Comparison Study of Atomic and Molecular Single Ionization in the Multiphoton Ionization Regime

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N₂⁺ to Ar⁺, a higher ion signal is seen in N₂⁺ compared to Ar⁺ for linear polarization but the difference vanishes in circularly polarized light. In comparing O₂⁺ to Xe⁺, we observe a suppression in O₂⁺ compared to Xe⁺ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O₂⁺ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O₂⁺ in the multiphoton ionization regime.