The Photoconductivity of Sodium Chloride in the Far Ultraviolet

Abstract

Measurements have been made on the wave-length dependence of the photoconductivity of an uncolored sodium chloride crystal in the far ultraviolet region of 900 to 1350 angstroms in order to test the theories of Peierls, Frenkel, and Mott regarding the existence of discrete energy levels, called exciton levels, lying just below the conduction bands. A region was found between 1300 and 1350 angstroms in which there was large absorption but very small if any photoconductivity or external photoelectric effect indicating the existence of exciton states. At wave-lengths shorter than 1300 angstroms both the photoconductivity and the photoelectric effect began to set in and these showed a doublet structure. The photoconductivity rises abruptly at the second absorption peak in the absorption spectrum and shows structure which locates energy levels above the ground state. The experiments of Podubnij on the iodides are a contradiction to this experiment inasmuch as the alkali halides should reasonably be expected to behave similarly.