What is Photochemistry

1
What is Photochemistry?

• Photochemistry a chemical interaction involving
  radiation
• Why mention photochemistry?
• Photochemistry plays an important role in
  atmospheric processes.
• The wavelengths in important atmospheric
  photochemical reactions are shortwave radiation
  from the sun

2
What is the Suns Source of Energy?

• Nuclear Energy neutrons protons minus binding
  energy
• Atomic Weight/(NeutronsProtons) does not equal 1
  because of the binding energy. More binding
  energy, lower ratio AW/(NP), lower total energy.
• If two light nuclei join to form a heavier
  nucleus with higher binding energy (lower total
  energy), the extra energy is released as
  radiation fusion.

3
Fusion

• Fusion is a thermonuclear reaction. It releases
  energy but needs a high enough temperature to
  bring the two nuclei together.
• 2H2H ? 4HeE
• 2x2.014?4.00260.0254(as energy)
• Requirement, high temperature for activation
  25,000,000oC
• This is the temperature in the suns interior,
  but not the temperature at the suns surface (the
  temperature at which the sun emits).
• Can calculate Tsurface based on a steady state
  assumption (heat flow from interior balances heat
  loss from surface)
• dT/dtC1MsTinterior-C2(S.A.)sTsurface0
• 4/3pR3C1Tinterior4pR2C2Tsurface
• TsurfaceCRTinterior6000oK
• Solar surface temperature is determined from the
  ratio of surface to volume.

4
Energy of Radiation

• Energy per photon
• Ehnhc/l h 6.626 x 10-34 J s c 3 x 108 m
  s-1

5
Energy Per One Mole of Photons (Einstein)

• Energy per one mole of photons at
• 100nm 290 kcal/mole
• 400nm 72.5 kcal/mole
• 700nm 41.5 kcal/mole
• 1000nm 29 kcal/mole
• Comparison to chemical bond strength
• N-N 225 kcal/mole Very strong
• O-O 120 kcal/mole Strong
• C-Cl 75 kcal/mole Intermediate
• O-O2 35 kcal/mole Weak
• HO..H5 kcal/mole Very weak

6
What Happens When Radiation Hits a Molecule?

• We learned in radiative transfer that two
  possible outcomes are
• scattering (no chemical interaction)
• absorption
• Following absorption, there are a number of
  possibilities.

7
Pathways Following Absorption
8
Pathways Following Absorption cont.

• Dissociation/photolysis breaking a chemical bond
  in the molecule
• Energy of radiation must be greater than bond
  energy.
• l100-1000nm is sufficient to break any chemical
  bond.
• Ionization removing an electron from the
  molecule
• In general ionization energy is greater than
  chemical bond strength
• He 552 kcal/mole l52.6 nm
• N2 398 kcal/mole l79.6 nm
• Na 120 kcal/mole l250 nm

9
Table of Ionization Energies
10
Pathways Following Absorption cont.

• Luminescence re-emission of photon

In atoms, re-emited photon is of same energy as
excitation lemlex. In molecules, it can be
less lemgtlex. Flourescence visible
wavelengths Phosphorescence non-visible
wavelengths
11
Pathways Following Absorption cont.

• Intramolecular energy transfer conversion of the
  absorbed energy to several forms of lower energy
  (vibration, rotation and eventually to heat
  typical for large molecules).
• Intermolecular energy transfer
• Quenching
• Reaction conversion to more active state and
  undergo selective chemical reactions

12
What Determines the Pathway?

• wavelength whether or not its possible
• population of excited states whether or not its
  probable
• conservation of orbital angular momentum and spin
  whether or not its probable

13
Rate of a Photochemical Reaction

• rate of formation of AB
• J for a photochemical reaction is the equivalent
  of a rate constant for a chemical reaction
• J can be treated as a first order rate constant
  (units of time-1) but it depends on light
  intensity and spectral distribution.

14
How is J Calculated?

• For a given wavelength
• JlPFl sl Yl
• PFl PhotoFlux
• sl Absorption cross section (population of
  excited states)
• Yl Quantum yield (conservation of orbital
  angular momentum and spin)
• Yl
• The quantum yield is sometimes also symbolized f.
• For a range of wavelengths (solar range)
• J ? PFl sl Yl dl

15
Photolysis Rate Example

• NO2hn ? NO O(3P) JNO2
• The rate of O(3P) formation
• dO(3P)/dt JNO2 NO2
• The rate of O formation will change diurnally
  even at constant NO2.

16
How Does J Vary With Latitude and Season?

• JJmax cos(f-d) cos(w)
• Jmax a SRI/R2 SRI solar radiation intensity
• w2pt/60x60x24
• Geographical Latitude
• Seasonal motion of the earth d23.5 cos(2p
  JD /365)
• JD Julian Day

17
The Solar Spectrum