Nuclear Dynamics in Linear and Non-linear X-ray Processes

Abstract: New generations of brilliant synchrotron radiation sources and intense X-ray and optical lasers have opened unprecedent opportunities in linear and non-linear spectroscopies of molecules. This together with new instrumentation calls for further development of theory to explain observations and suggest new experiments. These two goals are the main objectives of the present thesis.In the first part of the thesis we study resonant inelastic X-ray scattering of the oxygen molecule. Recent experimental data of O2 show the dependence of the vibrational profile on the polarization of the X-rays. This vibrational scattering anisotropy is explained by the interference of the resonant and nonresonant inelastic scattering channels in O2.The second part of the thesis explores X-ray lasing caused by ultrafast dissociation of the HCl molecule in a repulsive core-excited state pumped by an X-ray free-electron laser. We show that the lasing efficiency is drastically enhanced due to the self-trapping of the amplified pulse into the ridge of the gain.The third part of the thesis is devoted to another non-linear phenomenon - optical limiting. Our time-dependent simulations are in agreement with the experimental data and show that phthalocyanines with heavy central metals exhibit good optical limiting properties for nanosecond pulses.Symmetry and the symmetry breaking are important fundamental concepts in physics, chemistry and biology. In the next part of the thesis we study theoretically the novel symmetry breaking mechanism in resonant Auger scattering. Both theory and electron-ion coincidence measurements for the oxygen molecule evidence the symmetry breaking of the oxygen cation caused by momentum exchange between the fast Auger electron and counter propagating dissociating oxygen atoms.The last part of the thesis is devoted to the role of the rotational degrees of freedom in X- ray ionization of molecules. We present here new results related to the rotational Doppler and angular recoil effects in X-ray ionization using the N2 and HCl molecules as illustrating examples. The dependence of the ionization cross section on the direction of ejection of fast electrons explains the sensitivity of the rotational broadening to the final state and to the polarization of X-rays observed recently for the N2 molecule. The theory reveals the novel effect of site-selectivity of X-ray valence ionization caused by the excitation of the rotational degrees of freedom which is different for different ionization sites. Large recoil angular momentum related to the ejection of an electron from the light hydrogen atom results in a red shifted broad pedestal in the X-ray photoelectron spectrum of HCl.