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METO 637
LESSON 3
Photochemical Change
• A quantum of radiative energy is called a photon, and is given
the symbol hn . Hence in a chemical equation we write:
O3 + hn →O2 + O
• The energy of a photon in terms of its wavelength l is
E=119625/l kJ mol-1 or 1239.8/ l in eV
• To get enough energy to break up a molecule (dissociation) the
wavelength must be in or below the ultraviolet. Thus
dissociation typically occurs as the result of electronic
transitions
• Small, light chemical species generally have electronic
transitions at wavelengths shorter than those for more complex
compounds, e.g. l<200 nm for O2,.
Photochemical Change
• Atmospheres tend to act as filters cutting out short wavelength
radiation, since the absorptions of their major constituents are
generally strong at the short wavelengths.
• As a result, photochemically active radiation that penetrates
into an atmosphere is of longer wavelengths, and the chemistry
is characterized by lower energies. For example, the
dissociation of molecular oxygen, the ultimate source of ozone
in the stratosphere, is limited to altitudes above 30 km.
• Absorption of a photon of photochemically active radiation
leads to electronic excitation, represented as
AB + hn →AB*
• If the excited molecule then breaks apart, the quantum yield of
such a reaction is defined as the number of reactant molecules
decomposed for each quantum of radiation absorbed
Absorption in molecular lines and bands
• Molecules have three types of energy levels electronic, vibrational, and rotational
• Transitions between electronic levels occur
mainly in the ultraviolet
• Transitions between vibrational levels visible/near IR
• Transitions between rotational levels - far IR/ mm
wave region
• O2 and N2 have essentially no absorption in the IR
• 4 most important IR absorbers H2O, CO2, O3, CH4
Schematic of vibrational levels
Photochemical Change
Photodissociation
• Two main mechanisms are recognized for dissociation,
optical dissociation and pre-dissociation. These processes
will be illustrated in the O2 and O3 molecules.
• Optical dissociation occurs within the electronic state
to which the dissociation first occurs. The absorption
spectrum leading to dissociation is a continuum.
• At some longer wavelength the spectrum shows
vibrational bands. The bands get closer together as the
limit is approached – the restoring force for the vibration
gets weaker.
•The absorption from the ‘X’ to the ‘B’ state in O2 , is an
example.
Photodissociation
Photodissociation
Photodissociation
Photodissociation
• Note that when the B state dissociates, one of the two atomic
fragments is excited. One atom is left in the ground state (3P)
and the other in an excited state (1D).
• Some fragmentation occurs in the B→X (Schumann-Runge)
system before the dissociation limit. This occurs because a
repulsive state crosses the B electronic state and a radiationless
transition takes place. The repulsive state is unstable and
dissociation takes place. Note that both atomic fragments are
3P.
• Although molecular oxygen has many electronic states, not all
of the possible transitions between the states are allowed. The
magnitude of the photon energy is not the only criteria
• Consideration of things such as the need to conserve quantum
spin and orbital angular momentum indicate if the transition is
possible.
Predissociation
• Predissociation arises when two electronic
states cross.
• Usually one electronic state is stable - has a
well defined potential diagram. The other
state is typically a repulsive state.
• The transition is to the stable state, and then
moves across to the repulsive state, when
dissociation occurs.
Franck-Condon principle
• The time for a transition is extremely small,
and in this time the atoms within a molecule
can be assumed not to move.
• Franck and Condon therefore postulated
that on a potential energy diagram the most
likely transitions would be vertical
transitions
Franck-Condon principle
Predissociation
Photodissociation
• Let us take the reaction
O3 + hn → O2 + O
• The O2 molecule and atom can be left in several states if we
consider energy alone, because any ‘extra’ energy can be used
for kinetic energy of the products.
• For wavelengths about 310 nm or less, spin conservation
allows the transition
O3 + hn → O2(1Dg) + O(1D)
• The O(1D) atom formed in this reaction plays a major role in
atmospheric chemistry, for example
O(1D) + H2O → OH + OH
• OH, the hydroxyl radical, can break down hydrocarbons.
Wavelength Threshold for Dissociation of
Ozone
Quantum yield for Ozone
Quantum yield for Ozone
• Note that the onset of dissociation is not abrupt.
• The shape of the curve can be explained if the
internal energy of the molecule (vibration and
rotation) can be added to the photon energy to
induce transitions.
• Transitions from vibrationally excited states can
be important in the atmosphere.
• The solar spectrum shows a rapid increase above
310 nm, so any extension of the absorption cross
section above this limit can lead to a significant
increase in say the quantum yield of O1D