Anharmonic interatomic potentials of diatomic and linear triatomic molecules studied by extended x-ray-absorption fine structure

The first four cumulants of a radial distribution function of diatomic molecules and the first three cumulants for linear triatomic systems have been derived quantum statistically. These moments are directly related to the second- to fourth-order force constants of the chemical bond. The temperature dependence of extended x-ray-absorption fine structure spectra of diatomic Br-2 and linear triatomic HgBr2, HgCl2, AuBr2-, and CuBr2- systems has experimentally been investigated not only for the single-scattering paths but for the multiple-scattering paths. The resultantly obtained cumulants have successfully given the force constants, which are found to be in good agreement with the vibrational data. The differences of the force constants between the gas and solid phases are found even for the molecular crystals of HgBr2 and HgCl2.