Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities ($0.8\mathrm{PW}/{\mathrm{cm}}^2$ and $1.1\mathrm{PW}/{\mathrm{cm}}^2$, where $\mathrm{PW}\equiv {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$). 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.

• Received 13 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.133001