Coherent sources of extreme uv

Light sources in the extreme ultraviolet wavelength regime—from 104 nanometers down to the start of the x‐ray region at about 10 nm—are of particular interest for molecular spectroscopy and solid‐state surface studies. But this xuv domain poses special difficulties. The beams must be propagated in vacuum, but at these wavelengths there are no longer any solid materials available for vacuum windows or the harmonic generation of xuv light from longer wavelength sources. Crystalline lithium flouride, the last of the solids to maintain its transparency in the ultraviolet, becomes completely opaque at 104 nm. Beams of xuv can of course be generated by synchrotron light sources, but their usefulness is limited: The synchrotron beams do not provide coherent xuv light. Their spectral brightness (power per unit wavelength interval) is at present marginal for high‐resolution molecular spectroscopy. Furthermore, synchrotron beams are available only at large storage‐ring facilities; they cannot be brought into the experimenter’s home laboratory.