14
1 Introduction to Photochemistry
of O 2 declines. The photodissociation of O 3 , reaction (1.42), contributes to stabilize
the O 3 concentration at low levels.
Reaction (1.41) presents a small activation barrier and therefore can be speeded
up by catalyzers. The most important catalytic cycle is:
O 3 + Cl → O 2 + ClO
(1.43)
O + ClO → O 2 + Cl
(1.44)
Chlorine atoms are mainly supplied by chlorofluorocarbons, very stable synthetic
compounds that are not destroyed until they reach the stratosphere. Here the C-Cl
bonds are photodissociated by UV light, as in
CF 3 Cl
hν
−→ CF 3 · +Cl
(1.45)
This is the source of Cl atoms that catalyze the conversion of O 3 to O 2 . Reaction (1.44)
restores the Cl atom used by reaction (1.43). If a single Cl atom goes through the cycle
n times, the quantum yield of ozone destruction following the photodissociation of
CH 3 Cl is n, which can be of the order of thousands. Since the cycle (1.43)–(1.44)
also converts an oxygen atom to O 2 , thus eliminating a precursor of ozone, the real
loss of ozone molecules per absorbed photon is 2n.
1.6 Photochemical Kinetics
In the previous sections we provided some data about typical timescales of photochemical and photophysical phenomena. Actually it is useful to distinguish two
categories of time-dependent regimes, dynamics and kinetics. We can define dynamics as the time evolution of the molecular properties that are all connected to the
time-dependent wavefunction. Then, in principle, all photoinduced processes, such
as structural changes or electronic transitions, belong to the realm of dynamics.
However, in many cases a single molecule or molecular aggregate does not undergo
any important change, apart from small thermal fluctuations, for long-time intervals.
This occurs, for instance, in gas phase, where a molecule can remain in a timeindependent state until a collision triggers a reaction event or a transition to another
state. Moreover, when slow and gradual transitions between two stationary states
take place, the probabilistic nature of quantum mechanics allows us to treat a large
sample as composed of molecules that still occupy the initial state and molecules that
already populate the final one. If the reactive collisions or the population changes
occur according to well-defined rates, we can switch to kinetics and describe the
macroscopic phenomena by rate equations. Still, at molecular level, we shall need
dynamics for a detailed understanding of the processes and to compute their rates.
1 Introduction to Photochemistry
of O 2 declines. The photodissociation of O 3 , reaction (1.42), contributes to stabilize
the O 3 concentration at low levels.
Reaction (1.41) presents a small activation barrier and therefore can be speeded
up by catalyzers. The most important catalytic cycle is:
O 3 + Cl → O 2 + ClO
(1.43)
O + ClO → O 2 + Cl
(1.44)
Chlorine atoms are mainly supplied by chlorofluorocarbons, very stable synthetic
compounds that are not destroyed until they reach the stratosphere. Here the C-Cl
bonds are photodissociated by UV light, as in
CF 3 Cl
hν
−→ CF 3 · +Cl
(1.45)
This is the source of Cl atoms that catalyze the conversion of O 3 to O 2 . Reaction (1.44)
restores the Cl atom used by reaction (1.43). If a single Cl atom goes through the cycle
n times, the quantum yield of ozone destruction following the photodissociation of
CH 3 Cl is n, which can be of the order of thousands. Since the cycle (1.43)–(1.44)
also converts an oxygen atom to O 2 , thus eliminating a precursor of ozone, the real
loss of ozone molecules per absorbed photon is 2n.
1.6 Photochemical Kinetics
In the previous sections we provided some data about typical timescales of photochemical and photophysical phenomena. Actually it is useful to distinguish two
categories of time-dependent regimes, dynamics and kinetics. We can define dynamics as the time evolution of the molecular properties that are all connected to the
time-dependent wavefunction. Then, in principle, all photoinduced processes, such
as structural changes or electronic transitions, belong to the realm of dynamics.
However, in many cases a single molecule or molecular aggregate does not undergo
any important change, apart from small thermal fluctuations, for long-time intervals.
This occurs, for instance, in gas phase, where a molecule can remain in a timeindependent state until a collision triggers a reaction event or a transition to another
state. Moreover, when slow and gradual transitions between two stationary states
take place, the probabilistic nature of quantum mechanics allows us to treat a large
sample as composed of molecules that still occupy the initial state and molecules that
already populate the final one. If the reactive collisions or the population changes
occur according to well-defined rates, we can switch to kinetics and describe the
macroscopic phenomena by rate equations. Still, at molecular level, we shall need
dynamics for a detailed understanding of the processes and to compute their rates.