x 14 7 . 6 2 x 1 0 1
Since wave number = / ,\ , the minimum required wave number of the
1
2
radiation is 2.6 x 1 0 6 m - 1 or . 6 µ,m - 1 . (Dissociation of some N02 may
occur at somewhat smaller wave numbers than this due to internal
energy already present in the N0 molecules . )
2
.
a
o
a
Reaction 7 6 ) provides n example f one pathway that n excited
(
molecule may follow, that is, to dissociate chemically. This pathway
can be indicated in general by
XY* - A + B (dissociation) (7. 7 )
where A and B represent the products formed from the breakup of xy• ,
which may be chemically different from X and Y and which may or may
not be excited . This type of dissociation can occur if the energy of the
absorbed photon exceeds the binding energy of the chemical bond for
XY. The binding energy of chemical bonds are sometimes expressed in
electron-volts (eV), where l eV = . 6 x 1 0 - 19 J . 1 If l mole of a sub­
l
stance is considered, the units of binding energy are e V mole - 1 , where
1
e
1 eV mol - 1 = 1 . 6 x 1 0 - 19)NA = 9 . 6 x 1 0 4 J mole - .
(
There are other possible pathways that may be taken by the excited
molecule XY* in Reaction (7.7). These can be represented by
XY* + C D - E + F + - - (reaction) (7. 8 )
XY* - xy + + e - (photoionization) (7.9)
+
XY* - XY h v (luminescence) (7. 1 0 )
(intramolecular energy transfer) (7 . 1 1 )
XY + XY* - XY + XY§
XY* + G H - XY + G H * (intermolecular energy transfer) (7 1 2)
.
.
XY* + M - XY + M (quenching) (7 1 3)