It is well known that the absorption spectra of some crystals may be permanently changed if the crystals are exposed to photons with energies of tens or hundreds of electron-volts. It is therefore desirable, when studying absorption of far ultraviolet radiation in an ionic crystal, to expose it only to radiation of the wavelength for which its absorption is to be determined, rather than to a wide range of wavelengths. The usual techniques do, in fact, expose the crystal to the whole spectrum over which absorption measurements are equired - either simultaneously (before dispersion) or sequentially (as with a scanning monochromator).
Evidence for a Transient Emission Feature at 1524 Å in the Far Ultraviolet Spectrum of 21 Com
