In the past, I've done columns on source lamps and light-emitting diodes (LEDs), but there's one kind of light that is becoming
more popular in our everyday lives — the fluorescent light. Here, we'll look at how these lights are different, spectroscopically,
than the old-fashioned incandescent light bulb.
Thomas Edison wasn't the first person to work on incandescent light bulbs — indeed, as early scientists as Humphrey Davy and
Alessandro Volta tried to use electricity to heat a substance to glowing hot. However, Edison was the first to make a practical,
commercially viable light bulb. As culture-changing an impact as incandescent light bulbs were, they have one major problem:
inefficiency. As much as 90% of the energy given off by an incandescent light bulb is heat. That may be useful if you live
at the North Pole, but in most temperate climes it simply adds to the temperature increases that must be combated with air
conditioning. Incandescent light bulbs are not optimal sources of light.
There is more than one way to generate light, though. It uses ideas of quantum mechanics instead of thermal physics.
Fluorescence is a process by which a substance absorbs light and then emits light of a different wavelength. In most cases, fluorescing
materials emit light of lower frequency and energy than is absorbed, although there are occasionally two-photon emissions
in which the emitted light is a higher energy. The word "fluorescence" was coined by British physicist G.G. Stokes in 1852
after the mineral fluorite (crystalline CaF2), which fluoresced strongly because of impurities. It was observed as early as the 1560s, but it was only in the mid-19th
century that Stokes described the phenomenon after experimenting with ultraviolet light (which itself was only identified
as part of the spectrum in 1801).
Figure 1: Schematic of the fluorescence process: 1 = excitation, 2 = relaxation, and 3 = emission. The initial and intermediate
excited states can be different electronic states or even two different states within the vibrational manifold of the same
electronic state. See text for details.
The mechanism of fluorescence had to wait until the understanding of quantized energies in atoms in molecules, but a simplified
version of the mechanism is shown in Figure 1. An atom or molecule absorbs a photon of light (step 1 in Figure 1). In the
finite but brief time the system is in the excited state, it loses energy via some mechanism, like collisions with solvent
molecules or vibrational energy transfer to neighboring atoms or molecules. This step (step 2 in the figure) is generally
referred to as "nonradiative relaxation" or "radiationless decay." The energy loss stops at some intermediate but lower energy
state. Then, the system emits a photon and returns to the ground (or some other lower) state (step 3 in the figure). Because
the intermediate state is lower in energy than the initial excited state, the emitted photon is lower in energy than the exciting
photon, leading to an apparent wavelength or color shift; this is called the Stokes shift in honor of the aforementioned British physicist. Finally, in fluorescence processes, the energy states involved have the
same multiplicity (that is, overall electron spin), so the shifts between states are quantum-mechanically allowed and so are
fairly fast — on the order of nanoseconds. Thus, we experience fluorescence processes as immediately connected to the presence
of source of the exciting photons. (Contrast that to phosphorescence, which involves a spin-forbidden transition and therefore
is relatively slow, having lifetimes on the order of minutes or hours.)
Many minerals and organic molecules fluoresce. Geology uses fluorescence to help identify certain minerals and gemstones.
Quinine, a natural antimalarial compound found in the cinchona tree, fluoresces, as does petroleum jelly. Green fluorescent
protein (GFP) is a 238-amino acid protein used widely in molecular and cell biology; its developers won the 2008 Nobel Prize
in Chemistry as a tribute to its importance. Fluorescence spectroscopy is a major type of spectroscopy in its own right —
but that's another column.