Anharmonicity of the bending and stretching vibrations observed in extended X-ray absorption fine structure of tetrahedral molecules

We have measured and analyzed the temperature dependence of extended X-ray absorption fine structure (EXAFS) spectra of tetrahedral systems MBr(4) (M = C, Si, Ge). The EXAFS analysis by means of the cumulant expansion technique enables one to obtain information about force constants including the third-order anharmonicity. The second-order cumulants obtained experimentally are in excellent agreement with the values expected by the vibrational data and the third-order cumulants have been determined successfully. For the first nearest neighbor (NN) Br-M shells the stretching motions are apparently dominant to describe EXAFS, while for the second NN Br-Br shell the bending modes are found to contribute significantly to the cumulants especially for the third-order anharmonicity. The obtained force constants are compared to each other and the origin of observed bending anharmonicity is discussed.