8
1 Introduction to Photochemistry
The star suffix indicates an electronically excited species. The photon absorption
event is often described as extremely fast, although the real meaning of this simplified
view will be made clear in Chap. 3. Just after excitation, the molecule can be thought
of as keeping the same geometry it had in the ground state, which is a classical
view of the Franck–Condon principle. Then, in the new electronic state, the nuclei
experience different forces and will start moving, which can give place to a small
rearrangement of the molecular geometry or to more important changes, including
chemical reactions. The processes undergone directly by the excited species A
∗ are
labeled as “primary.” The competition among all the possible primary processes is
basically determined by their relative rates. The primary products may give place
to further “secondary” reactions or photophysical events. Here is a list of primary
processes:
• Ionization:
A
∗
→ A
+
+ e
−
(1.24)
Ionization normally requires high energies, needed to extract an electron from
the neutral molecule, and is an extremely fast process. The ionization energy or
ionization potential is at least 8–10 eV for molecules in gas phase not containing
metal atoms. The corresponding wavelengths are about 120–150 nm. The excess
energy is mostly converted into kinetic energy of the emitted electron. In fact,
when a molecule produces two fragments of any nature the conservation of both
energy and momentum requires that their translational energies E 1 and E 2 in a
common center of mass reference frame are
E 1 =
m 2
m 1 + m 2
E tot
E 2 =
m 1
m 1 + m 2
E tot
(1.25)
where m i is the mass of fragment i and E tot = E 1 + E 2 is the total kinetic energy.
• Luminescence:
A
∗
→ A + hν
(1.26)
Luminescence is the spontaneous emission of a photon of frequency ν
, associated
with a transition of the excited molecule to a lower state, usually the ground state.
ν
can be different from the exciting frequency ν and usually ν
< ν, because
other processes convert part of the excitation energy into heat (i.e., mostly into
vibrational energy) before and after photon emission. Rates of emission can vary
over a very wide range, depending on whether the transition is spin- and symmetryallowed or forbidden. When luminescence is due to a spin-allowed transition then
it is called fluorescence, otherwise, phosphorescence. Large rate constants are only
found for fluorescence and can be of the order of 1 ns
−1 .
• Radiationless decay:
A
∗
→ A
(1.27)
In this case the electronic transition is not accompanied by photon emission, nor by
a chemical reaction. While the molecule switches to the ground state or to a lower
1 Introduction to Photochemistry
The star suffix indicates an electronically excited species. The photon absorption
event is often described as extremely fast, although the real meaning of this simplified
view will be made clear in Chap. 3. Just after excitation, the molecule can be thought
of as keeping the same geometry it had in the ground state, which is a classical
view of the Franck–Condon principle. Then, in the new electronic state, the nuclei
experience different forces and will start moving, which can give place to a small
rearrangement of the molecular geometry or to more important changes, including
chemical reactions. The processes undergone directly by the excited species A
∗ are
labeled as “primary.” The competition among all the possible primary processes is
basically determined by their relative rates. The primary products may give place
to further “secondary” reactions or photophysical events. Here is a list of primary
processes:
• Ionization:
A
∗
→ A
+
+ e
−
(1.24)
Ionization normally requires high energies, needed to extract an electron from
the neutral molecule, and is an extremely fast process. The ionization energy or
ionization potential is at least 8–10 eV for molecules in gas phase not containing
metal atoms. The corresponding wavelengths are about 120–150 nm. The excess
energy is mostly converted into kinetic energy of the emitted electron. In fact,
when a molecule produces two fragments of any nature the conservation of both
energy and momentum requires that their translational energies E 1 and E 2 in a
common center of mass reference frame are
E 1 =
m 2
m 1 + m 2
E tot
E 2 =
m 1
m 1 + m 2
E tot
(1.25)
where m i is the mass of fragment i and E tot = E 1 + E 2 is the total kinetic energy.
• Luminescence:
A
∗
→ A + hν
(1.26)
Luminescence is the spontaneous emission of a photon of frequency ν
, associated
with a transition of the excited molecule to a lower state, usually the ground state.
ν
can be different from the exciting frequency ν and usually ν
< ν, because
other processes convert part of the excitation energy into heat (i.e., mostly into
vibrational energy) before and after photon emission. Rates of emission can vary
over a very wide range, depending on whether the transition is spin- and symmetryallowed or forbidden. When luminescence is due to a spin-allowed transition then
it is called fluorescence, otherwise, phosphorescence. Large rate constants are only
found for fluorescence and can be of the order of 1 ns
−1 .
• Radiationless decay:
A
∗
→ A
(1.27)
In this case the electronic transition is not accompanied by photon emission, nor by
a chemical reaction. While the molecule switches to the ground state or to a lower