Electromagnetic radiation whose wavelength is smaller (and the frequency higher) than that of visible light, is called "ultraviolet radiation" or UV radiation. The name refers to the radiation that comes beyond the "violet", the shortest wavelength of light that is just visible. The boundary between violet and UV radiation, which is invisible, is 380 nm.
The range of UV radiation extends to the range of X-rays whose wavelength is less than 4 nm (less than 1 nm according to other sources). UV radiation is a part of solar radiation. However, radiation with a wavelength smaller than 280 nm does not reach the surface of the earth but is absorbed in the high atmosphere. This short-wave radiation is called “ionising radiation” because it can excite atoms so heavily that they are transformed to become ions; it also can split molecules and transform them to become radicals. UV radiation is absorbed in this process. In the layer of air around the earth, O2 molecules are split into oxygen radicals by ionising UV radiation. These oxygen radicals can recombine to become O2 or O3 molecules. O3 molecules are referred to as “ozone”. This reaction absorbs practically all ionizing UV radiation in very high air layers. Therefore, there is no significant radical formation in deeper layers and thus no significant ozone formation. For this reason, the "ozone layer" is limited to high atmospheric layers. Conversely, UV radiation, in particular ionizing radiation, is emitted when positive ions capture free electrons and thus recombine them into neutral atoms. The recombination to the atom in the basic state takes place in the plasma parallel to ionization, excitation and radical formation. This is why UV radiation is also generated continuously (when excited atoms return to the basic state, less energy-rich visible radiation is generated, the typical plasma luminescence). The UV radiation generated in the plasma also causes radicals to form on substrate surfaces, converting solid particles into volatile particles. This process is of major importance in cleaning, activation and etching in the plasma. von wesentlicher Bedeutung.