We present numerical solutions of the time-dependent Schrödinger 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.

Phys. Rev. Lett.

Muller, H. G. (1999). Tunneling excitation to resonant states in helium as main source of superponderomotive photoelectrons in the tunneling regime. Phys.Rev.Lett., 83, 3158–3161.