THEORY OF THE ABSORPTION SPECTRA OF HALIDE IONS IN SOLUTION
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Publisher:Ohio State University
The characteristic ultraviolet absorption spectra of the halide ions in aqueous solution are perhaps the simplest and most important examples of an electronic spectrum characteristic of an ion in solution. Many mechanisms have been proposed in explanation of these spectra. Most of the models do not give any insight into details of the spectra, and are not in harmony with recent photochemical evidence; all have a strong element of ad hoc assumption. A new theory of the mechanism of the light-absorption process which is responsible for the spectra is presented. The absorption act transfers an electron from the state in which it is bound in the negative ion to one in which it is bound principally to a thick sheath of water molecules adjacent to the halogen atom. By virtue of the alignment of the water dipoles in the field of the halide ion, these water molecules give rise to an attractive field for the excited electron which is pseudo-Coulombic with effective charge roughly one-half the electronic charge. The state to which the electron is brought by light absorption is one approximating a 2s-Coulombic state, modified slightly by the electronic polarization of the environment and distorted in the region in the interior of the halogen atom. This model makes possible a satisfactory calculation of the positions of the absorption bands, no arbitrary assumptions being necessary, and agrees with a number of other features of the absorption spectra, and with the photochemical consequences of light absorption as well.
Author Institution: Department of Physics, Purdue University
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