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.

© 1997 Optical Society of America