Ultrafast imaging of coherent molecular dynamics at the linac coherent light source [electronic resource]
Keywords: Applied Physics

In this thesis I will describe some of the techniques we have developed to extract coherent vibrational and rotational wavepacket motion from stochastic ultrafast free electron x-ray light sources such as the Linac Coherent Light Source. In the initial experiments we performed, the ultrafast x-rays themselves created vibrational wavepackets that were probed by an ultrafast optical laser, and to the best of our knowledge this is the first example of ultrafast coherent preparation of a wavepacket using x-rays. The x-rays were also utilized to probe rotational wavepackets formed by the optical laser. These findings led to a second series of experiments where we sought to use the x-rays as a probe of non Born-Oppenheimer dynamics. The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features produced by coherent rotational wavepackets were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. To overcome this limitation we analyzed the ion time of flight traces using a manifold embedding and nonlinear singular value decomposition techniques in collaboration with Abbas Ourmazd and Russell Fung at the University of Wisconsin Milwaukee. This analysis automatically separated the alignment and dication dissociation dynamics from the data, and it revealed fast dynamics that we can attribute to coherent vibrational wavepackets that were created by the ultrafast x-rays and probed by the optical laser. A second study we performed looked at the feasibility of using the LCLS as a fast Coulomb explosion probe of systems undergoing fast laser induced dynamics beyond molecular alignment. As the primary mechanism of energy transfer in natural chemical systems, developing a fundamental understanding of non-radiative excited-state transfer via conical-intersections is of utmost importance for many fields such as biochemistry, alternative energy research, and quantum control. In general, the relevant chemical systems are complex and the exact energy transfer pathways are hard to discriminate from other dynamics. We conducted an experiment to use ultrafast optical and x-ray lasers to induce and observe time resolved molecular dynamics of an optically induced conical-intersection in a prototype system of molecular iodine. Few-femtosecond x-ray pulses from the LCLS then rapidly ionized the molecules without additional strong-field dressing of the potential energy surfaces under investigation. We used optically-dressed molecular iodine as a well controllable analog of a natural conical-intersections. Additionally, we implemented a molecular alignment technique that should prove generally applicable to numerous future LCLS experiments and may compliment condensed-phase x-ray imaging studies of molecular dynamics.