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Processes in the Oceans
The excited molecule can react by rearrangement:
M* → P
(10.12)
or by reaction with another species N:
M* + N → P
(10.13)
These steps involving M* are called the primary photochemical process. The products P
may undergo further reactions that lead to final, stable products. The excited molecule can
lose its energy by the loss of heat:
C* → C + Heat
(10.14)
by the loss of light energy:
C* → C + hν
(10.15)
or by ionization:
C* → C + + e –
(10.16)
where e – is an electron. Secondary reactions of the primary photoproducts can frequently
occur with the components of the solution. For example, when the solutions contain O 2 ,
the free radical O 2
– can be formed. This radical can react as both an oxidizing and a reducing agent.
Before we discuss various photochemical reactions important in seawater, it is useful to
briefly review the energy states of molecules. The types of excited molecules depend on
the amount of energy absorbed. Rotational excitation of a molecule requires the smallest
amount of energy. This results in a spinning of the molecule around a preferred axis; however, the molecule is not changed chemically. With higher energies, the molecule may be
promoted to a vibrationally excited state. The energy is in the form of vibrations of various
parts of the molecule; again, the molecule is not changed chemically. With higher energies,
the molecule can be excited to an electronically excited state. One or more electrons can be
promoted to higher- energy orbitals.
Photochemical reactions occur from such electronically excited states of molecules. If a
very high amount of energy is absorbed, a continuum is reached, and the molecule is ionized or dissociated:
M M e
h
→
+
+
−
ν
(10.17)
M X Y
h
→ +
ν
(10.18)
The electronic states of polyatomic molecules are shown in Figure 10.1. In the electronic
ground state and various electronically excited states, there are sublevels ν containing
various vibrational states, and each vibrational state can have various rotational sublevels.
Processes in the Oceans
The excited molecule can react by rearrangement:
M* → P
(10.12)
or by reaction with another species N:
M* + N → P
(10.13)
These steps involving M* are called the primary photochemical process. The products P
may undergo further reactions that lead to final, stable products. The excited molecule can
lose its energy by the loss of heat:
C* → C + Heat
(10.14)
by the loss of light energy:
C* → C + hν
(10.15)
or by ionization:
C* → C + + e –
(10.16)
where e – is an electron. Secondary reactions of the primary photoproducts can frequently
occur with the components of the solution. For example, when the solutions contain O 2 ,
the free radical O 2
– can be formed. This radical can react as both an oxidizing and a reducing agent.
Before we discuss various photochemical reactions important in seawater, it is useful to
briefly review the energy states of molecules. The types of excited molecules depend on
the amount of energy absorbed. Rotational excitation of a molecule requires the smallest
amount of energy. This results in a spinning of the molecule around a preferred axis; however, the molecule is not changed chemically. With higher energies, the molecule may be
promoted to a vibrationally excited state. The energy is in the form of vibrations of various
parts of the molecule; again, the molecule is not changed chemically. With higher energies,
the molecule can be excited to an electronically excited state. One or more electrons can be
promoted to higher- energy orbitals.
Photochemical reactions occur from such electronically excited states of molecules. If a
very high amount of energy is absorbed, a continuum is reached, and the molecule is ionized or dissociated:
M M e
h
→
+
+
−
ν
(10.17)
M X Y
h
→ +
ν
(10.18)
The electronic states of polyatomic molecules are shown in Figure 10.1. In the electronic
ground state and various electronically excited states, there are sublevels ν containing
various vibrational states, and each vibrational state can have various rotational sublevels.