A framework for understanding molecular ionization in strong laser fields

The aim of this paper 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 iodine molecules 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 led us to examine closely the individual ionization steps. 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. 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 molecule, this pair consists of the ground and first excited state whereas in a two-electron molecule this corresponds to the lowest lying pair of ionic states. In this paper, we propose a framework for organizing the numerous ionization pathways based on the electronic configuration of the initial charge state. (author)