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In chemistry, chemiluminescence is the emission of light caused by a chemical reaction. The effect is due to one or more product molecule leaving a chemical reaction in an excited state (state of energy higher than the ground—or normal—state). Consecutively, the product molecule loses its excess energy by emitting photons (light quanta). The radiation of a molecule is called luminescence and luminescence caused by a prior chemical reaction is chemiluminescence.

An example is the reaction of nitrogen monoxide with ozone giving nitrogen dioxide (NO2) (in an excited state) and dioxygen (O2) in its ground state. The star indicates an excited state of NO2:

NO + O3 → NO2* + O2
NO2* → NO2 + hν

Here h is Plancks' constant and hν is the energy of the photon emitted by NO2*. Further, ν is the frequency of the emitted light; it is related to wavelength by λ = c/ν where c is the speed of light. Due to vibronic interactions (interactions of electronic with vibrational motions) the electromagnetic radiation emitted by nitrogen dioxide has a range of wavelengths; however, the emission is centered around λ = 1.2 micrometer (μm), which is in the near-infrared.

The first ever observation of chemiluminescence was the glow of phosphorus in the dark.