Infrared - X-ray pump probe spectroscopy

Abstract: The present thesis concerns theoretical studies of molecular interactions investigated by infrared and X-ray spectroscopic techniques, with emphasis on using the two technologies combined in pump probe experiments. Three main types of studies are addressed: the use of near-edge X-ray absorption fine structure spectra (NEXAFS) to manifest through-bond and through-space interactions; the role of hydrogen bonding on the formation of X-ray photoelectron spectra as evidenced by simulations of the water dimer; and the development of theory, with sample applications, for infrared X-ray pump probe spectroscopy - the main theme of the thesis. Ab initio calculations indicate that NEXAFS spectra give direct information about the through-bond and through-space interactions between vacant non-conjugated π' orbitals. It is found that the X-ray photoelectron spectrum of the water dimer differs strongly from the monomer spectrum in that two bands are observed, separated by the chemically shifted ionization potentials of the donor and the acceptor. The hydrogen bond is responsible for the anomalously strong broadening of these two bands. The studies show that X-ray core electron ionization of the water dimer driven by an infrared field is a proper technique to prove the proton transfered state contrary to conventional X-ray photoelectron spectroscopy. Our simulations of infrared X-ray pump-probe spectra were carried out using wave packet propagation techniques. The physical aspects of the proposed new X-ray spectroscopic method - phase sensitive Infrared - X-ray pump probe spectroscopy - are examined in detail in two sample applications - on the NO molecule and on the dynamics of proton transfer in core ionized water dimer. It is found that the phase of the infrared pump field strongly influences the trajectory of the nuclear wave packet on the ground state potential. This results in a phase dependence of the X-ray pump probe spectra. A proper choice of the delay time of the X-ray pulse allows to directly observe the X-ray transition in the proton transfered well of the core excited potential.