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

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 Electronic Energy Levels

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

The N2 Visible Absorption Spectrum

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 Diatomic Molecules

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

Ozone Absorption Spectrum – Hartley and Huggins Bands

lecture 6 nitrogen and ozone photochemistry

Chappuis Band

Ozone Absorption Spectrum – Chappuis Band

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

Quantum Yield of O(1D)

lecture 6 nitrogen and ozone photochemistry

Quantum Yield of O2(1g)

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

lecture 6 nitrogen and ozone photochemistry

Another Example of an Excited State Reaction

Excited state of N2:

N2* + O2 N2O + O

Source of N2O at altitudes above 20 km