The main goal of this dissertation is to examine the role of excited states and multi-electron interactions in molecular ionization by strong laser fields. We present new data on the ionization and dissociation of homonuclear diatomic molecules (N₂, I₂, and O₂) that reveal important aspects of the strong field-molecule interaction in the short pulse regime. Our data, along with previous studies, is inconsistent with the simplest and commonly accepted model of molecular ionization in a strong laser field and this has lead us to examine closely the individual ionization steps. By retracing the fundamental analytic methods, we have developed a framework for understanding molecular dynamics in strong laser fields. We have found that a molecule can ionize into several distinct configurations predominantly through multi-electron interactions, and, the abundance of such configurations is dependent on internuclear separation. In the case of homonuclear molecules, the ionization appears to be dominated by pairs of states with gerade and ungerade symmetry as they have a large dipole coupling and the transition is near resonant with the strong laser field. In a one-electron homonuclear molecule, this pair consists of the ground and first excited state whereas in a two-electron homonuclear molecule this corresponds to the lowest lying pair of ionic states. In this dissertation, we propose a framework for organizing the numerous ionization pathways based on the electronic configuration of the initial charge state.

In addition, we have also extended our analysis to heteronuclear diatomic molecules. Such molecules do not have identical nuclei and hence the gerade-ungerade inversion symmetry vanishes. Our analysis of the CO molecule shows that multi-electron effects do indeed play an important role in the ionization process, though the exact nature of the dominant molecular states remains unknown.