Photoelectron spectroscopy in the vicinity of a core-ionization threshold

Sammanfattning: The fundamental interaction between photons and free molecules/atoms has been studied by means of soft x-ray photoelectron spectroscopy. Special interest has been paid to the non-resonant and resonant processes in the vicinity of a core-ionization threshold. The C1s threshold of carbon monoxide (CO) was chosen as the prototype case, and a synchrotron radiation light source together with a hemispherical electron energy analyzer were used as the main experimental tools. Observations made in resonant Auger spectra, of both a dissociative (HCl) and a bound (CO) core excited state, demonstrate that such spectra are strongly frequency dependent. This is explained in terms of a generalized duration time for the resonant process, where the lifetime of the core excited state and the inverse of the detuning frequency play equal roles. Results obtained from vibrationally selective core-to-Rydberg resonant Auger spectra have been successfully interpreted within the single-particle hydrogenic model. Also, an angular anisotropy is reported which depends on the vibrational quantum number of the final state.The non-resonant studies have been focused on the influences from post collision interaction (PCI) and shape resonances on the Auger decay. It is shown that the lifetime broadening of the 4d-15/2 and the 4d-13/2 core ionized states of Xe depends on the photon energy. The C1s shape resonance in CO modifies the vibrational progression of the Auger spectrum. This result is interpreted by means of numerical simulations, and it is shown that the de-excitation step is described by the Franck-Condon principle even at the shape resonance.The vibrational structures of large hydrocarbons have been studied by both core photoelectron spectroscopy and ab initio calculations. The results provide information on, for instance, changes in bond lengths and angles upon core ionization.