Fluorescence is one of the possible mechanisms by which a substance that has absorbed light gets rid of the energy — by emitting light. It sounds simple. Light goes in, light comes out. But it can be tricky because of the large number of molecular states available.

You are probably familiar with some objects that display fluorescence, such as white T-shirts that glow under UV light sources. The reason that these materials appear to “glow” is that they are able absorb ultraviolet light (which the human eye cannot see) and re-emit it as light of a longer wavelength, which is in the visible spectrum, which we can see! Fluorescent materials give our eyes access to light that would normally be invisible to us by this absorption-emission process.

But not all fluorescent processes require UV light. Some, like chlorophyll, can absorb light in the violet/blue region and emit light in the red region. As you may have seen before in a biology course, if a violet or blue light is shone through a sample of spinach extract, the solution turns red in color.

The important observation about fluorescence is that the emitted photon of light is of lower energy (has a lower frequency — a longer wavelength) than the photon of light that was initially absorbed.

You might reasonably ask, “Well, if the emitted photon is lower in energy than the absorbed photon, what happened to the rest of the energy?” 

For absorption on molecules, the answer lies in the combination of two different types of energy levels: electronic states, that have to do with changing the state of individual electrons, and molecular states (vibrational and rotational), which have to do with changing the relative states of motion of the whole atoms or larger pieces of the molecule.

Excitations of electrons in atoms (electronic states) tend to be fairly large, typically on a scale of an electron volt. Vibrational and rotational states deal the periodic motion of the atoms within a molecule, just like the mass-spring model discussed in the Quantum oscillators page. These tend to have smaller energy gaps than electron states (typically on a scale of milli-electron volts).

For each electronic state, a molecule can be in one of a number of closely spaced multiple vibrational and rotational states. So each electronic energy level gets split into many nearby levels. (For more discussion, see the page on Molecular Spectra.)

Transitions between close vibrational energy levels tend to involve interactions with other molecules or atoms, whereas electronic transitions between groups of levels tend to involve photons.

This is modeled in the diagram (at right), where an incoming blue photon is absorbed, causing a transition from ground state into a high vibrational level of the 1st excited state. The molecule then undergoes collisions with other molecules, resulting in several transitions between vibrational/rotational levels. This essentially shares the vibrational energy with neighboring atoms or molecules, a process which takes a few picoseconds, comparable to the typical time for vibrations between atoms. This is part of the thermalization of the energy — losing energy to "heat". The key to fluorescence is that this equilibration has enough time (much longer than picoseconds) to take place, long before the next step happens — the emission of a photon (that typically happens after a few nanoseconds ).

This difference in timescales between the picosecond equilibration of vibration energies and nanosecond time it takes to emit a photon is the reason we can draw the diagram above. The vibration equilibrium typically happens faster than the fluorescent photon emission. As a result, the fluorescent photon is emitted from the band of states in thermal equilibrium states. The molecule can return back to the ground level, by emitting a photon.

The overall process therefore results in the absorption of a high energy photon into the internal motions of the molecule, the sharing of some of that energy with the thermal bath through collisions, and the emission of a low energy photon. The whole process is called fluorescence.

A more realistic situation can involve additional transitions, including flipping the spins of coupled electrons from single (S) to triplet (T). A diagram of the transitions associated with the fluorescence of a typical molecule is referred to as a Jablonski diagram and one is shown below. 

For some fluorescences, such as that for phosphorous, another transition labeled "intersystem crossing" has to take place before a photon is emitted. This occurs at a much longer time scale — milliseconds to minutes later. A number of materials with such huge time delays between light absorption and light emission were discovered in the last few years and have been used in  useful applications such as glow in the dark clothes.

Kim Moore, John Giannini, Wolfgang Losert, & Joe Redish 4/30/13

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Last Modified: July 10, 2019