Modeling light induced dynamics in molecules: X-ray spectroscopy and laser control

The interaction of light and molecules is a fundamental phenomenon which plays a crucial role in so diverse fields as physics, chemistry, biology as well as in many technological applications. A long standing dream of generation of scientists have been to capture and understand light-induced molecular dynamics in act, by recording a molecular movie. To achieve this, time-resolved spectroscopy has been developing at a tremendous pace, embracing wider and wider spectral ranges (from IR to X-ray), while at the same time addressing shorter and shorter time-scales (from 10−9 to 10−18 seconds) and increasing spatial resolution. Despite these remarkable achievements, experimental spectra always require to be interpreted. Theoretical calculations are therefore a key ingredient to successfully understand the recorded results and guide the experimental effort. The work presented in this thesis takes up the challenge of predicting and interpreting molecular movies by developing innovative and efficient simulation methodologies that combine state-of-the-art electronic structure theory with highly accurate quantum dynamics simulations. These have been applied to design from scratch and measure the performance of novel non-linear experiments based on the combination of optical and X-ray pulses. Even though simulating molecular movies is already a formidable challenge... why stopping there? Why not trying to rewrite the whole movie script to give it the desired (happy) ending? The final part of the thesis explores the idea to move from being passive spectators to active directors of the molecular movie itself. This is achieved by proposing a paradigmatic shift in laser control strategies, and demonstrate its ability to steering photochemical events towards the wanted direction with unprecedented efficiency.