Extreme ultraviolet

Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 124 nm down to 10 nm, and therefore (by the Planck–Einstein equation) having photons with energies from 10 eV up to 124 eV (corresponding to 124 nm to 10 nm respectively). EUV is naturally generated by the solar corona and artificially by plasma and synchrotron light sources.

Its main uses are photoelectron spectroscopy, solar imaging, and lithography.

In air, EUV is the most highly absorbed component of the electromagnetic spectrum, requiring high vacuum for transmission.

EUV generation

Neutral atoms or condensed matter cannot emit EUV radiation. Ionization must take place first. EUV light can only be emitted by electrons which are bound to multicharged positive ions; for example, to remove an electron from a +3 charged carbon ion (three electrons already removed) requires about 65 eV. Such electrons are more tightly bound than typical valence electrons. The existence of multicharged positive ions is only possible in a hot dense plasma. Alternatively, the free electrons and ions may be generated temporarily and instantaneously by the intense electric field of a very-high-harmonic laser beam. The electrons accelerate as they return to the parent ion, releasing higher energy photons at diminished intensities, which may be in the EUV range. If the released photons constitute ionizing radiation, they will also ionize the atoms of the harmonic-generating medium, depleting the sources of higher-harmonic generation. The freed electrons escape since the electric field of the EUV light is not intense enough to drive the electrons to higher harmonics, while the parent ions are no longer as easily ionized as the originally neutral atoms. Hence, the processes of EUV generation and absorption (ionization) strongly compete against each other.