Determination of the I2 bond-length distribution in liquid, solid and solution, by extended x-ray absorption fine structure spectroscopy

Abstract

A systematic extended x-ray absorption fine structure (EXAFS) study of the structural parameters of the I2 molecule has been performed for I2 liquid, solid and in low density gaseous phases, as well as for I2 dissolved in organic solvents. Measurements on gaseous HI have also been performed in order to refine the structural analysis. A complex atomic double-electron excitation background at the I K-edge has been observed. A careful data analysis with full statistical evaluation of the results has indicated undoubtedly that, in condensed phases, the average bond length can increase by as much as 0.03 Å with respect to the free molecule value. The expansion, known to occur from gas to liquid, and solid I2, was confirmed by the present EXAFS analysis while in the case of I2 in solution this is the first determination of the bond length. In particular we find that the bond length increases monotonically while decreasing the ionizing potential of the solute. A linear correlation between the blue-shift of the optical absorption I2 band and the increase of the bond length has been found.