Environmental Photochemistry
Stefan Franzen
NC State University
Excited State Processes
Photophysics
• Fluorescence
• Phosphorescence
• Internal conversion
• Intersystem crossing
Photochemistry
• Electron transfer
• Isomerization
• Photolysis
The fate of electronically
excited states
• A radiative decay process involves
emission of a photon.
• Non-radiative decay involves the
transfer of excess energy into
vibration, rotation, and translation
of surrounding molecules. The result
is to heat the surroundings.
- Vibrational relaxation
- Internal conversion processes
Vibrational relaxation
Absorption populates the Franck-Condon
active modes of the excited state. This is
NOT the equilibrium population. Therefore,
relaxation among the vibrational levels occurs.
Singlet State
Ground State
Internal conversion
A singlet excited state can decay directly
back into the ground state by a process
known as internal conversion.
Excited State
No photon is emitted
Ground State
Fluorescence
Fluorescence is the spontaneous emission
of radiation from an excited singlet state.
1. Absorption leads to population of
Franck-Condon active vibrational modes
of the excited state.
2. Vibrational relaxation results in a
a change of the excited state levels.
3. Emission occurs from an equilibrium
population of excited state vibrational levels.
Intersystem crossing
The singlet and triplet state potential
surfaces may cross and therefore a change
of electron spin due to spin-orbit interactions
gives rise to the triplet state.
Singlet State
Triplet State
Ground State
Phosphorescence
Phosphorescence is the emission of radiation
from a triplet state.
1. Absorption leads to population of the FC
active modes of the singlet.
2. After vibrational relaxation in the singlet
intersystem crossing produces the triplet state.
3. Vibrational relaxation continues in the
triplet state.
4. Emission occurs from the triplet. Because
it is spin-forbidden, the emission rate is very
low. Consequently, it is long-lived.
Jablonski diagram
The radiative and non-radiative decay processes
can be represented using a diagram. Horizontal shifts
represent changes in nuclear position.
T1
S1
S0
IC
ISC
Fluorescence
Phosphorescence
ISC
Photochemistry
Excited state processes that result in changes
in bonding are photochemical processes.
• Photodissociation or photolysis is the breaking
of a chemical bond. IR → I*R → I + R
• Electron transfer results in a change in bond
order due to a process. DA → D*A → D+A• Isomerization results in a change in molecular
structure.
B
A
B
A
Photolysis
If the excited state is anti-bonding with respect
to a particular coordinate the result is dissociation
of a chemical bond.
Dissociative State
Ground State
Tropospheric photochemistry
OH and HO2 radicals are produced primarily by photolysis
of atmospheric trace gases under the influence of solar
UV radiation (hν). OH radicals oxidize and thereby remove
air pollutants such as carbon monoxide (CO), nitrogen
dioxide (NO2) or volatile organic compounds (VOC, e.g.
methane). Part of these reactions generate peroxy
radicals (HO2 and RO2) which can recycle OH by reactions
with nitrogen monoxide (NO). As a result new products are
formed (for example oxygenated VOC, ozone and particles),
which influence strongly air quality and climate.
The rate of the degradation and transformation processes
depends critically on the concentration of radical species.
Tropospheric photochemistry
Partially oxidized
VOCs, Particles
VOC = Volatile organic species
Photochemical radical formation
We can consider the reactions of nitrate and nitrite as
specific examples of photochemical radical formation.
Although the absorption cross sections are relatively small,
(nitrate, λmax = 303 nm, ε = 7 and nitrite λmax = 355 nm, ε = 22)
-
NO3 + H2O + hν
NO2- + H2O + hν
. NO + HO. + OH2
. NO + HO .+ OH-
The pathlength in water is long enough that significant
absorption can occur. The quantum yield for OH radical
formation is φ ~ 0.015. Therefore the rate of radical formation
is:
rate = Iφ
where I is the intensity of light.
Stratospheric Ozone Formation
For an introduction to ozone see:
http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm
The stratosphere is heated by the formation of ozone
according to the following series of reactions:
O2 + hν (<242 nm) → 2 O .
O2 + O . → O3 + 100 kJ
O3 + O . → 2 O2 + 400 kJ
O3 + hν (240 – 290 nm) → O2 + O .
Although the process generates ozone, there is also a process
that destroys ozone. Thus, ozone is formed at steady state.
Destruction of Ozone
An ozone molecule's life ends when it reacts with one of a
variety of chemicals in the stratosphere such as chlorine,
nitrogen, bromine or hydrogen. These loss reactions generally
occur in a catalytic process. A catalyst is a substance that
facilitates a chemical reaction, but which itself remains
unchanged or is reformed by the end of the reaction, so that it
can take part in a similar reaction again. In this catalytic
process, the ozone molecule is lost while the catalyst (chlorine,
nitrogen, bromine or hydrogen) is reformed to potentially
destroy another ozone molecule. The catalyst will react until
it is carried out of the stratosphere. Over its lifetime in the
stratosphere, an individual Cl atom can destroy about 100,000
ozone molecules.
Catalytic Degradation of O3
An example of catalytic O3 loss is shown below where ClO
(chlorine monoxide) reacts with an oxygen atom to form Cl
(free chlorine) and O3 (molecular oxygen). The Cl atom then
reacts with an ozone molecule to reform ClO and another O2.
O3 + hν (240 – 290 nm) → O2 + O .
ClO + O . → O2 + Cl .
Cl . + O → O + ClO
3
2
Net: 2O3 → 3O2
The net effect of this reaction is the formation of 2 oxygen
molecules from an oxygen atom and an ozone molecule, while
the ClO molecule is unaffected. At 40 km, this Cl-ClO catalytic
chain can destroy nearly 1000 ozone molecules before the Cl
or ClO is converted to a benign chlorine form such as HCl
(hydrochloric acid) or ClONO2 (chlorine nitrate).
Oxidation of terpenes in the rain forest
The photooxidation of isoprene
results in an annual production
of about 2 million tons of the new
compounds. This represents 25%
of the organic aerosol formed by
atmospheric photochemistry from
biogenic precursors.
Emission of isoprene by the Amazon
forest and oxidation into the
2-methyltetrol compounds.
There is a between isoprene emitted
by forest vegetation and formation of
water soluble fine particles. These
aerosol particles give rise to the
formation of haze and positively
effect cloud formation and rainfall.
Formation of pyrimidine dimers
Irradiation in the ultraviolet leads to crosslinking of nucleotides in DNA and RNA. The pyrimidines (thymidine, cytidine
and uracil) are the most susceptible to these reactions.
The photochemical pathway involves intersystem crosslinking
to the triplet state.
Common pyrimidine-pyrimidine crosslinks
Pyr(6,4)Pyr
Pyr<>Pyr
Repair of pyrimidine dimers
Two types of repair
Are shown in the diagram.
Photoactivated repair by
the enzyme photolyase
occurs in a single step
by electron transfer.
Excision repair is the
mechanism for repair in
humans.