Prominent absorption lines are seen when X-rays eject or promote or (inner shell) electrons. The resulting cascade of electrons to fill the vacated low energy state produces photons whose wavelengths are element-specific. The two most common forms of X-ray spectroscopy are powder diffraction and X-ray fluorescence. In the early 1980's, the Stanford and Brookhaven synchotrons came on line, producing X-ray intensities on the order of 105 times as large as previously available. The high flux gave enough signal to do X-ray spectroscopy on solids, including the identification of individual atoms. H, He, and Li have peaks which are too hard to see.
Electron Microprobe, X-Ray Fluorescence Spectroscopy
