Temporal Studies of Molecular Processes using Resonant Spectroscopy : Femtosecond experiments with synchrotron radiation and pulsed lasers

Sammanfattning: The physical and chemical properties of matter are to a high degree determined by the electronic structure. One of the most powerful experimental techniques to study the electronic structure is electron spectroscopy. Conventionally most of the investigations on molecules, carried out using this technique, have dealt with static information such as binding energies and localization of the electron. This thesis focuses on the currently expanding field more concerned with dynamic effects such as lifetimes and dissociation times. In particular, resonantly excited molecules have been studied. As an example, we have used the "core hole clock" method to determine the dissociation times of core excited water and ammonia molecules. These experiments are possible using a synchrotron light source and the experiments are based on ultra-fast dissociation of resonantly excited states. A similar experiment on doubly core-excited nitrogen molecules have revealed no sign of ultra-fast dissociation as the resulting spectral features contain vibrational progressions and therefore result from molecular decay.Molecules that dissociate on an ultra-fast timescale, i.e. in the low femtosecond regime, are interesting for several reasons. For instance, decay occurring in core excited oxygen fragments can be used to demonstrate a localization of the core hole through the existence of a Doppler effect. Also, the nature of the molecular intermediate state leading to ultra-fast dissociation can be of interest. For the oxygen molecule there is a debate concerning the assignment of the large absorption structure near the O1s ionization energy. We are able to disclose which photon wavelengths lead to ultra-fast dissociation.Direct information on the time dependence of electronic structure and molecular dynamics during the ionization process can be obtained through the use of fast pulsed lasers in pump-probe experiments. For such studies we have used the terawatt laser at the Lund Laser Center providing pulses down to 110 fs long to observe the decay of two different states in valence excited acetylene.