Sources of monochromatic ionizing radiation

When neon is used in a discharge lamp radiation is produced predominantly with two close wavelengths, 74.4 nm and 73.6 nm, corresponding to energies of 16.67 eV and 16.85 eV, making this source rather less useful than the more tmly monochromatic, and more highly energetic. He I source. 

A monochromator is useful not only for removing unwanted lines from the X-ray source but also for narrowing the otherwise broad lines. For example, each of the MgXa and AlXa doublets is unresolved and about 1 cY wide at half-intensity. A monochromator can reduce this to about 0.2 cY This reduction of the line width is very important because in an XPS specttum, unlike an ultraviolet photoelectron specttum, the resolution is limited by the line width of the ionizing radiation. Unfortunately, even after line narrowing to 0.2 cY 

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