Non-Sequential Double Ionization of Nitrogen

Abstract

Although the non-sequential double ionization (NSDI) of atoms has been well documented experimentally, there is still no consensus as to the physical mechanism.1 However, in an atom the only information easily obtained about the ionization process comes from ion yields versus laser intensity. For this reason, we have begun to study the multiphoton ionization of nitrogen in order to learn more about the double ionization process as there are more final state products to analyze. Specifically, if we consider the rescattering model2 for NSDI the electron ionized from N2 → N2+ would return to and further ionize the molecule creating N22+. However, the molecular double ion can then remain as N22+, dissociate into N+ + N++ K.E., or N22+ → N + N2++ K.E. or be further ionized to N23+ and again dissociate. By analyzing the ion yields for N2+, N22+, N+, and N2+ and the kinetic energies of the N+ and N2+ ion fragments we have observed the following: 1) NSDI does occur in nitrogen with approximately the same rate as in argon. 2) NSDI does not lead to the fragmentation of N22+ into any dissociation channel. From these observations, we conclude that the NSDI process in N2 always leaves the N22+ molecular ion in the metastable ground state. Furthermore, it would seem to be unlikely that the rescattering model for NSDI would lead to such final state specificity.