1. Chemistry 2
Lecture 13
Everything

2. Energy is the most important thing

3. Extraterrestrial solar spectrum
6000 K thermal spectrum

4. Atmospheric absorption
Absorption by water,
Carbon dioxide and ozone. Scattering too! O3
H2O
H2O & CO2
overtones!

5. Blue ice is due to overtone absorption
overtones!

6. Absorption of light by the earth
Chlorophyll fluorescence
in satellite image
ISC IC
Absorption S2 T2
While the earth fluoresces a little, the majority of incoming energy is internally converted into heat, and re-radiated in vibrational infrared transitions of water, rocks, asphalt…
ISC IC S1
Fluorescence
ISC T1
Phosphorescence S0

7. Greenhouse
(average)

8. Absorption and re-emission of infrared radiation by atmospheric molecules
ATMOSPHERE
E/2 E
E/4 E
E/2 2E
EARTH

9. The greenhouse effect is due to IR absorption
With no atmosphere, average temperature on earth is T0. If earth was blackbody with albedo of 0.40, then E
E=168
But, with single layer blackbody atmosphere absorbing outgoing radiation, Earth heats to irradiate twice the incoming energy. E 2E
EARTH
But the atmosphere does not absorb all outgoing radiation…. And is best represented as a multlayer. Clouds and weather complicate matters.

10. Greenhouse gases
If atmosphere was purely N2 and O2, all surface-emitted radiation would escape into space. Gases which have oscillators that overlap the emission spectrum of earth’s 300K blackbody convey blackbody behaviour to the atmosphere (statistical emission/absorption). Peak emission of 300K object is 590cm-1.

11. Green plants
Green plants absorb CO2 and synthesize sugars using light energy from the sun.
S2-S0
S1-S0 S2 S1 S0 IC T1 T2
Absorption
Fluorescence
ISC
Phosphorescence
Can be modelled as a particle on a ring system.

12. Light harvesting
The energy in photosynthesis is transferred from chlorophyll to chlorophyll and funnelled into the reaction centre.
chlorophylls
S2-S0
S1-S0

13. Energy transfer is internal conversion in bichromophoric molecule*
S2-S0 IC
Absorption S2 T2 IC S1
Fluorescence hn T1 S0 hn

14. Photodynamic cancer therapy
Cytotoxic singlet oxygen is produced from energy transfer after intersystem crossing. Specific tissues can be targeted by laser irradiation of triplet sensitizer.
ISC S1 T1 hn S1
energy transfer T0 S0 O2
triplet sensitizer
Skin transmits red and near infrared light effectively (800nm is great), but absorbs most visible and UV. Much effort goes into finding near IR absorbing triplet sensitizers for cancer therapy.

15. what is wrong with this picture?

16. triplet-triplet annihilation (TTA)
sensitizer
sensitizer S1 S1 S1 S1
ISC
ISC T1
TTA T1 T1 T1 S0 S0 S0 S0
emitter
emitter

17. spin states of two tripletsQ
1/9 collisions statistically gives singlet which can decay into excited S1 of one chromophore, and S0 opf the other.

19. requirements for TTA upconversion
sensitizer S1
Step down by >>kBT S1
ISC
More than half S1 T1
TTA T1 S0
emitters

20. single threshold solar cells
unoccupied
energy
levels
IC!
~32% max V
electrons

21. Up-conversion cell Up-conversion unit Limiting efficiency of an
Upconversion cell is about 50%
cell
Up-conversion unit

23. Nothing wrong with this picture!

25. Good Luck! Week 13 homework
Electronic spectroscopy worksheet in the tutorials
Complete the practice problems at the end of the lectures
Note: ALL of the relevant past exam problems have been used as practice problems. Other questions on past papers include parts which are no longer part of the course.