Electronic Processes in Ultraviolet and Pulse Irradiated Halide Ion Systems. The Effect of Heavy Atoms

Abstract

Air-free, concentrated, aqueous solutions of alkali halides at ∼ 20°C containing small concentrations of heavy metal ions or I2 have been uv irradiated in the induced absorption bands. There was no evidence of chemical change unless an electron acceptor, e.g., N2O was also present when halide oxidation occurred. Luminescence from 1M KCl with 10−4M Tl+ was quenched by N2O and by I−, both systems obeying Stern-Volmer kinetics. In 8M LiCl glass saturated with TlCl, there were three absorption bands at 235, 217, and 187 nm at 77°K. They resemble, both in position and relative intensity, the A, B, and C bands of KCl:Tl+. Analogous results were observed for Tl+-doped aqueous glasses of LiBr and LiI, and all may involve the known halocomplexes of Tl+. The absorption bands in the systems examined are attributed to charge transfer from halide ion to heavy metal cation or to iodine. The relevance of these results to Tl+-doped alkali halide crystals is also considered. The absorption spectra of the diatomic halides MX (GaX, InX, and TlX) with 1Σ+ ground states have in common Π30+, 3π1, and 1π1 excited states. These may correlate with A1, A2, and C bands in ionic lattices. If MX is nearly substitutional in the alkali halide lattice, there are three distinguishable metal-halogen internuclear separations which may account for structure in the C band through Franck-Condon effects. γ-Irradiated LiCl glass at 77°K shows absorption by Cl2− and by trapped electrons and recombination luminescence, induced by optical bleaching of trapped electrons, has both 1 and 9 sec lifetimes. The luminescence and chemical reactivity are attributed to excited halide ions. Pulse radiolysis of neutral chloride ion aqueous solutions is known to yield Cl2−. It has been found that aeration affects G(Cl2−) and, in solutions saturated with N2O or CO2 instead of N2, yields of Cl2− are considerably enhanced.