FIG. 8-18 A ruby laser. A ruby is a crystal that contains Cr⁺³ ions, which are chromium atoms that have lost three electrons each. A Cr⁺³ ion has a metastable level whose lifetime is about 0.003 s. The xenon flash lamp excite the Cr⁺³ ions to a level of higher energy from which they fall to the metastable level by losing energy to other ions in the crystal. Photons from the spontaneous decay of some Cr⁺³ ions cause other excited Cr⁺³ ions to radiate. The result is a large pulse of single-frequency, coherent red light from the partly silvered end of the rod.