The interaction between light and matter and the resulting photochemical reactions are of paramount importance, in natural processes like photosynthesis or the radiation damage in our genetic code, as well as in artificial systems such as photocatalysts or molecular machines. Excitation of molecules by light opens up the possibility to survey the ultrafast dynamics of these processes with the help of a variety of spectroscopic techniques. Attosecond pulses in the extreme ultraviolet (XUV) region can be used to create electron wave packets in highly excited states of molecules. They also enable deeper insights due to their element, charge and electronic state sensitivity by accessing the inner valence (in the XUV) and the core level states (in the soft X-ray region) of elements. Build on top of these properties, especially attosecond transient absorption spectroscopy (ATAS), could be established as a powerful tool, capable of resolving coupled non-adiabatic electronic-nuclear dynamics with great spectral and temporal resolution. But the interpretation of these spectra can be quite complicated. Here the simulation of both the ultrafast dynamics after excitation and the corresponding XUV/X-ray absorption spectrum can help in the understanding of these complex interaction between light and matter.

Using ab initio non-adiabatic molecular dynamics (NAMD), we are able to simulate the ultrafast processes after laser excitation, making it possible to resolve both the changes in the electronic structure and the nuclear motion over time. Based on this information we are able to calculate the time-dependent XUV/X-ray absorption spectra, with the help of our protocol based on the multi-reference methods RASSCF and RASPT2.