Optical absorption spectra of I3 - molecular ions in KI crystals and aqueous solutions

A semiempirical molecular orbital method including spin-orbit interaction is used to study optical absorption spectra of I3 - molecular ions in (KI + I2) aqueous solutions and in KI crystals coloured additively or electrolytically. The splitting of two strong absorption bands observed for these specimens is considered to be due to the spin-orbit interaction in the iodine atoms. The values of the spin-orbit coupling constants for I3 - in aqueous solutions and coloured KI crystals are estimated to be 0·56 and about 0·47 eV, respectively. The oscillator strengths of the two strong bands for these specimens is also discussed.