Chemiluminescence

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.