The effect of thermal vibrations on extended x-ray absorption fine structure. II

Abstract

A formalism is introduced to describe the dependence of the EXAFS amplitude and phase on thermally excited vibrations. The EXAFS expression for a general three‐atom system of C_s symmetry is discussed in detail. To lowest order, thermal vibrations induce a change only in the scattering path length and give rise to the standard EXAFS expression for each scattering path damped by the appropriate Debye‐Waller factor. A change in scattering angle, however, is a higher order effect and gives rise to additional EXAFS terms called modification factors. There is a modification factor associated with each multiple scattering path. Within this formalism, the correlation which exists between a change in scattering path length and a change in scattering angle is explicitly calculated. The normal modes which contribute to the modification factors are discussed, and shown to be dependent on the symmetry of the system. The nature and temperature dependence of the Debye–Waller and modification factors for the three‐atom system Br_2O are discussed as a function of bridging angle. The modification factors are shown to make a small contribution to the total EXAFS of the system. The magnitude of this contribution is dependent on the degree of correlation that exists between changes in scattering path length and scattering angle.

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