What happens to a molecule once it has absorbed UV or visible light? How does the molecule release or convert the extra energy it just received? Answering these questions clearly goes beyond a pure theoretical curiosity, as photochemical and photophysical processes are central to numerous domains like energy conversion and storage, radiation damage in DNA, or atmospheric chemistry. A plethora of theoretical tools have been developed over the past decades to address these questions by simulating the excited-state dynamics of molecules. These methods are often tested and theoretically validated on reduced-dimensionality models or rather simple molecules.

In this seminar, I will present a series of examples in which studying the photophysics and photochemistry of molecules in the gas phase has highlighted the limitations of current theoretical methodologies and stimulated the development of new strategies for excited-state dynamics. More specifically, I will focus on the formation of complex photoproducts, related to atmospheric chemistry and time-resolved spectroscopy, and on the description of photoexcitation.