ABSTRACT
FactorThe two parameters f H and PH (coherent fraction and coherent position, respectively) are related to the XRD structure factor FH, which represents (neglecting dispersion corrections) the Fourier transform of the total electron density function re(r). By contrast, f AH and PAH are amplitude and phase, respectively of the (complex) Fourier coefficient of an particle density function specific for an element A, which is selected by spectroscopy. In fact, even a particular chemical species of an element can be selected by spectroscopy. The analogy and differences of XSW and XRD results had already been pointed out [2]. The two parameter coherent fraction f H and coherent position PH can be directly compared with the more familiar X-ray structure factor, defined by the following equation: F c Fj
= = Â 11 and F N f f if ij j j
Q Q r= + + - ◊ -
= Â1 01( ' '')exp( ) . (5.18) Here, Q is a reciprocal lattice vector, the index j counts the
different elements and fj0, f j¢ and f j¢¢ are the atomic form factor, dispersion correction, and absorption coefficient of a particular element labeled by j. With 2pH = Q, the XSW “structure factor” GH can be identified as follows: G F f f f F iPj j j jH Q H H= + + =/( ' '') exp( )0 2p , (5.19)where the element j is selected by spectroscopy. Thus, an XSW measurement directly determines an atomic distribution whereas an X-ray diffraction experiment measures the electron density distribution.
