Phys. Rev. Lett. (in the press); preprint at https://arxiv.org/abs/1802.04421

Ultrafast X-ray diffraction promises dynamic information about molecular structure at an unprecedented spatio-temporal resolution. But when contributions from optically excited valence electrons are considered, only a limited share of the total electron density generates useful signals. Being able to visualize electron density changes would help us to obtain structural and electronic information from molecular photophysics. Now, Jérémy Rouxel and co-workers have proposed a variant of X-ray spectroscopy that is especially sensitive to optically active electrons.

Leaning on nonlinear signal generation from interfaces, Rouxel and colleagues propose the combination of an X-ray field with a visible field to create signals through a so-called sum-frequency generation. As electronic coherences contribute to the process, the emitted light is directly sensitive to the transition charge density, which can be viewed as an interference between charge densities from the contributing electronic states. With additional delays and frequency extensions, the method might serve as a flexible tool for experiments at next-generation free-electron lasers.