Evidence of decrease of potential energy in nonsequential double ionization above threshold for electron impact excitation

We study the CEP (carrier envelope phase) dependent correlated electron momentum distributions for nonsequential double ionization of Ar in near-single-cycle short pulses at the intensity of 3.0 × 1014 W/cm2 with a central wavelength of 750 nm measured by Bergues et al [Nat. Comm. 3, 813 (2012)]. A kinematical analysis on the constraints for the correlated electron momenta reveals that the excitation potentials of the parent ion Ar+ are reduced at the instant of scattering.