Light matter interactions are usually described in the dipole approximation.

This approximation relies on the fact that the molecule is much

smaller than the wavelength of the incoming light. However, in the

X-ray regime where the wavelength becomes comparable to the dimensions

of the molecule, this approximation begins to break down.

A more exact treatment is given by the minimal coupling Hamiltonian,

which involves the current densities and charge densities, rather than transition dipoles.

 

Mukamel's postdocs have developed a framework, which extends the

the well known, diagram based, pertubative formalism for the use with current

densities and charge densities.

This new method allows to work in the intermediate regime between diffraction and optical excitation.

From a quantum chemist's perspective working with charge densities is even more

intuitive than working in the dipole approximation, explains Markus Kowalewski.

 

 

Jeremy. R. Rouxel, Markus Kowalewski, Shaul Mukamel,

"Current vs Charge Density Contributions to Nonlinear X-ray Spectroscopy",

J. Chem. Theor. Comput., 12, 3959 (2016).