Lecture 6 nitrogen and ozone photochemistry Regions of Light Absorption of Solar Radiation.
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Transcript of Lecture 6 nitrogen and ozone photochemistry Regions of Light Absorption of Solar Radiation.
lecture 6 nitrogen and ozone photochemistry
Absorption by Small Molecules
Small, light chemical species (N2 and H2) generally absorb via electronic excitation at shorter wavelengths ( <~ 100 nm) than more complex compounds.
As symmetric linear diatomic molecules, they also do not absorb much IR radiation (cannot induce a dipole moment by vibration or rotation – no dipole allowed transitions).
Most of their influence is in the upper atmosphere.
lecture 6 nitrogen and ozone photochemistry
N2 Absorption Regions
1. ionization continuum: < 800 Å
2. Tanaka-Worley bands: 800-1000 Å
3. Lyman-Birge-Hopfield bands: 1000-1450 Å
lecture 6 nitrogen and ozone photochemistry
Light absorption begins at 120 nm
Dissociation:
N2+h(80<<91nm) 2N. (N(4S) + N(2D))Ionization:
N2+h(80nm) N2+ + e
At 91nm =4x10-20 cm2
The atmospheric absorption of a layer 1 km deep is:
Beer-Lambert law: I = I0exp(-n z) Why can we use this?
D = ln(I0/I) = n z =(9x1012)(4x10-20)(1x105) = 0.036
I/I0 = 0.92; T = 0.92; A=1-T = 0.08T: transmissionA: absorption
Result: 8% of the light is absorbed by the 1km layer at 100km
Nitrogen Photochemistry
lecture 6 nitrogen and ozone photochemistry
Ozone Absorption
• mixing ratio: ~0.3 ppm• only absorber to absorb damaging radiation at 230290 nm• high absorption cross section at 230290 nm
lecture 6 nitrogen and ozone photochemistry
O-O2 is very weak
Minimal dissociation energy (=1180nm)
O3+h(<1180nm) O(3P)+O2
Light absorption:
At 250nm =10-17cm2
The atmospheric depth of O3 is equivalent to 0.3 cm at STP:
D{250nm]=10-17x0.3x2.7x1019=81; T=10-D=10-81
Ozone Photochemistry
lecture 6 nitrogen and ozone photochemistry
Energy Level Diagrams for Polyatomic Molecules
Instead of potential energy curves, in triatomic systems have potential energy surfaces, since need to represent three distances:
With more than three atoms have a multi-dimensional potential energy hypersurfaces.
lecture 6 nitrogen and ozone photochemistry
Energy Levels of Polyatomic Molecules
Although the energy level diagrams are more complicated, the same types of transitions can occur:
• Allowed Transitions/Optical Dissociation: The molecule jumps to higher vibrational states and eventually to dissociation within the same electronic energy state.
• Forbidden Transitions
• Pre-Dissociation: The molecule jumps from its ground electronic energy state to a higher electronic energy state, followed by intramolecular energy transfer to the energy level of dissociation into two ground state species.
lecture 6 nitrogen and ozone photochemistry
Explanation of Ozone Absorption Regions• Hartley band: spin allowed transitions
• Huggins and Chappuis bands: spin forbidden transitions (weaker)
lecture 6 nitrogen and ozone photochemistry
Ozone Dissociation Products
• Depending on photon energy, the dissociation products O and O2 can be in excited states.
• According to spin conservation, allowed transitions have O and O2 both as singlets (2S+1 = 1) or both as triplets (2S+1 = 3).
• Lowest energy singlet pair: O(1D) and O2(1g)
What is the threshold for allowed O(1D) production?
lecture 6 nitrogen and ozone photochemistry
O3+h(<X nm) O(tY)+O2()
O(3P) O(1D) O(1S)
O2(3) 1179nm 411 237
O2(1) 611 310 199
O2(1) 462 267 181
Ozone Dissociation Products cont.
lecture 6 nitrogen and ozone photochemistry
Ozone Dissociation Products cont.
What is the threshold for allowed O(1D) production? ~310 nm
However, O3+h( < 411 nm)O(1D) + O2(3) is also an important source of O(1D).
Why?
How does the reaction occur?
lecture 6 nitrogen and ozone photochemistry
Why is the Quantum Yield Not a Step Function?
Energy in internal vibrations and rotations can assist dissociation.
Quantum yield depends on temperature as well.
lecture 6 nitrogen and ozone photochemistry
The most reactive atmospheric reagent (chicken and egg story):Selective reactions
O(1D) + H2O 2HO.
O(1D) H2 HO + H.
O(1D) + N2O 2NOO(1D) + CFC’s Products
Also
O(1D) + N2 O(3P)+ N2
In fact:O(1D) + M O(3P) + M
O(1D) Reactions
lecture 6 nitrogen and ozone photochemistry
Formation
O2+h (<175nm) O(1D)+O(3P) J{O2}
O3+h (<410nm) O(1D)+O(3) J{O3}
Removal
O(1D) + N2 O(3P)+ N2 k3=5.4x10-11
O(1D) + O2 O(3P) + O2 k4=7.4x10-11
[O(1D)]ss=(J{O2}+J{O3})/(k3[N2 ]+k4[O2])
=1/(k3[N2 ]+k4[O2])Height (km) sec
20 10-10
40 10-7
80 5x10-5
100 2x10-3
O(1D) Lifetime
lecture 6 nitrogen and ozone photochemistry
Reactivity and Electronic State
Why is O(1D) more reactive than O(3P)?
1. energy: excitation energy contributes to energy of reaction (reaction may switch from endothermic to exothermic)
2. kinetics: the dependence of reaction rates on temperature can often be written exp(-Ea/RT): Arrhenius expression
R: universal gas constant
Ea: activation energy
excitation energy reduces Ea
3. electronic configuration: different electron arrangement may favor reaction by making it easier to conserve spin angular momentum