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