High-Energy Cutoff in the Spectrum of Strong-Field Nonsequential Double Ionization

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/cm² and 1.1 PW/cm², where PW≡10¹⁵ W/cm²). 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 5Uₚ cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.