Light is a transverse wave. Therefore, light in the optical range (i.e. visible light) couples to transverse collective excitations of a material when measuring the optical conductivity for instance. Likewise, X-rays are also transverse waves, but somehow couple to longitudinal collective excitations of a material.
I understand that it is because X-ray photons couple to the electron density as detailed in this paper, but what I am looking for is a plausible intuitive explanation with some physical insight as to why this might be so.
(Sorry if you can't view the paper from the publisher without incurring a fee, but I couldn't find it anywhere else. I think the question is clear enough without the link, though.)