Three-wave grazing-incidence X-ray diffraction from thin crystal surface layers: determination of triplet phase invariants

Numerical calculation of the angular and spectral distributions of the intensities of the specularly diffracted waves in the case of three‐wave grazing‐incidence X‐ray diffraction is carried out using the dynamical theory. The angular and spectral distributions are shown to be uniquely and continuously dependent upon the value of the triplet phase invariant. A method of determining the value of the triplet phase invariant for thin crystal surface layers is developed, based on the comparison of experimentally measured three‐wave peak profiles with the profiles calculated for different values of the triplet phase invariant. An analysis scheme of the phase sensitivity of the reflection coefficients is proposed taking into account the interference of the directly excited and the `Umweg'‐excited specularly diffracted waves.