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Electromagnetic radiation

Electromagnetic radiation consists of two components: the electric and the magnetic field. It can be described as electromagnetic waves, which are synchronized oscillations of magnetic and electric fields and propagate with the speed of light through vacuum. Electromagnetic radiation can be classified into classes comprising different types of interaction with matter. Distinction between the different classes can be made by the frequency f , wavelength l or energy E of the radiation according to the PLANCK-EINSTEIN correlation:

EReq1

with h being PLANCK’s constant (h = 6.6261034 Js) and c being the speed of light (c = 299,792,458 m s-1). In general the following main classes of radiation are distinguished:

  • γ (gammma) radiation,
  • X-radiation,
  • ultraviolet radiation,
  • visible radiation (light),
  • infrared radiation and
  • radio waves.

The way matter interacts with electromagnetic radiation depends on the wavelength of the radiation. The essential requirement for an interaction is that the energy of the radiation is equal to the difference of certain energy levels of the molecule. These can be either electronic or vibrational energy levels. The interacting molecule is excited to a higher energy level. Especially electronic excitations are of interest for photochemical conversions. A change of the electronic level corresponds to a change of the occupation of the molecular orbitals by valence electrons. This results in a change of the chemical nature and reactivity. This can be utilized beneficially to enter reaction pathways not accessible through thermal initiation, namely heating of the reactants.

spektrum

Figure 1:The electromagnetic spectrum. The magnification illustrates the wavelength range relevant for photochemical reaction. Based on[1].

 

  1. PHILIP RONAN: EM spectrum, https://commons.wikimedia.org/wiki/File:EM_spectrum.svg.



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