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

Abstract

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 PW≡1015 W/cm^2). Numerical solutions of the full-dimensional time-dependent helium Schrödinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U_p cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.