The effect always competes with a diamagnetic response of opposite sign due to all the core electrons of the atoms.

This beam uses the X-ray's inherent abilities to interact at the level of atomic nuclei and core electrons.

Usually, only the valence electrons are treated quantum mechanically while the role of core electrons is to reduce the nuclear charge.

Furthermore, the core electrons shield the ion charge magnitude "seen" by the conduction electrons.

The excitation of core electrons is possible, but requires much higher energies generally corresponding to x-ray photons.

Pseudopotentials replace the atomic nucleus and the core electrons by an effective numeric potential.

In resonant excitation, the core electron is promoted to a bound state in the conduction band.

Non-resonant excitation occurs when the incoming radiation promotes a core electron to the continuum.

This pulls the electrons closer to the center, especially the core electrons (closest to the center).

If that electron is removed, presumably the remaining core electrons experience slightly less screening of the nuclear charge and therefore can come closer to the nucleus.