1. OPTICAL ABSORPTION

2. LEARNING OBJECTIVES Each student should be able to:
Describe photon absorption;
Explain the basic principles that create an absorption spectra;
Explain a broaden absorption spectra;
Describe the possible fate of optically excited states;

3. LESSON OUTLINE Absorption/Transmittance; Photon Absorption;
Franck-Condon Principle
Absorption/Transmittance Spectra;
Fate of Excited States

4. TRANSMITTANCE L a = absorption coefficient I0 (lx ) It(lx )
© Thomson/Brooks Cole L
It(lx ) = I0 (lx ) e-aL
a = absorption coefficient
I0 (lx )
It(lx )
Tunable Laser (lx )
Detector
Sample

5. ABSORPTION & TRANSMISSION SPECTRA

6. LESSON OUTLINE Absorption/Transmittance; Photon Absorption;
Franck-Condon Principle
Absorption/Transmittance Spectra;
Fate of Excited States

7. FRANCK-CONDON PRINCIPLE

8. ABSORPTION SPECTRA
When absorption occurs to unbound states of the upper electronic state, the molecule dissociates and the absorption is a continuum.
Above the dissociation limit a continuous band of absorption occurs because the final state is an un-quantized translation of the fragments.
Below the dissociate limit the electronic spectrum shows a normal vibrational structure.

9. ABSORPTION STATES
As the optical transition becomes less vertical the absorption spectra shift due to the change in the Franck-Condon patterns.

10. POTENTIAL ENERGY CURVES FOR MOLECULAR OXYGEN

11. MOLECULAR ORBITALS FOR BENZENE

12. THE ABSORPTION SPECTRUM OF CHLOROPHYLL
Red
Blue
Green

13. TRANSMITTANCE
- Aromatic Compounds -

14. TRANSMITTANCE
- Carbonyl Compounds -

15. BROADENED ABSORPTION SPECTRA

16. CHROMOPHORES
Chromophores are a group of atoms in a molecule that produces color in dyes and other compounds through selective absorption of light, for example, the azo group.
Chromophores are components of molecules which absorb light.
The wavelength of absorption is related to the size of the chromophores.
Smaller chromophores, higher energy (shorter wavelength).
They are generally aromatic rings.

17. LESSON OUTLINE Absorption/Transmittance; Photon Absorption;
Franck-Condon Principle
Absorption/Transmittance Spectra;
Fate of Excited States

18. FATE OF EXCITED STATES (Primary Photochemical Processes)

19. PREDISSOCIATION
Predissociation is the relaxation to a state with enough energy to break a bond.
Predissociation arises when two electronic states cross.
Usually one electronic state is stable, and has a well defined potential diagram. The other state is typically a repulsive state.
The transition is initially to the stable state, and then transfers across to the repulsive state, where dissociation occurs

20. DO YOU KNOW WHAT YOU KNOW?

21. LEARNING OBJECTIVES Each student should be able to:
Describe photon absorption;
Explain the basic principles that create an absorption spectra;
Explain a broaden absorption spectra;
Describe the possible fate of optically excited states;