Determination of the I-2 bond-length distribution in liquid, solid and solution, by extended x-ray absorption fine structure spectroscopy

A systematic extended x-ray absorption fine structure (EXAFS) study of the structural parameters of the I-2 molecule has been performed for I-2 liquid, solid and in low density gaseous phases, as well as for I-2 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 Angstrom with respect to the free molecule value. The expansion, known to occur from gas to Liquid, and solid I-2, was confirmed by the present EXAFS analysis while in the case of I-2 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 I-2 band and the increase of the bond length has been found. (C) 1997 American Institute of Physics.