Luminescence spectra and vibrations in crystal lattices
A general survey of the known luminescence spectra of crystals is given. Those which give discrete spectra have been divided into seven classes. The luminescence spectrum of each of these crystals is examined to see what evidence it gives regarding the nature of the vibrations in the crystal lattices. Wherever possible the information thus obtained is supplemented by the evidence furnished by the complementary absorption spectra. It is shown that there is a mass of striking evidence indicating that vibrational frequencies are sharp and monochromatic in all crystals. Those luminescence spectra, which are not discrete are due to an electronic transition in which one or both the electronic states involved are not sharp and hence no evidence regarding the vibrational frequencies can be gained from them. It is remarkable that over the whole range of crystal structure—from the typical covalent crystal diamond to the typical Van der Waals crystal solid nitrogen— the vibrational frequencies are all sharp and monochromatic. The ionic crystals,e.g., the uranyl salts are also no exception to this rule. In many crystals some of the constituents,e.g., N2 in solid nitrogen, C6H6 in solid benzene, the UO2++ molecule in uranyl salts and so on, behave as if they are in the gaseous state. For example, the vibrational frequency observed in the fluorescence spectrum of solid benzene at very low temperatures is thus the same as that of the ground state of the C6H6 molecule, probably the only difference being that the observed bands are more discrete in the crystal state owing to the absence of the rotational lines.