Tunneling Excitation to Resonant States in Helium as Main Source of Superponderomotive Photoelectrons in the Tunneling Regime

We present numerical solutions of the time-dependent Schr\"odinger equation in the single-active-electron approximation and calculate wave functions for photoionization of helium exposed to 800 nm light. Electron spectra show ~200 peaks up to 272 eV, due to above-threshold ionization. The simulations confirm the existence of a wide, flat plateau in the electron spectrum due to backscattering, between 3 and 8 times the ponderomotive energy. Electrons in this range originate almost exclusively through resonance enhancement by quivering excited states, which in turn are populated by light-induced tunneling from the ground state.