>>> Jacob Keller <j-kell...@fsm.northwestern.edu> 10/13/10 2:50 PM >>> Isn't the simplest answer that this is not really an Argand diagram with real and imaginary axes, but simply a diagram showing the graphical addition of the component phasors of the atom's scattering components? And aren't real and imaginary relative terms anyway (in more ways than one, I guess!)?
Can we say: scattering factors are relative to a non-anomalously scattered wave from the center of the atom. f' is horizontal, f" vertical Structure factors are relative to a non-anomalously scattered wave from the origin of the (any) unit cell. so both f' and f" are shifted by the same amount, which depends on the component of the vector from the origin along the scattering vector. They are still perpendicular, but no longer horizontal and vertical. So when you set your detector distance, you need to make sure there is an even number of wavelengths between the origin of a unit cell and the center of detector, so you get the phase right (at least in the center). No wait, I guess the detector cant detect phase. At this point. phase is really relative, it just matters whether the waves scattered by the different units of electron density are in phase or out of phase with each other. Move the detector back another half-angstrom and you rotate the argand diagram 90*. But we don't care since we can't measure phase. As we move back along the beam the relative phases of the different scatterers remain the same, and it is convenient to pretend we are at a point in phase with non-anomalously scattered wave from the origin of the unit cell in order to calculate the intensity.
