Fluorescence Versus Phosphorescence

Photoluminescence Basics

Photoluminescence occurs when molecules absorb energy. If the light causes electronic excitation, the molecules are called excited. If light causes vibrational excitation, the molecules are called hot. Molecules may become excited by absorbing different types of energy, such as physical energy (light), chemical energy, or mechanical energy (e.g., friction or pressure). Absorbing light or photons may cause molecules to become both hot and excited. When excited, the electrons are raised to a higher energy level. As they return to a lower and more stable energy level, photons are released. The photons are perceived as photoluminescence. The two types of photoluminescence ad fluorescence and phosphorescence.

How Fluorescence Works

In fluorescence, high energy (short wavelength, high frequency) light is absorbed, kicking an electron into an excited energy state. Usually, the absorbed light is in the ultraviolet range, The absorption process occurs quickly (over an interval of 10^-15 seconds) and does not change the direction of the electron spin. Fluorescence occurs so quickly that if you turn out the light, the material stops glowing.

The color (wavelength) of light emitted by fluorescence is nearly independent of the wavelength of incident light. In addition to visible light, infrared or IR light is also released. Vibrational relaxation releases IR light about 10^-12 seconds after the incident radiation is absorbed. De-excitation to the electron ground state emits visible and IR light and occurs about 10^-9 seconds after energy is absorbed. The difference in wavelength between the absorption and emission spectra of a fluorescent material is called its Stokes shift.

Examples of Fluorescence

Fluorescent lights and neon signs are examples of fluorescence, as are materials that glow under a black light, but stop glowing once the ultraviolet light is turned off. Some scorpions will fluoresce. They glow as long as an ultraviolet light provides energy, however, the exoskeleton of the animal does not protect it very well from the radiation, so you shouldn't keep a black light on for very long to see a scorpion glow. Some corals and fungi are fluorescent. Many highlighter pens are also fluorescent.

How Phosphorescence Works

As in fluorescence, a phosphorescent material absorbs high energy light (usually ultraviolet), causing the electrons to move into a higher energy state, but the transition back to a lower energy state occurs much more slowly and the direction of the electron spin may change. Phosphorescent materials may appear to glow for several seconds up to a couple of days after the light has been turned off. The reason phosphorescence lasts longer than fluorescence is because the excited electrons jump to a higher energy level than for fluorescence. The electrons have more energy to lose and may spend time at different energy levels between the excited state and the ground state.

An electron never changes its spin direction in fluorescence, but can do so if the conditions are right during phosphorescence. This spin flip may occur during absorption of energy or afterwards. If no spin flip occurs, the molecule is said to be in a singlet state. If an electron does undergo a spin flip a triplet state is formed. Triplet states have a long lifetime, as the electron won't fall to a lower energy state until it flips back to its original state. Because of this delay, phosphorescent materials appear to "glow in the dark".

Examples of Phosphorescence

Phosphorescent materials are used in gun sights, glow in the dark stars, and paint used to make star murals. The element phosphorus glows in the dark, but not from phosphorescence.

Other Types of Luminescence

Fluorescent and phosphorescence are only two ways light may be emitted from a material. Other mechanisms of luminescence include triboluminescence, bioluminescence, and chemiluminescence.