X-ray Spectroscopy of Molecules Driven by Strong IR Fields

Sammanfattning: The current thesis deals with one important branch of the physics of ultrafast processes, namely modeling of femtosecond nuclear dynamics. We suggest a new type of time resolved spectroscopy, the phase sensitive infrared-x-ray pump probe spectroscopy, which combines rich opportunities of IR laser techniques in quantum control of molecular systems with the site selectivity of x-rays. We have developed and applied a dynamical theory of x-ray pump-probe spectroscopy to study different molecular systems. Special attention is paid to design of the wave packets of desirable shape and spectral composition. Such a quantum control of the nuclear wave packet enables the study of molecular properties in regions that are unavailable by standard x-ray spectroscopies. The IR - x-ray pump probe spectroscopy is nicely suited to perform mapping of wave packet trajectories, to study revival phenomena, femtosecond chemical dynamics, and proton transfer, to mention a few examples.Our simulations show that the phase of the infrared pulse strongly influences the trajectory of the nuclear wave packet, and hence, the x-ray spectrum. Such a dependence is caused by the transfer of the phase of the IR field to the wave packet through the interference of the one (x-ray) and two-photon (IR + x-ray) excitation channels. The time resolved x-ray spectra are sensitive to the shape, duration and delay time between the pulses. The phase of the IR pulse influences the molecular dynamics also when the Rabi period becomes comparable with the period of vibrations, breaking down the rotating wave approximation. We predict a phase memory effect which is a promising technique in studies of chemical dynamics on different time scales. It is shown that the final state interaction with the pump affects the probe spectrum when the pump and probe pulses overlap.In a further step, we explore the electronic recoil effect in x-ray photoelectron spectroscopy, which has recently attracted attention of experimentalists due to its sensitivity to intramolecular interaction. We show that an IR field enhances the manifestation of the recoil effect through the formation of extensive vibrational wave packets. The theory of x-ray Raman scattering from molecules with strong spin-orbit coupling accompanied by electron-hole interaction is developed and applied to simulations of resonant x-ray Raman scattering of the HCl molecule. Special attention is paid to the theoretical methodologies to reduce the computational cost of our wave packet codes.