X-ray Standing Waves
The Bragg X-ray standing wave approach measures the modulation of X-ray
fluorescence as the crystal is rotated through a substrate Bragg reflection.
Due to the
coherent superposition of the incident and reflected Bragg beams, an X-ray standing
wave (XSW) is formed both above and below the crystal surface. The modulation
of
X-ray fluorescence by the XSW field is directly sensitive the location of the
fluorescing
atoms and provides a powerful probe of interfacial ion site distributions. With
a known
substrate lattice, the fluorescent yield, Y(q), can
be described by two parameters:
coherent fraction, FH, and coherent position, PH,
where H = (hkl) indicates the Bragg
reflection. Because the fluorescing atom is located in the near field, the XSW
measurement
does not suffer from the well-known ‘phase problem’ of X-ray crystallography.