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 (N 2 and H 2 ) 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 N 2 Electronic Energy Levels

lecture 6 nitrogen and ozone photochemistry N 2 Absorption Regions 1.ionization continuum: < 800 Å 2.Tanaka-Worley bands: Å 3.Lyman-Birge-Hopfield bands: Å

lecture 6 nitrogen and ozone photochemistry Light absorption begins at 120 nm Dissociation: N 2 +h (80< <91nm)  2N. (N( 4 S) + N( 2 D)) Ionization: N 2 +h  (  80nm)  N e  At 91nm  =4x cm 2 The atmospheric absorption of a layer 1 km deep is: Beer-Lambert law: I = I 0 exp(-n  z) Why can we use this? D = ln ( I 0 /I ) = n  z =(9x10 12 )(4x )(1x10 5 ) = I/I 0 = 0.92; T = 0.92; A=1-T = 0.08 T: transmission A: absorption Result: 8% of the light is absorbed by the 1km layer at 100km Nitrogen Photochemistry

lecture 6 nitrogen and ozone photochemistry The N 2 Visible Absorption Spectrum

lecture 6 nitrogen and ozone photochemistry Ozone Absorption mixing ratio: ~0.3 ppm only absorber to absorb damaging radiation at 230  290 nm high absorption cross section at 230  290 nm

lecture 6 nitrogen and ozone photochemistry O-O 2 is very weak Minimal dissociation energy ( =1180nm) O 3 +h ( <1180nm)  O( 3 P)+O 2 Light absorption: At 250nm  = cm 2 The atmospheric depth of O 3 is equivalent to 0.3 cm at STP: D{250nm]= x0.3x2.7x10 19 =81; T=10 -D = 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 O 2 can be in excited states. According to spin conservation, allowed transitions have O and O 2 both as singlets (2S+1 = 1) or both as triplets (2S+1 = 3). Lowest energy singlet pair: O( 1 D) and O 2 ( 1  g )  What is the threshold for allowed O( 1 D) production?

lecture 6 nitrogen and ozone photochemistry O 3 +h ( <X nm)  O( t Y)+O 2 (   ) O( 3 P)O( 1 D)O( 1 S) O2(3)O2(3) 1179nm O2(1)O2(1) O2(1)O2(1) Ozone Dissociation Products cont.

lecture 6 nitrogen and ozone photochemistry Ozone Dissociation Products cont.  What is the threshold for allowed O( 1 D) production? ~310 nm However, O 3 +h  ( < 411 nm)   O( 1 D) + O 2 ( 3  ) is also an important source of O( 1 D). Why? How does the reaction occur?

lecture 6 nitrogen and ozone photochemistry Quantum Yield of O( 1 D)

lecture 6 nitrogen and ozone photochemistry Quantum Yield of O 2 ( 1  g )

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( 1 D) + H 2 O  2HO. O( 1 D)  H 2  HO + H. O( 1 D) + N 2 O  2NO O( 1 D) + CFC’s  Products Also O( 1 D) + N 2  O( 3 P)+ N 2 In fact: O( 1 D) + M  O( 3 P) + M O( 1 D) Reactions

lecture 6 nitrogen and ozone photochemistry Formation O 2 +h ( <175nm)  O( 1 D)+O( 3 P) J{O 2 } O 3 +h ( <410nm)  O( 1 D)+O( 3  ) J{O 3 } Removal O( 1 D) + N 2  O( 3 P)+ N 2 k 3 =5.4x O( 1 D) + O 2  O( 3 P) + O 2 k 4 =7.4x [O( 1 D)] ss =(J{O 2 }+J{O 3 })/(k 3 [N 2 ]+k 4 [O 2 ])  =1/(k 3 [N 2 ]+k 4 [O 2 ]) Height (km)  sec  x x10 -3 O( 1 D) Lifetime

lecture 6 nitrogen and ozone photochemistry Reactivity and Electronic State Why is O( 1 D) more reactive than O( 3 P)? 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(-E a /RT): Arrhenius expression R: universal gas constant E a : activation energy excitation energy reduces E a 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 N 2 : N 2 * + O 2  N 2 O + O Source of N 2 O at altitudes above 20 km