Influence of autoionizing states on the pulse-length dependence of strong-field Ne+ photoionization at 38.4 eV

Using the time-dependent R-matrix approach, we investigate ionization of ground-state Ne+, irradiated by laser light with a photon energy of 38.4 eV at intensities 1013 W cm-2, 2 x 1013 W cm-2 and 1014 W cm-2 as a function of pulse length. Although the photon energy is below the threshold for single-photon ionization, we obtain a significant contribution from single-photon ionization to the ionization probability due to the finite duration of the pulse. The two-photon ionization rates deduced from the calculations are consistent with those obtained in R-matrix-Floquet rate calculations. The ionization probability oscillates with pulse length, which is ascribed to population and depopulation of autoionizing states just above the Ne2+ ground state, reached after absorption of a single photon. At an intensity of 1014 W cm-2, pulse lengths longer than 50 cycles are required for two-photon ionization to dominate the ionization probability.