How Do Materials Emit Light? Incandescence Atomic Emission Molecular Fluorescence Phosphorescence Photoluminescence

Incandescence

Thermal Radiation Spectrum Blackbody (thermal) radiation Stefan-Boltzmann Law: I(λ) = σT 4 “Ultraviolet catastrophe” led to quantum mechanics

Atomic Emission HydrogenMercury Neon

Atomic hydrogen energy levels

Molecular emission Fluorescence Phosphorescence Luminescence –Photoluminescence (PL) –Chemical –Triboluminescence –Others

Fluorescence

Luminescence: emission of light from a substance Fluorescence/phosphorescence: emission of light by a substance following excitation of that substance by electromagnetic radiation. Photoluminescence: used in condensed phases when distinction between fluorescence and phosphorescence becomes blurred. ↓ ↓

Fluorescence vs. Phosphorescence Fluorescence: emission between electronic states with same spin multiplicity. Results in short radiative lifetimes. Phosphorescence: emission between electronic states with different spin multiplicities. Results in long radiative lifetimes.

Jablonski Diagram (yah-blon´-ski)

Molecular Spin States Pauli Exclusion Principle: no two electrons in a system can have the same set of quantum numbers. Spin quantum numbers, s = ½, m s = +/- ½. Represent m s = +/- ½ by up arrow, down arrow: ↑ or ↓ ↓↑ ▬

Molecular Spin States, continued How many ways can you put two electrons in two states? ↓▬↓▬ ↓▬↓▬ ↓▬↓▬ ↑▬↑▬ ↑▬↑▬ ↑▬↑▬ ↑▬↑▬ ↓▬↓▬ Singlets { Triplet ↓↑▬↓↑▬

Molecular Spin States Most molecules have even number of electrons, so results in singlets, triplets, quintets, etc. Radicals (molecules with odd number of electrons) will result in doublets, quartets, etc.

Absorption and Fluorescence Typically electronic transitions occur between singlet states in organic molecules. Ground state is labelled S 0. Absorption then occurs to excited electronic states: S 1, S 2, etc. Fluorescence occurs from excited electronic states to ground state: typically S 1 → S 0

Phosphorescence If excited triplet state is populated, phosphorescence can occur: T 1 → S 0 Note: this is a spin forbidden transistion, results in weaker emission strength, longer emission lifetimes. Can distinguish experimentally by doing time resolved spectroscopy.

Fluorescence Decay [S 1 ] = [S 1 ] initial e -t/τ where τ = τ f is the fluorescence lifetime τ f = τ rad + τ nrad