Fluorescent Lights

Aug 31, 2012

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


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.
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).

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.