Ion time-of-flight and vacuum ultra-violet fluorescence spectroscopy of highly ionized nitrogen.

摘要

This thesis investigates dissociative ionization in ultrafast laser fields. We review a possible mechanism for enhanced excitation that is exclusive to molecular systems and may provide a connection between multiphoton excitation and the phenomenon of enhanced ionization. Following our discussion of the excitation model, we present a study of the vacuum ultraviolet fluorescence produced by molecules in strong laser fields. These results show, that not only is enhanced excitation exclusive to molecules, but also that the excitation is selective, creating only 2s-1(-2) configurations. Such excitations correlate to charge-transfer states that are fundamental to both the electron localization of enhanced ionization and to the multiphoton susceptibility model. To gain a deeper insight into the details of the molecular dissociation we compare the fluorescence with results of an ion time-of-flight study and find that an extension of the excitation model to include odd-charged molecules may be in order. We then show that pump-probe experiments show clear evidence of resonant enhancement of the ionization process.