lecture 6 nitrogen and ozone photochemistry regions of light absorption of solar radiation

lecture 6 nitrogen and ozone photochemistry

Regions of Light Absorption of Solar Radiation


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.


N2 Electronic Energy Levels


N2 Absorption Regions

1. ionization continuum: < 800 Å

2. Tanaka-Worley bands: 800-1000 Å

3. Lyman-Birge-Hopfield bands: 1000-1450 Å


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


The N2 Visible Absorption Spectrum


Ozone Absorption

• mixing ratio: ~0.3 ppm• only absorber to absorb damaging radiation at 230290 nm• high absorption cross section at 230290 nm


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


Energy Level Diagrams for Diatomic Molecules


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.


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.


Ozone Absorption Spectrum – Hartley and Huggins Bands


Chappuis Band

Ozone Absorption Spectrum – Chappuis Band


Explanation of Ozone Absorption Regions• Hartley band: spin allowed transitions

• Huggins and Chappuis bands: spin forbidden transitions (weaker)


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?


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.


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?


Quantum Yield of O(1D)


Quantum Yield of O2(1g)


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.


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


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


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


Another Example of an Excited State Reaction

Excited state of N2:

N2* + O2 N2O + O

Source of N2O at altitudes above 20 km

lecture 6 nitrogen and ozone photochemistry regions of light absorption of solar radiation

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light chemical species

electronic excitation

shorter wavelengths