Light-induced Ultrafast Dynamics in Molecules Probed by NonlinearX-ray Spectroscopic Techniques

The breakdown of the Born-Oppenheimer approximation drives rich dynamics of the coupled vibronic states. Conical intersections are well-known examples of where such complex processes can be probed spectroscopically. They operate as nonradiative ultrafast decay channels for electronically excited molecules with at least two degrees of freedom. Spectroscopic detection of such dynamics, however, still poses a great challenge due to the sub-100 femtosecond timescale of such events, and the rapidly changing energy separation between potential energy surfaces. Current approaches, including transient vibrational and electronic spectroscopic techniques, provide indirect signatures of conical intersections but face limitations in temporal and spectral resolution. This thesis aims to address these challenges by using a variety of modern X-ray techniques. A full quantum approach is preferred for simulating the underlying molecular dynamics. This dynamics is then probed with both static and time-resolved spectroscopic tools, including X-ray absorption, time-resolved X-ray stimulated Raman spectroscopy, and photoelectron spectroscopy with UV and XUV photons. These techniques enable direct monitoring of the coupled electronic and nuclear dynamics, capturing changes in the molecular geometry and in the electronic structure in real time. Key findings include the investigation of an off-resonant X-ray stimulated Raman scheme, which employs attosecond pulse trains to probe ultrafast vibronic coherences generated at conical intersections; the real-time tracking of internal conversion and subsequent bond cleavage with sub-100 femtosecond time resolution in nitrogen dioxide, a small molecule with a big environmental impact; and novel insights into the nonadiabatic relaxation mechanisms in larger molecular systems such as benzophenone and meta-methyl benzophenone. These results highlight the potential of ultrafast X-ray methods to overcome existing limitations, providing high-resolution and direct access to elementary processes, with applications ranging from biochemistry to energy-storage technologies.