Properties of a detuned non-dispersive double crystal monochromator

Detuning of a non-dispersive double crystal X-ray monochromator is e.g. used to suppress higher order harmonics in the monochromatic beam. The applied offset can be a significant fraction of the rocking curve width and results in a deviation of the reflected monochromatic beam both in angle and energy. Here the wavelength distribution was studied by measuring the Au-4f photoemission while detuning either the first or second crystal of a Si-1 1 1 monochromator at 3500 eV. While the angular deflection corresponds to twice the relative detuning angle in either case, the energy distribution exhibits two components with the same bandwidth, but with a different dispersion depending on the detuned crystal. For large angles, the components are clearly separated and yield well resolved replicas of all spectral features. The observed behavior is in agreement with model calculations. For a reduced energy shift, the detuning of the second crystal is preferable.