molecular luminescence spectrometry chap 15. three related optical methods fluorescence...
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MolecularMolecularLuminescence SpectrometryLuminescence Spectrometry
Chap 15
Three Related Optical Methods
Fluorescence
Phosphorescence
Chemiluminescence
} From excitation throughabsorption of photons
Emission from an excitedspecies formed in a chemicalreaction
Fluorescence
• From atomic sodium vapor
• 3p → 3s transition at 589.6 and 589.0 nm
3pʹ
3s
3pʺ
3pʹ
3s
3pʺ
Resonancefluorescence
Fluorescence excitation spectrum of the two sodium D-lines
Fluorescence Phosphorescence
Excited States ProducingExcited States Producing
Fluorescence & PhosphorescenceFluorescence & Phosphorescence
Electron Spins can be:
• “Paired” → diamagnetic
• “Unpaired” → paramagnetic (e.g., free radicals)
Fig. 15-1
Energy Level Diagram for a Photoluminescent System
Fig. 15-2
allowed forbidden
Rates of Absorption and EmissionRates of Absorption and Emission
In general:
Absorption: 1 – 10 fs
Fluorescence: 1 – 100 ns
Phosphorescence: 0.1 us – 10 s or longer
Deactivation ProcessesDeactivation Processes
Luminescence competes with:
• Vibrational Relaxation
• Internal Conversion
• External Conversion
• Intersystem Crossing
Deactivation ProcessesDeactivation Processes
Vibrational Relaxation
• Result: Emission lines are Stokes shifted
• Longer λ’s and lower v’s
Energy Level Diagram for a Photoluminescent System
Fig. 15-2
Deactivation ProcessesDeactivation Processes
Internal Conversion
• Molecule passes to a lower energy
electronic state without emission
• Bound State (e.g., S2 → S1)
• Unbound State:
• Predissociation
• Dissociation
Energy Level Diagram for a Photoluminescent System
Fluorescent Excitation and Emission Spectra of Quinine
Fig. 15-3
Deactivation ProcessesDeactivation Processes
External Conversion
• Excited state molecule collides with solute
or another molecule
• Energy is transferred: collisional quenching
Energy Level Diagram for a Photoluminescent System
Fig. 15-2
Deactivation ProcessesDeactivation Processes
Intersystem Crossing
• Excited state electron spin flips
• S1 → T1
• Common in molecules with heavy atoms: I, Br, etc.
Energy Level Diagram for a Photoluminescent System
Fig. 15-2
Quantum YieldQuantum Yield
Method to express amount of luminescence
• Ratio of number of molecules that luminesce
to total number of excited molecules
• Expressed in terms of rate constants:
where knr = kvib + kic + kec +kisc + kpd + kd
• varies from 0 to 1
nrf
f
kkk
φ
φ