Page 155 - Basic physical chemistry for the atmospheric sciences

Photochemistry 1 -1 1

Reactions (7. 7) to (7 1 3) are referred to as primary photochemirnl
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steps. Reaction (7. 8 ) represents a chemical reaction between xv· ant.I
a molecule CD leading to products E, F , - -, any of which can be in
an excited state. In Reaction (7.9), the excited electron in XV* escapes
to become a free electron leaving XV ionized ; this is called photoioni­
zation. The electrons in the Earth' s upper atmosphere (which permit
long-distance radio communications) are produced primarily by solar
radiation photoionizing molecules and atoms in the air. In Reaction
(7. 1 0 , the radiation absorbed in the initial step (7.5) is re-emitted ; this
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is called luminescence. If the radiation is re-emitted very quickly it is
called .fluorescence; if it i s re-emitted s l owly ( � 1 0 - 3 to 1 0 - 2 s) it is
called phosphorescence. Fluorescence is primarily responsible for the
phenomenon of airglow, that is, the emission of a faint glow from the
Earth' s upper atmosphere. In Reaction (7 1 1 ) , the collision of xv·
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with another XV molecule transforms xv• to a new excited state
(indicated by XV§). In Reaction (7 1 2 , xv· interacts with another
)
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molecule GH which it excit s . In Reaction (7. 1 3 , xv· interacts with a
e
)
chemically unreactive molecule (M) to which it transfers its excited
energy and is itself reduced to the electronic ground state.
In addition to the molecules that are specifically indicated as being
in an excited state in Reactions (7.7) to (7. 1 3 ) , the other molecules and
atoms in these reactions may or may not be excited. The energy
change associated with the initial step in a photochemical process
[Reaction (7. 5 )] generally involves the excitation of one electron in the
molecule (or atom) from a lower to a higher energy level , although
s
longer wavelengths may excite molecular vibrations or rotation .
s
Tran i tions in the electronic, vibrational, and rotational energy states
of a molecule are governed by the rules of quantum mechanic , which
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determine the characteristic frequencies of em radiation that a mole­
cule can absorb or emit (e.g. , the characteristic yellow light emitted
by sodium compounds when they are heated in a flame is produced by
one of these transitions). The excited state of an atom or a molecule
can be indicated by spectroscopic notation, which is often placed in
3
brackets following the chemical symbol . For example, 0( P) indicates
the ground state of the oxygen atom and 0 ( 1 D ) an excited state.
However, since it is beyond the scope of this book to explain the exact
meaning of this notation, we will not use it.