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Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0. 8 PW/cm^2 and 1. 1 PW/cm^2, where PW10^15 W/cm^2). Numerical solutions of the full-dimensional time-dependent helium Schr\"odinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.
Parker et al. (Thu,) studied this question.